Note On Chemistry Practice On Energetics

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Chemistry practice on energetics

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PMT The Born-Haber cycle for the formation of sodium chloride from sodium and chlorine may be represented by a series of stages labelled A to F as shown. Naw (g) + Cl(g) + e— Na+(g) 4 C12(g) + e- Na(g) + (g) c Na(s) + (g) CV(g) NaC1(s) Write the letters A to F next to the corresponding definition in the table below (i) (ii) definition ionisation energy of sodium I st electron affinity of chlorine the enthalpy of atomisation of sodium the enthalpy of atomisation of chlorine the lattice enthalpy of sodium chloride the enthalpy of formation of sodium chloride letter AH/kJ mol- +494 -364 109 +121 -770 Calculate the enthalpy of formation of sodium chloride from the data given.
PMT The lattice enthalpies can be calculated from theory as well as determined experimentally. (b) (c) Sodium chloride Silver iodide Expefimental AH/kJ mol- -770 _889 Theoretical AH/kJ mor -766 -778 Why is the experimental value of the lattice enthalpy of silver iodide (—889kJmorl) so different from the value calculated theoretically? Explain the trend in first ionisation energies of the elements ofGroup I in the Periodic Table. (Total 10 marks)
2. (a) PMT The Born-Haber cycle for the formation of sodium chloride is shown below. Na+(g) + Cl(g) + e 2 C12 (g) + e Na(s) NfCV(s) Use the data below to calculate the lattice enthalpy of sodium chloride. Enthalpy change Enthalpy of atomisation of sodium I st ionisation energy of sodium Enthalpy of formation of sodium chloride Enthalpy of atomisation of chlorine Electron affinity of chlorine Value of the enthalpy change /kJ mol- +109 +494 _411 +121 -364 3
3. (b) (c) (a) (b) PMT Sodium chloride and magnesium oxide have very similar crystal lattices. Suggest why the lattice enthalpy of magnesium oxide is very much larger than that of sodium chloride. The lattice enthalpy of silver iodide can be calculated but the experimental value does not match the calculated value as well as those for sodium chloride match each other. Explain why the calculated and experimental values for silver iodide are different. (Total 6 marks) State Hess's Law. Define the term standard enthalpy change of combustion.
PMT (c) The equation for the combustion of ethanol in air is C2H50H(1) + 302(g) 2C02(g) + 3H20(l) and the structural representation of this is: H—C—C (i) Calculate the enthalpy change for this reaction using the average bond enthalpy values given below. Bond Average bond enthalpy/kJ mol- +412 +360 +496 Bond Average bond enthalpy/kJ mol- +348 +463 +743 4. Draw and label an enthalpy level diagram to represent this reaction. entha Ipy (Total 10 marks) In an experiment to find the enthalpy change when copper is displaced from a solution of copper ions excess zinc was added to 50.0 cmg of 1.00 mol dm- aqueous copper(ii) sulphate in a plastic cup. 5
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(a) (b) (c) PMT Suggest two reasons why a series of temperature readings is taken rather than sinply initial and final readings. First Second reason. Use the graph to calculate the maximum temperature change, AT, as the reaction takes place. Calculate the enthalpy change for the reaction using the formula below, giving your answer to an appropriate number of significant figures. -4.18 x AT mor (Total 6 marks)
5. (i) (iii) PMT Define the term standard enthalpy of formation, AHf The following table shows some values of standard enthalpy of formation. Name ethene hydrogen bromide bromoethane Formula C2H4(g) HBr(g) C2H5Br(g) AHfØ/kJ mor +52.3 -36.2 Use the data in the table above to calculate the standard enthalpy change forthe following reaction. C2H4(g) + HBr(g) —s C2HsBr(g) State the significance of the sign of the value obtained in part (a)(ii) above. (b) Enthalpy changes can also be calculated usingaverage bond enthalpy data. 3
Bond c c PMT Average bond enthalpy/kJ mol-I +612 +348 +412 +276 +366 H—Br 6. (c) Use the data in the table above to recalculate the enthalpy change for thereaction in part (a)(ii). C2H4(g) + HBr(g) C2H5Br(g) Suggest why the value obtained in part (b) above is likely to be less accurate than that obtained in part (Total I marks) The formation of magnesium chloride from magnesium and chlorine may be represented bythe following Born-Haber cycle:
PMT 2C1 (g) (a) (b) 42C1(g) 2e MgHg) + C12(g) + 2e Mg(g) + Cl 2(g) Mg(s) 4 (g) MgC12(s) Define the terms: Lattice enthalpy. Enthalpy of atomisation. (i) Identify on the diagram the chance representing the enthalpy of atomisation of magnesium. Use the data below to calculate the first electron affinity of chlorine. Value of the enthalpy Enthalpy change
Enthalpy of atomisation of magnesium I st Ionisation energy of magnesium 2nd Ionisation energy of magnesium Enthalpy of formation of magnesium chloride Enthalpy of atomisation of chlorine Lattice enthalpy of magnesium chloride PMT change / k] mol-I +150 +736 +1450 _642 +121 -2493 (c) Hydrogen gas reacts with sodium metal to form an ionic solid, NaH, which contains sodium cations. Draw a Born-Haber cycle which could be used to determine the electron affinity of hydrogen. (Total I marks)
7. PMT The reaction of an acid with a base to give a salt is an exothermic reaction. In an experiment to determine the enthalpy of neutralisation of hydrochloric acid with sodium hydroxide, 50.0cn13 of I .00 mol dm-g HCI was mixed with 50.0 cmg of l. 10 mol dm-g NaOH. The temperature rise obtained was 6.90 DC. (a) (b) (c) Define the term enthalpy Of neutralisation. Assuming that the density ofthe final solution is 1.00 g cm- and that its heat capacity is 4.18 J K 1 g- , calculate the heat evolved during the reaction. 0.0500 mol of acid was neutralised in this reaction; calculate kJ mol-
PMT Suggest why sodium hydroxide is used in slight excess in the experiment. 8. (d) (a) (i) (Total 7 marks) Define the term standard enthalpy of combustion. The values for the standard enthalpy of combustion of graphite and carbon monoxide are given below: AHc C (graphite) CO(g) /kJ mol- -394 -283 Use these data to find the standard enthalpy change of formation of carbon monoxide using a Hess's law cycle. C(graphite) + — 02(g) —+ CO(g) 13
(iii) (iv) PMT Suggest why it is not possible to find the enthalpy of formation of carbon monoxide directly. (1) Draw an enthalpy level diagram below for the formation of carbon monoxide from graphite. (b) Natural gas consists of methane, CHA. When methane burns completely in oxygen the reaction occurs as shown in the equation CH4(g) + 202(g) C02(g) + 2H20(l) Methane does not burn unless lit. AHc = -890 k] mori 9. Use this information to explain the difference between thermodynamic and kinetic stability. (Total 12 marks) A student was required to determine the enthalpy change for the reaction between iron and copper sulphate solution.
PMT The student produced the following account of their experiment. A piece of iron, mass about 3 g, was placed in a glass beaker. 50 cm3 of 0.5 mol drn-3 aqueous copper sulphate solution was measured using a measuring cylinder and added to the beaker. The temperature of the mixture was measured immediately before the addition and every minute afterwards until no further change took place. Fe + + Cu Timing before addition I min Temperature/ OC 22 2 mins 29 3 mins 26 4 mins 24 5 mins 22 (a) Suggest two improvements you would make to this experiment. Give a reason for each of the improvements suggested. Improvement I . Reason I Improvement 2 Reason 2 15
(b) PMT In an improved version ofthe same experiment a maximum temperature rise of 15.2 oc occurred when reacting excess iron with 50.0 cn13 of0.500 moldm- aqueous copper sulphate solution. (i) (iii) Using this data and taking the specific heat capacity of all aqueolö solutions as 4.18 Jg-l deg- calculate the heat change. (1) Calculate the number of moles of copper sulphate used. Calculate the enthalpy change of this reaction in kJ mol-I. (Total 8 marks)
10. (a) (b) (c) PMT Define the term standard enthalpy of combustion, making clear the meaning of standard in this context. Use the enthalpies of combustion given below to find the enthalpy change for the reaction: 2C(graphite) + 2H2(g) + 02(g) —s CHgCOOH(1) C(graphite) H2(g) CHgCOOH(l) mol-I -394 -286 -874 With reference to ethanoic acid, CH3COOH, what is the enthalpy change obtained in (b) called?
11. (d) (a) PMT Draw an enthalpy level diagram to represent the enthalpy change for the combustion of graphite. Show both the enthalpy levels of the reactants and products and an energy profile which represents the activation energy for the reaction. (Total 10 marks) This question is about finding the formula of copper hydroxide. The method is as follows: 18
PMT 20.0 cm-g of an aqueous solution of a copper salt ofconcentration 1.00 mol dnf was placed in a polystyrene cup and its temperature measured using a thermometer graduated in 0.1 oc intervals. A burette was filled with aqueous sodium hydroxide, of concentration 2.00 mol dm 2.00 cm of sodium hydroxide solution was run into the solution of the copper salt and the temperature was measured immediately. As soon as possible a further 2.00 cmg of sodium hydroxide solution was run in and the temperature measured again. This process of adding 2.00 cm; portions of sodium hydroxide solution and measuring the temperature was continued until a total of36.0 cm ofthe sodium hydroxide solution had been added. The temperature readings are shown in the graph below. 30 29 Temperature / oc 28 27 26 25 24 23 22 21 20 o 4 12 16 20 24 28 32 36 40 Volume of NaOH(aq) / cm
(i) (ii) (iii) (v) (vi) PMT Explain why the temperature reaches a maximum and then falls slightly on addition of further sodium hydroxide solution. From the graph, what volume ofthe aqueous sodium hydroxide was required for complete reaction? Calculate the amount (number of moles) of sodium hydroxide in this volume of solution. Calculate the amount (number of moles) of copper ions that have reacted. Write the ratio of moles of copper ions to hydroxide ions reacting. Write the formula of the copper hydroxide that is produced. (b) The data call be used to find the enthalpy change for the reaction between sodium hydroxide and the copper salt. 20
(i) (iii) PMT Use the graph to find the temperature rise that occurs for complete reaction. Find the heat change, q, that occurs in the polystyrene cup for complete reaction. Use the formula q = 168 x AT joules Use your results from (a)(iv) and (b)(ii) above, to find the molar enthalpy change, AH, for the reaction. Give the correct sign and units to the answer. (c) Identify one potential source of error in this experiment, and say what you would do to reduce its effect. (Total 14 marks)
PMT An excess of zinc powder was added to 20.0 cm; of a solution of copper(ll) sulphate of concentration 0.500 mol dm- . The temperature increased by 26.3 oc. 12. 13. (a) (b) How many moles of copper(ll) sulphate were used in this experiment? (1) Calculate the enthalpy change, AH, in kJ mol-I for this reaction given that: energy change = specific heat capacity x mass of solution /g x temperature change Assume that the mass of solution is 20.0 g and the specific heat capacity of the solution is 4.18 J SK (Total 5 marks) Urea, which is used as a fenillser in much of mainland Europe, Asia and Africa, is manufactured by the reaction of ammonia and carbon dioxide. 2NHg(g) + C02(g) -s NH2CONH2(s) + H20(l)
PMT , of urea. (a) (b) Define the term standard enthalpy of formation, Affr Calculate the enthalpy change, AH for the reaction above, given the following standard enthalpies of formation. Substance NHg(g) C02(g) NH2CONH2(s) H20(l) 0/ k] mor _46.2 -393.5 -632.2 -285.8 (Total 6 marks)
14. PMT Halogenoalkanes react with many nucleophiles. Define the term nucleophile. (a) (b) (i) Identify the reagent and conditions necessary for the conversion of iodoethane to ethylamine, C2H5NH2. Reagent.... . State why the rate of reaction would be slower if bromobutane were used in place of iodoethane, with all other conditions remaining the same.
PMT Iodoethane reacts with water to form ethanol and hydrogen iodide. (c) (d) C2H51 + H20 C2H50H + HI = +36 kJ mol- Use some or all ofthe data below to calculate the bond enthalpy. Bond Bond enthalpy / kJ mol- 413 347 464 Bond HA Bond enthalpy / kJ mol- 298 358 Ethanol was heated under reflux with an excess of a mixture of potassium dichromate(VI) and dilute sulphuric acid. Draw the full structural formnula of the orgamc product. (Total 10 marks)
15. PMT The equation below shows the reaction which occurs when ammonia is dissolved in water. (a) (b) (i) NHg(g) + H20(1) (aq) + OW(aq) Explain why water is classified as an acid in this reaction. The ammonia is acting as a weak base in this reaction. What is the difference between a weak base and a strong base? Ammonia reacts with oxygen to form the gases nitrogen(ll) oxide and steam. (i) Complete the Hess cycle below so that can be calculated using standard enthalpy changes of formation. Include state symbols. 4NH3(g) + 502(g) 4NO(g) + 6H20(g)
16. PMT Calculate for this reaction using the following data. [NH3(g)] = - 46.1 k] mori AH, [NO(g)] = + 90.2 kJ mor AH, [H20(g)] = - 241.8 kJ mori Include a sign and units in your answer and give your answer to three significant figures. (Total 7 marks) The apparatus shown in the diagram below may be used to find the enthalpy of combustion of alcohols. water spirit lamp ethanol
PMT Using the apparatus, a student recorded the results included in the table below. Alcohol = ethanol, C 2H50H Molar Mass (C2HsOH) = 46.0 g mol - Volume of water in beaker = 200 cm . mass of water in beaker = 200 g Weighings Spirit lamp + ethanol before combustion = 198.76g Spirit lamp + ethanol after combustion = 197.68 Temperatures Water before heating = 19.5 oc Water after heating = 38.1 oc Specific heat capacity of water = 4.18 J g (a) (b) What assumption is the student making about water to be able to state that its mass is numerically equal to its volume? Calculate the heat gained by the water. Give your answer in kJ.
(c) (d) (e) PMT Calculate the amount (number of moles) of ethanol used. Using your values from (b) and (c), calculate the enthalpy of combustion of ethanol. Give your answer to a number of significant figures consistent with the readings in the table. Include a sign and units in your answer. The student's evaluation ofthe experiment is given below. My calculated value of the enthalpy of combustion was 2 3 numerically much less than the data book value. The reasons for my low value include: heat losses to the surrounding air; when I re-checked the mass of the spirit lamp and ethanol after combustion, I noticed that it had lost mass even when it was not being used; a black solid which formed on the base of the beaker. (i) Explain why the spirit lamp and ethanol lost mass even when not in use. 29
17. PMT Suggest the identity ofthe black solid. Explain why its formation will lead to a low value for the enthalpy of combustion. Identity Explanation . (Total I marks) This question is about a self-heating can of coffee. The bottom of the can has a compartment containing copper(ll) nitrate solution. When a button on the bottom of the can is pressed, magnesium powder is released into the compartment where it reacts with the copper(ll) nitrate solution. (i) Write an ionic equation for the reaction between magnesium powder and copper(ll) ions. Include state symbols, but omit any spectator ions.
(b) PMT Show how the standard enthalpy change for this reaction could be calculated from the standard enthalpies of formation of copper(ll) ions and magnesium ions. You should include a Hess cycle in your answer. The can contains 150 g ofa solution of coffee in water. The temperature of the solution needs to increase by 60 oc to produce a hot drink. (i) Calculate the energy change needed to produce a temperature increase of 60 oc in the coffee, using the relationship Energy change = 4.2 x mass of solution x temperature change. Remember to include a unit in your answer. The standard enthalpy change for this reaction is —530 kJ mol- Calculate the number of moles of reactants needed to produce the energy change in (1) 31
(iii) PMT A solution of copper(ll) nitrate of concentration 8.0 mol din- is used. Use your answer to (ii) to calculate the volume, in cm , of coppe«ll) nitrate solution needed. Your answer should be given to two significant figures. (c) Suggest TWO reasons why the temperature ofthe coffee may not increase by as much as 60 pc. (Total Il marks) 18. The reaction between chlofine and methane, in the presence of ultraviolet light, involves the formation of free radicals and includes the following steps: A B c D E CHA + cr -s C12 + CHs• cr + CHg• + CHg• -s cr + CHg• -s 2C1• HCI + CHgC1 + cr C12 CHgCHg 242 kJ mol-I AHO = +4 kJ mol I AM = -97 k] mor AT = -339 kJ mori
(i) (iii) PMT What is meant by a free radical? Draw a 'dot-and-cross' diagram, showing outer shell electrons only, for a chlorine free radical. What type ofbond breaking occurs in step A? (b) (d) Which of the steps, A to F, are chain propagation steps? (i) (ii) (i) Write the equation for the overall reaction between one mole of chlorine and one mole of methane molecules. Calculate the standard enthalpy change, AM', for this reaction. What is the value of Al-r for step D? — 33
(e) PMT Would you expect step E to be exothermic or endothermic? Justify your answer. The overall reaction was repeated using bromine gas instead of chlorine gas. Would you expect step A for bromine to be more or less endothermic than step A for chlorine? Justify your answer. (Total I I marks)
19. PMT In two similar, separate experiments the enthalpy changes for the reactions of sodium hydrogencarbonate and sodium carbonate with excess dilute hydrochloric acid were determined. (a) The first experiment was to find the enthalpy change, AHI , for the reaction NaHCOg(s) + HC1(aq) —s NaCl(aq) + C02(g) + H20(l) Measurement Mass of solid sodium hydrogencarbonate added to hydrochloric acid. Volume of hydrochloric acid Temperature of hydrochloric acid before addition of solid sodium hydrogencarbonate Final temperature of solution Molar mass of sodium hydrogencarbonate Specific heat capacity of solution Reading 500 g 50.0 cm 22.0 oc 15.5 oc 84.0 g mol- 4.18 J g-l (i) Calculate the amount (moles) of sodium hydrogencarbonate used. 35
(iii) PMT Calculate the heat absorbed in the reaction in kJ. [Assume that I cm of solution has a mass of I g] Calculate the value ofMIl in kJ mol-I. Include a sign in your answer expressing it to a number of significant figures suggested by the data in the table.
(b) PMT In the second experiment the enthalpy change for the reaction between sodium carbonate and dilute hydrochloric acid was measured. NazC03(s) + 2HCl (aq) 2NaC1(aq) + C02(g) + H20(l) The molar enthalpy change, AH2, was calculated to be —35.6 kJ mol- (i) Give TWO ways in which the temperature change differs when equal molar amounts of sodium hydrogencarbonate and sodium carbonate react separately with the same volume of hydrochloric acid. Give ONE assumption that has been made in calculating the values of AHI , and AH2 from experimental results. (Total 8 marks)
20. PMT In the manufacture ofbeer, brewers often add small amounts of salts of Group 2 elements to the water used. These salts influence the chemical reactions during the brewing process. Two such salts are calcium sulphate and magnesium sulphate. (a) A flame test can be used to confirm that a sample of a salt contains calcium ions. (i) (iii) Describe how you would carry out a flame test. A positive test results in a brick-red flame colour. Describe the changes that occur in calcium ions to produce a colour. Impurities in the salt may lead to other colours being observed in the flame. What metal ion is likely to be present ifa yellow flame is seen?
(b) (c) PMT Magnesium sulphate can be used in its anhydrous form, MgS04(s), or in its hydrated form, MgS04.7H20(s). An experiment was carried out to find the enthalpy change when hydrated magnesium sulphate dissolved completely in water. excess water •F 7 H20(l) 12.3 g of hydrated magnesium sulphate was added to 100 g ofwater in a simple calorimeter and the temperature was found to fall by I. I oc. (i) (ii) (iii) Calculate the energy change, in joules, that occurred in the experiment, using the relationship Energy change (J) = 4.18 x mass of water x temperature change Calculate the number of moles of hydrated magnesium sulphate used in the experiment. Use the Periodic Table as a source of data. Use your answers to (i) and (ii) to calculate the enthalpy change for the reaction. Include a sign and units in your final answer, which should be given to 2 significant figures. The enthalpy change as hydrated magnesium sulphate is converted to anhydrous magnesium sulphate is very difficult to measure. The Hess Cycle below can be used to
find this enthalpy change, AHr. MgS04 _ 7H20(s) excess H20 PMT MgS04(aq) + 7H20(1) excess HIC) (i) (ii) MgS04(s) + 7H20(1) Use the cycle to write an expression for Affr using AHI and AH2. (1) Use your expression in (c)(i) and your answer from (b)(iii) to calculate AHr. Include a sign and units in your final answer, which should be given to 2 significant figures. (Total IS marks) 21. Phosphine, PHg, is a hydride of the Group 5 element, phosphorus. (i) Draw a 'dot-and-cross' diagram of a phosphine molecule. You should include only outer shell electrons. 40
(b) (iii) (i) PMT Draw the shape you would expect for the phosphine molecule, suggesting a value for the HPH bond angle. HPH bond angle Explain the shape of the phosphine molecule you have given in your answer in (ii). Justify your value for the HPH bond angle. Write a balanced equation, including state symbols, for the atomisation of phosphine gas.
(ii) (iii) PMT Use your answer to (i) and the data below to calculate the standard enthalpy change of atomisation of phosphine at 298 K. Include a sign and units in your answer. = + 5.4 kJ mol- = +218.0kJ mori = +314.6 k] mori Calculate a value for the bond energy of the bond between phosphorus and hydrogen, using your answer to (ii). (Total 10 marks) 22. Methane, CH4, is used as a domestic and industrial fuel and as a reagent in the petrochemical industry. (a) Define the term standard enthalpy Of combustion.
(b) PMT Methane burns in oxygen according to the equation: Use the average bond enthalpy data shown below to calculate the enthalpy change of this reaction. Bond Bond enthalpy/kJ mol- +435 +498 +805 +464
(e) PMT Methane is the feedstock in the manufacture of hydrogen according to the equation: CH4(g) + 2H20(g) —s C02(g) + 4H2(g) Given the enthalpy of formation data below, draw a labelled Hess's law cycle and use it to calculate the enthalpy change of this reaction. Substance Enthalpy of formation/kJ mol- CH4(g) C02(g) H20(g) -75 -394 -242 (Total 10 marks) 23. In an experiment to find the enthalpy of neutralisation of a monobasic acid, HX, with an alkali, the following procedure was followed: Step 1 Step Il Step Ill 25.0 cmg of I .00 mol dm-3 dilute aqueous acid, HX, was measured into a polystyrene cup. A 0-100 oc thermometer was placed in the acid. The temperature of the acid was immediately read and recorded. 5.00 cmg poltions of aqueous sodium hydroxide were added to the acid from a burette. After each addition, the temperature of the solution was read and recorded. The thermometer was removed and rinsed with water between each addition. A total of 50.0 cm of aqueous sodium hydroxi& was added.
(a) (b) PMT Suggest ONE change that could be made at Step Il and ONE change that could be made at Step Ill to improve the accuracy of the experiment. Step Il Step Ill . The readings of temperature and volume are plotted on the grid. Draw two separate straight lines of best fit, extending the two lines so that they intersect. 25 Temperature 20 15 10 20 30 40 Volume of sodium hydroxide added / cni 50 45
PMT From the graph, read off the maximum temperature rise, AT, and the volume of aqueous sodium hydroxide added at neutralisation, VN. (c) (d) AT- (i) Use the formula below to calculate the heat evolved in the neutralisation. (VN +25b ATx 4.18 Heat evolved — kJ 1000 Given that the amount (moles) of acid neutralised was 0.025 mol, calculate the enthalpy of neutralisation, AHneut, in units ofkJ mol- „ kJ mol (Total 9 marks)
PMT A reaction of ammonium dichromate(VI) is shown by the following equation. 24. What is the value of (a) (b) N2(g) + 4H20(g) + Cr20g(s) What type of reaction is this? The enthalpy change for this reaction can be calculated from standard enthalpy changes of formation. State fully what is meant by the standard enthalpy change of formation AH/ of a compound. Complete the Hess cycle for the reaction so that you can calculate the enthalpy change of the reaction from standard enthalpy changes of formation. N2(g) + 4H20(g) + CT203(s) N2(g) + (iii)
(iv) PMT Calculate Afr,. for the reaction using the following data. Remember to include a sign and units in your answer. - -1810kJ mori -242kJ mol I - -1140kJ mori (c) In this reaction, water vapour is formed which condenses to liquid water on cooling. Is this reaction H20(g) —9 H20(1) exothermic or endothermic? Justify your answer. (Total 13 marks)
25. PMT Chlorine can be converted to the gas chlorine(l) oxide, CEO. The standard molar enthalpy change of formation of chlorine(l) oxide and the standard molar enthalpy changes of atomisation of chlorine and oxygen are given below: AH/ [C120(g)] AH: ['hC12(g) AHu,H [Ih02(g)] + 80.3 kJ mori +121.7k] mori +249.2 kJ mol-I A partially completed Hess cycle involving chlorine(l) oxide is shown below: [ChO(g)] C120(g) 2C1(g) + O(g) (i) (ii) elements in their standard states Insert the appropriate formulae, showing the correct quantities of each element, into the box above. Include state symbols in your answer. Insert arrows between the boxes, writing the correct numerical data alongside the appropriate arrows. 49
PMT 26. (iii) (iv) (a) Use the cycle to calculate Allure Calculate the CI—O bond energy in chlorine(l) oxide. (Total 14 marks) Enthalpy changes can be calculated using average bond enthalpy data. (i) The enthalpy change to convert methane into gaseous atoms is shown below. CH4(g) C(g) + 4H(g) +1664 kJ mol- Calculate the average bond enthalpy of a C—H bond in methane.
PMT Use the data in the table below and your answer to (a)(i) to calculate the enthalpy change for 2C(g) + 2H2(g) + Br2(g) CH2BrCH2Br(g) Bond c_c Br—Br Average bond enthalpy / kJ mol- +348 +193 Bond C_Br Average bond enthalpy / kJ mol- +436 +276 (b) The standard enthalpy of formation of I ,2-dibromoethane, CH2BICH2Br, is -37.8k.' Suggest the main reason for the difference between this value and your calculated value in (Total S marks) 51
27. PMT In an experiment to find the enthalpy change for the reaction Zn(s) + —s Zn (aq) + Cu(s) a student was given the following list of instructions: weigh out 5.0 g of zinc powder into a weighing bottle use a measuring cylinder to transfer 50 cm of I mol dm- aqueous copper(ll) sulphate into a polystyrene cup, firmly held in a 250 cm beaker stir the solution with the thermometer and record the temperature to the nearest 0.5 oc continue to stir the solution, recording its temperature every minute at exactly 3.5 minutes, add the zinc powder to the aqueous coppe<ll) sulphate, stiffing continuously record the temperature ofthe solution every minute from 4.0 to 9.0 minutes. The temperature readings obtained are shown in the table below. Time/min o 1.0 2.0 3.0 4.0 20.0 20.0 20.0 20.0 63.0 60.5 6.0 59.0 7.0 57.0 8.0 55.5 9.0 53.0
(i) PMT Plot a graph of temperature against time on the grid below. 70.0 60.0 50.0 Temperature 40.0 30.0 20.0 10.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 Time / minutes 9.0 10.0 (ii) Use the graph to calculate the maximum temperature change, AT. Show clearly on the graph how you obtained your answer.
(b) (iii) (i) (ii) (iii) PMT Give ONE reason why a series of temperature readings is obtained instead of just the starting and maximum temperatures. (1) Calculate the heat change, in joules. The specific heat capacity of the solution is 4.18 J g-l oc What assumption have you made about the solution in your calculation in (i)? Calculate the amount (moles) of copper (Il) sulphate, CuS04, in 50 of a I .0 mol dm-g solution.
(iv) PMT Calculate the enthalpy change for this reaction in kJ mol-I. (c) Suggest TWO improvements that could be made to the experimental procedure. Give a reason for each. Improvement I . Reason . Improvement 2 Reason (Total 14 marks) 55
28. PMT The values ofthe lattice energies of potassium iodide and calcium iodide experimentally determined from Born-Haber cycles and theoretically calculated from an ionic model are shown below. Potassium iodide, Kl(s) Calcium iodide, Ca12(s) Experimental lattice energy /kJ mol- -651 -2074 Theoretical lattice energy /kJ mol- - 636 -1905 (i) Explain why the experimental lattice energy of potassium iodide is less exothermic than the experimental lattice energy of calcium iodide.
(ii) PMT Explain why the experimental and theoretical values of the lattice energy are almost the same for potassium iodide, but are significantly different for calcium iodide. (Total 6 marks) 29. When solutions of potassium carbonate and calcium chloride are mixed together, the following reaction takes place (a) CaC12(aq) + K2C03(aq) Cacog(s) + 2KC1(aq) Re-write the above equation as an ionic equation. Include state symbols, but omit any spectator ions. 57
(b) PMT An experiment was carried out to measure the enthalpy change for this reaction. 50 cm; of a 1.00 moldm- solution of potassium carbonate was added to 50 of a 1.00 moldm- solution of calcium chloride. The temperature fell by 1.5 oc. (i) (iii) Calculate the energy taken in from the surroundings using the relationship energy = mass of x specific heat capacity x temperature solution 'g You may assume that of solution change 1.0 cm of solution has a mass of 1.0 g. • The specific heat capacity ofthe solution is 4.2 J g Energy taken in = How many moles of calcium chloride are used in this experiment? Calculate the enthalpy change for the reaction, giving your answer to two significant figures. Include a sign and units in your answer.
(v) PMT Which measurement is likely to have caused the major source of error in this experiment? Explain your answer. What apparatus should be used to contain the reaction mixture during this experiment? 30. (a) If the experiment in (b) was repeated, but using only 25 cmg of each solution, predict what the fall in temperature would be. (Total 9 marks) State Hess's Law.
PMT Methane burns in oxygen. (b) 0=0 c=o (i) (iii) CH4(g) + 202(g) —s C02(g) + 2H20(g) Calculate the enthalpy change for this reaction, using the bond enthalpies given below. Bond enthalpy k] mol- +435 +498 +805 +464 State the name of this enthalpy change. The value of this enthalpy change, under standard conditions, is—890 kJ mol- . State the meaning of standard conditions. Suggest, with a reason, why the enthalpy change calculated in (i) is different from the standard value quoted in (iii). 60
(c) PMT Although the reaction between methane and oxygen is exothermic, it does not occur unless the mixture is ignited. Use these facts to explain the difference between thermodynamic and kinetic stability. (Total 14 marks) 31. A student carried out an experiment to find the concentration of a solution of nitric acid and also its enthalpy of neutralisation. The solutions of nitric acid and sodium hydroxide were allowed to reach the same temperature. 50.0 cm ofthe nitric acid was pipetted into a polystyrene cup. A burette was filled with a solution of 2.0 mol dm sodium hydroxide, NaOH. The initial temperature of the acid was recorded. The sodium hydroxide was added to the acid in 5.0 portions. After each addition, the mixture was stirred and the maximum temperature recorded. This was repeated until 45 cmg of the sodium hydroxide solution had been added. 61
PMT The student plotted the results, as shown below. 30 28 26 Temperature 20 18 16 10 15 20 Volume of 2.0 mol din-g 25 30 35 40 45 50 sodium hydroxide solution added/ cni (a) Complete the graph by drawing two intersecting straight lines of best fit. 62
(b) (c) (d) PMT The point where the lines cross represents the neutralisation of the nitric acid by the sodium hydroxide solution. Use the graph to find: (i) the volume of 2.0 mol dm sodium hydroxide solution, NaOH, that reacts exactly with the 50 cm of the nitric acid. the maximum temperature change, AT, in the reaction. (1) The information in (b)(i) and the equation below can be used to calculate the concentration of the nitric acid. (i) (ii) (iii) (i) NaOH(aq) + HNOg(aq) —s NaN03(aq) + H20(l) Calculate the amount (moles) of sodium hydroxide used to neutralise the 50 cm; of nitric acid. Write the amount (moles) of nitric acid in 50.0 cm of the solution. (1) Hence calculate the concentration of nitric acid, HNOg, in mol dmg Use the data from (b) to calculate the heat change for this reaction. 63
PMT and the specific The density of the mixture produced at neutralisation is I .0g cm- heat capacity of the mixture is 4.2 J g-l oc- . Heat change = mass x specific heat capacity x AT (e) Use your answer from (d)(i) and (c)(iii) to calculate the enthalpy of neutralisation per mole of nitric acid, HN03. Include a sign and units with your answer. The enthalpy of neutralisation found by this method may be less exothermic than the data book value because of heat loss. Suggest ONE way to reduce the error due to heat loss. (Total 13 marks) 64
32. (a) (b) PMT Calculate the number of atoms in 3.50 g of lithium. Use the Periodic Table as a source of data. [The Avogadro constant, L = 6.02 x 1023 mol-I] The equation for the reaction of lithium with hydrochloric acid is shown below. (i) 2Li(s) + 2HC1(aq) 2LiC1(aq) + H2(g) Rewrite this equation as an ionic equation, omitting the spectator ions. (1) 65
(iii) PMT Draw a 'dot and cross' diagram of lithium chloride showing all the electrons. Indicate charges clearly on your diagram. The value of the standard enthalpy change for the reaction, AT, is —557 kJ mol—I. State TWO of the reaction conditions necessary for this enthalpy change to be standard. (Total 7 marks) 33. (a) (b) Define the term standard enthalpy Of formation. The dissociation of phosphorus pentachloride is a reversible reaction. 66
PC15(g) PMT PCIg(g) + C12(g) (i) (iii) Use the values of enthalpy of formation given to calculate MI for the forward reaction. PC15(g) PCIg(g) AHf kJ mol-I - 399 - 306 Explain, with reasons, the effect that raising the temperature would have on the composition of the equilibrium mixture. Other than by changing the temperature, suggest how the amount of PC15 present at equilibrium could be increased. Give a reason for your answer. (Total 8 marks) 67
34. PMT The enthalpy change for the thermal decomposition of calcium carbonatecannot be measured directly, but can be found by carrying out two reactions as shown in the Hess cycle below. CaC03(s) AH3 CaO(s) + CO (g) Elements in their standard states Suggest ONE reason why it is difficult to measure A H (a) (b) directly by experiment. In an experiment to find A HI a student added 2.00 g of finely powdered calcium carbonate to 20.0 cm of 2.50 mol dm- hydrochloric acid solution (an excess) in a polystyrene container. The temperature rose from 20.5 oc to 23.0 oc. (i) Why is the calcium carbonate used in this experiment finely powdered, rather than in lumps? Explain why this is important for an accurate result.
PMT Calculate the energy change using the relationship below. Energy change 4.2 x mass of solution x temperature change /g (iii) Assume that the mass of the solution is 20 g. Calculate the enthalpy change, AHI. Include a sign and units in your answer. [The molar mass ofCaCOg is 100 g mol-I] 69
(iv) PMT In another experiment, the value of H2 was found to be —181 kJ mol- Use this result and your answer to (iii) to calculate the value of A H (c) The student checked the experimental results using information from the Book of data in another Hess cycle. AH3 CaCO(s) + CO (g) Elements in their standard states Name the enthalpy change represented by AH3. (Total 10 marks)
35. PMT The Hess cycle below can be used to estimate the enthalpy change of formation, AHf, of the unstable gaseous compound with the formula HOCl(g). 667 k] mol-I HOCl(g) Gaseous atoms of the elements 4589 kJ mol- (i) Elements in their standard states Insert formulae, with state symbols, into the appropriate boxes, to show the correct quantities of each element. Use the cycle to calculate a value for the enthalpy change of formation, AH f [HOCl(g)].
PMT (iii) Assuming that the H O bond energy is +464 kJ mop l, calculate a value for the O—CI bond energy.
(b) (i) PMT Draw a 'dot and cross' diagram for the HOCI molecule showing outer electrons only. Predict the HOCI bond angle. Justify your answer. Angle Justification 73
36. (c) (a) PMT HOCl(g) can be made from chlorine(l) oxide by the reversible reaction C120(g) + H20(g) 2HOCl(g) What effect, if any, would an increase in pressure have on the proportion of HOCl(g) at equilibrium? Justify your answer. (Total 9 marks) Define the term standard enthalpy of formation.
(b) PMT In the Haber process, ammonia is manufactured from nitrogen and hydrogen as shown in the equation. (i) N2(g) + 3H2(g) 2NH3(g) Use the bond enthalpies below to calculate the standard enthalpy of formation of ammonia. Bond NZN in N H-H in H N-H in NH Bond enthalpy / k.' mol-I +945 +436 +391 Draw a labelled enthalpy level diagram for the formation of ammonia in the Haber process. Enthalpy 75
(iii) (iv) (v) PMT State the temperature used in the Haber process and explain in terms of the rate of reaction and position of equilibrium, why this temperature is chosen. Temperature . Identify the catalyst used in the Haber process and state what effect, if any, it has on the equilibfium yield of ammonia. Catal»t Effect on yield Explain why it is necessary to use a catalyst in this process.
(c) PMT The pressure used in the Haber process is 250 atmospheres. (i) State and explain an advantage of increasing the pressure to 1000 atmospheres. Suggest a disadvantage ofusing a pressure of 1000 atmospheres. (Total 18 marks) 37. The enthalpy change for the reaction of anhydrous aluminium chloride, AIC13, with water can be found as follows: Add about 100 cm; of distilled water to a weighed polystyrene cup. Measure the steady temperature of the water. Add anhydrous aluminium chloride to the polystyrene cup, with stirring. (i) Measure the highest temperature reached. Re-weigh the polystyrene cup and contenb; Data Mass of anhydrous aluminium chloride Mass of solution Initial temperature Highest temperature reached Specific heat capacity of the solution Calculate the heat change in this expeliment. = 4.00 g = 104g = 17.5 oc = 43.5 oc = 4.09 J g-
PMT Assuming that 100 of water is a large excess, calculate the enthalpy change, in kJ mol-I , when one mole of aluminium chloride reacts. Include a sign and unit in your ans wer. 38. (ii) (a) (i) (Total 5 marks) Write the equation which represents the change occurring when the standard enthalpy of atomisation of bromine is measured. Define lattice energy. 78
(b) PMT A Born-Haber cycle for the formation of magnesium chloride is shown below. Mg2+(g) + 2Cl(g) 4 2e- Mg2+(g) + C12(g) + 2e Mg+(g) 4 C12(g) + e Mg(g) 4 C12(g) Mg(s) + C12(g) MgC12(s) Enthalpy of atomisation of magnesium I st ionisation energy of magnesium 2nd ionisation energy of magnesium Enthalpy of atomisation of chlorine Enthalpy of formation of magnesium chloride Lattice energy of magnesium chloride 4 2CV(g) Value / kJ mol- +150 +736 +1450 +122 _642 -2526
(c) PMT The theoretically calculated value for the lattice energy of magnesium chloride is -2326 kJ Explain, in terms of the bonding in magnesium chloride, why the experimentally determined value of—2526 kJ mol-I is significantly different from the theoretical value. The table shows values for the lattice energies of the metal chlorides of some Group 2 metals. Group 2 metal chloride Lattice energy/ kJ mol- MgC12 -2526 caC12 -2237 srC12 -2112 BaC12 -2018 Explain why these lattice energies become less exothermic from MgC12 to BaC12. (Total 10 marks) 80
39. PMT An experiment was can•ied out to find the enthalpy change for the reaction of zinc powder with copper(ll) sulphate solution. zn(s) + CuS04(aq) ZnS04(aq) + cu(s) 50cmg of copper(ll) sulphate solution, ofconcentration I .0 mol dm- , was put into a polystyrene cup and the temperature of the solution measured. After one minute, 5.0 g of zinc powder was added, the mixture stirred with a thermometer and the temperature measured every 30 s. (i) (ii) (iii) What is meant by a spectator ion? Give the formula of the spectator ion in this reaction. (1) Write the equation for this reaction, omitting the spectator ion. (b) (c) (d) How would you measure the 50 cmg of copper(ll) sulphate solution? Give TWO reasons why it is better to use a polystyrene cup, rather than a metal container, to obtain more accurate results. Calculate the number of moles of each of the reactants and hence deduce which reactant 81
(e) is completely used up. Use the Periodic Table as a source of data. Moles of zinc powder Moles of copper(ll) sulphate Reactant used up The following results were obtained. Time /s Temperature PC o 22 60 22 90 60 120 65 150 63 180 61 PMT 210 59 82
PMT On the graph paper below, plot the results ofthis experiment. (i) 70 50 Temperature 30 20 10 (ii) (iii) 40 80 120 Time / s 160 200 Explain the shape of your graph The maximum recorded temperature in this experiment was 650C. Use your graph to estimate a more accurate maximum temperature. 240 83
(t) (i) PMT Calculate the energy change in this experiment using your answer to (e)(iii) and the relationship energy change = mass of x specific heat capacity x temperature rise solution /g You may assume that of solution 1.0 cm ofsolutionhas a mass of 1.0 g The specific heat capacity of the solution is 4.2 J g Use your answers to (d) and to calculate AH for this reaction. Include a sign and units in your answer. (Total 18 marks) 84
40. PMT This question is about some of the chemicals used in car engines and their reactions. (a) Compound X, shown below, is one component of petrol. (i) (iii) (iv) H CHs H CHs H Name X. Give the empirical formula of X. X can be made by cracking decane, C10H22. Assuming only one other product forms in a cracking reaction, deduce the molecular formula of this other product. (1) What is the sign of the enthalpy change for the reaction in which decane is cracked? Give a reason for your answer. 85
(v) PMT Ifthe air supply in a car engine is poor, there is not enough air for carbon dioxide to be produced. Use this information to suggest ONE possible equation for the combustion ofX in this engine. Use the molecular formula of X in your equation. (b) When air enters a car engine, as well as the fuel buming, nitrogen and oxygen can react to form nitrogen(ll) oxide. (i) (iii) N2(g) + 02(g) 2NO(g) AH = + 180 k] mor What, if any, is the effect on the percentage of nitrogen(ll) oxide in an equilibrium mixture of these three gases if the pressure and temperature are increased? Explain your answers. Increase in pressure Increase in temperature In a car exhaust pipe, nitrogen(ll) oxide passes over a catalytic converter. The following reaction occurs. AH = -746 kJ mori 2NO(g) + 2CO(g) N2(g) + 2C02(g) Explain why this reaction speeds up when the car engine has been running for a few minutes. A textbook says "The catalytic converter converts the gases coming out ofthe engine into less harmful ones". 86
(iv) PMT State, with a reason, which of the four gases in the equation in (ii) you consider to be least harmful The diagram below shows the reaction profile for the change which occurs in the catalytic converter. Energy 2NO O(g) Progress of reaction On the diagram, show the activation energy, EA. Add a line showing the reaction profile if no catalyst is present. (Total 12 marks) 87
41. (a) (b) PMT Define the term standard enthalpy of combustion. The following standard enthalpies of combustion are needed to calculate the standard enthalpy of formation of ethanol, C2H50H. Substance carbon, C (s, graphite) hydrogen, H2(g) ethanol, C2H50H (l) Standard enthalpy of combustion /kJ mol— -394 -286 -1371 (i) Complete the Hess's Law cycle by tilling in the box and labelling the arrows with the enthalpy changes. 2C(s) + 3H2(g) + 3'/202(g) + 302(g) 88
42. PMT Use your Hess's Law cycle to calculate the standard enthalpy of formation of ethanol. (Total 8 marks) The apparatus used and the recordings made by a student, carrymg out an experiment to determine the enthalpy of combustion of methanol, are shown below. Diagram thermometer beaker water spirit lamp methanol Results Molar mass (methanol) = 32 g mol- Volume of water in beaker = 50 cmg Mass of water in beaker = 50 g Weighings Spirit lamp + methanol before combustion = 163.78 g Spirit lamp + methanol after combustion = 163.44 g Temperatures Water before heating = 22.0 oc Water after heating = 43.5 oc Specific heat capacity of water = 4.18 J g-l OC-I
PMT Observations When the spirit lamp was being weighed its mass was continually falling. A black substance formed on the bottom of the beaker as the methanol burned. (i) (iii) Calculate the amount (moles) of methanol, CHgOH, burned. Calculate the heat gained by the water. Give your answer in kJ. Use your values from (i) and (ii) to calculate the enthalpy of combustion of methanol in kJ mol-I. Include a sign with your answer. kJ mol
(b) (i) (ii) (i) PMT The thermometer used in the experiment can be read to an accuracy of±0.5 oc. Calculate the percentage error in the temperature change. Calculate the maximum temperature change that could have occurred during the exp eriment. Give a reason why the mass of the spirit lamp fell as it was being weighed. Suggest the identity ofthe black substance that forms on the beaker. State the effect on the value of the enthalpy of combustion obtained. (Total I marks)
PMT An incomplete Born-Haber cycle for the formation of magnesium oxide, MgO, from its constituent elements is shown below. All numerical values are in kJ mori. 43. AH, = 4657 - +150 (a) Mg2 (g) + O(g) + 2e enthalpy Of atomisation Of oxygen — +249 Mg2V(g) + ä02(g) first plus second ionisation energy Of magnesium — +2186 Mg(g) + {02(g) lattice energy of magnesium oxide (i) Mg(s) + AHI - -602 Mgo (s) Complete the empty box on the cycle by writing in the formulae of the missing species. State symbols are required. Identify each of the following enthalpy changes by name:
(iii) PMT Use the Born-Haber cycle to calculate the lattice energy of magnesium oxide. (b) Magnesium iodide is another compound ofmagnesium. The radius of the magnesium ion is 0.072 nm, whereas the radius of the iodide ion is much larger and is 0.215 nm. (i) Describe the effect that the magnesium ion has on an iodide ion next to it in the magnesium iodide lattice. What TWO quantities must be known about the ions in a compound in order to calculate a theoretical lattice energy?
(iii) PMT Suggest how the value ofthe theoretical lattice energy would compare with the experimental value from a Born-Haber Cycle for magnesium iodide. Give a reason for your answer. (Total I marks) 44. Calcium hydroxide decomposes on strong heating to form calcium oxide and water. CO(s) + H20(l) Two samples of calcium hydroxide were taken, each weighing exactly I .00 g. The first sample was cautiously added to 25.0 cm-3 of dilute hydrochloric acid contained in a glass beaker. The temperature rise was measured and found to be 16.5 The other sample was heated for some time. It was then allowed to cool and then added to another 25.0 cm3 portion of hydrochloric acid as before. In this case the temperature rose by 25.5 In both cases, the acid used was an excess.
(i) PMT Calculate the energy produced by the reaction of each solid with the acid. Use the relationship Energy produced mass of solution x 4.2 x temperature rise g You may assume that 1.0 cm ofsolution has a mass of 1.0 g. Ignore the mass of the solid. For the solid calcium hydroxide For the solid calcium oxide 95
(iii) PMT How many moles of calcium hydroxide were used in each experiment? [Molar mass = 74.0 g mol-I] Using your answers to (a)(i) and (ii), calculate the enthalpy changes for each reaction. Give your answers to two significant figures. Include the sign and units for each answer. For the solid calcium hydroxide, AHI For the solid calcium oxide, AH2 (b) A Hess cycle for all these reactions is shown below. CaO(s) + H20(l) 2HC1(aq) 2HC1(aq) CaC12(aq) + 2H20(1)
PMT . Include a (i) (iii) Use this Hess cycle and your answers in (a)(iii) to calculate AH sign and units. Apart from the approximations involved in using the equation given in (a)(i), give TWO other potential sources of error which are likely to affect the accuracy of the results. Suggest why is difficult to determine directly by experiment. (Total 9 marks)
45. PMT This question is about ammonia, NH3, which is produced as shown in the following equation. N2(g) + 3H2(g) e 2NHg(g) Use oxidation numbers to explain why this is a redox reaction. (a) (b) (i) Use the average (mean) bond enthalpy data to calculate a value for the enthalpy change for this reaction. You are reminded to show all your working. Bond NEN Average bond enthalpy / kJ mol 944 436 388 The actual standard enthalpy change for this reaction is —92 kJ mol- . Explain why the value you calculated in (b)(i) is not the same as this.
(iii) PMT At room temperature, a mixture of nitrogen and hydrogen is thermodynamically unstable with respect to ammonia, but is kinetically stable. Use the data in (b)(i) and (ii) to help you explain why this mixture is thermodynamically unstable kinetically stable
(c) PMT The manufacturer of ammonia would like to achieve a high rate of reaction and a high equilibrium yield of product. (i) State and explain, in terms of collision theory, TWO ways to increase the rate of the reaction. An increase in pressure does not alter the rate in this process.
46. PMT State and explain TWO ways to increase the equilibrium yield of ammonia. (Total 19 marks) The enthalpy change for the reaction between aqueous sodium hydroxide solution and aqueous hydrochloric acid was determined by the following method: Aqueous hydrochloric acid was titrated against 25.0 cmg of 1.50 mol din- aqueous sodium hydroxide solution using a suitable indicator. The mean (or average) titre was 22.75 cm . 25.0 cm of the sodium hydroxide solution was carefully measured into a polystyrene cup and 22.75 cmg of the hydrochloric acid was transferred to a clean dry beaker. Both solutions were allowed to stand for five minutes before their temperatures were noted. The hydrochloric acid was then added to the sodium hydroxide solution, the mixture stirred thoroughly and the highest temperature noted. The experiment was repeated three times giving an average temperature change of +10.50C.
PMT and its (i) (ii) Calculate the heat produced in the reaction, in joules. Use the approximations that the density of the final solution is I .00 g cm- specific heat capacity is 4.18 J g-l K Calculate the enthalpy change for the reaction, in kJ mol- . (b) State ONE assumption made when calculating this enthalpy change, other than those stated in (Total 6 marks) 102
47. PMT The Born-Haber cycle below represents the enthalpy changes when calcium hydride, CaH2, is formed from its elements. Cay (g) + 2e- + 2H(g) ca2+(g) + 2e- + H2(g) ca(g) + H2(g) ca(s) + H2(g) ca2+(g) + 2W(g) (a) CaH2(s) Write down in terms of one of the symbols AHI to M16 (i) the lattice energy of calcium hydride . (1) the first electron affmity of hydrogen
PMT Use the data below to calculate the standard enthalpy offormation of calcium hydride, (b) (c) CaH2(s). enthalpy of atomisation of calcium first plus second ionisation energies of calcium enthalpy of atomisation of hydrogen first electron affinity of hydrogen lattice energy of calcium hydride Calculation: value /kJ mol- +178 +1735 +218 -73 -2389 Explain why the lattice energy of magnesium hydride, MgH2(s), is more exothermic than the lattice energy of calcium hydride, CaH2(s). (Total 7 marks)
48. PMT Two expeliments were carried out in order to calculate the enthalpy change of formation of magnesium carbonate, MgCOg. A Hess cycle for these reactions is shown below. Mg Experiment 1 +2HCl(aq) AHI MgC12(aq) + H2(g) + C 02 02 AH2— —680 W mol-I MgC03(s) + 2HCl(aq) Experiment 2 (a) (b) MgC12(aq) + H20(l) + C02(g) Complete the Hess cycle above for the formation of magnesium carbonate from its elements by balancing the equations and adding state symbols. In Experiment 1 the temperature of 100 cm-3 of hydrochloric acid was measured. After one minute, 0.100 g of magnesium was added to the excess acid and the temperature measured every minute. The following results were obtained: Time / min Temp / oc 21.0 21.0 2 25.3 3 25.1 4 24.9 24.8 6 24.7 (i) How many moles of magnesium were used in this experiment? Use the Periodic Table as a source ofdata. (1)
(iii) PMT The initial concentration ofthe hydrochloric acid was 2.00 mol dm- Calculate the number of moles of hydrochloric acid at the start and hence the number remaining at the end of the experiment. Plot the graph of temperature against time.
(v) (vi) PMT Calculate the energy change in this experiment assuming the temperature fise is 4.5 oc. Use the expression Energy change (J) = 4.2 x mass of solution x temperature change [Assume that I cm of solution has a mass of I g] Use your answer to (iv) to calculate AHI for one mole of magnesium reacting with hydrochloric acid. Include a sign and units in your answer. Suggest why a temperature rise of 4.5 oc was used in the calculation in (iv). 107
(c) PMT 2.2 g of magnesium camnate was added to 100 cm; of the same acid in Experiment 2. The temperature changed from 21.0 oc to 23.5 oc resulting in an energy change of 1.05 k]. (i) Calculate the mass of one mole of magnesium carbonate, MgCOg and hence the number of moles of magnesium carbonate used in this experiment. Use the Periodic Table as a source of data. Using the method in pan (b)(v), calculate AHS.
(d) (e) PMT Using your answers to (b)(v) and (c)(ii), calculate the enthalpy change of formation, AHf, of magnesium carbonate, MgCOg. Include a sign and units in your answer. Why is it impossible to measure AHf0fMgCOg(s) directly? (Total 18 marks) 49. Calculate the standard enthalpy change of formation of gaseous silicon tetrachlorde, AWf [SiC14(g)]. Your answer should include a sign and units. Use the Hess cycle below and the following data at 298 K. AH \ LSi(s)] Bond energy, E (Si-Cl) = +455.6 IJ mori = +121.7 mol-I = +407.4 IJ mol-I Sic 14 (g) Si(g) Si(s) + 2C12(g) 4C1(g) (Total 3 marks) 109
50. PMT An experiment was carried out to measure the enthalpy change for the reaction of zinc with aqueous copper(ll) sulphate. The equation for the reaction is Zn + cusoa znS04 + cu A measuring cylinder was used to transfer separate 50 cm3 samples of 1.25 mol dm—3 copper(ll) sulphate solution into polystyrene cups. Weighed amounts of zinc powder were added to each sample in turn. Each mixture was stirred thoroughly and the temperature rise noted with a thermometer accurate to 0.5 oc. The results ofthis experiment are summarised on the graph below. 70.0 60.0 50.0 40.0 Temperature change P' C 30.0 20.0 10.0 0.0 0.00 1.00 2.00 3.00 4.00 5.00 Mass of zinc/ g 6.00 7.00 (a) Explain why the graph initially shows a rise in temperature and then levels off. 110
(b) (i) (iii) PMT Suggest why the mass of metal is not used in the calculation of the heat change. The graph shows that the maximum temperature change is 63.5 oc. Use this value to calculate the maximum heat change, in joules, in this reaction. You should assume that the density ofthe solution is 1.00 g cm- and its heat capacity is the same as water, 4.18 J . From the heat change calculated in (b)(ii) calculate the enthalpy change, in k] mol- for the reactiom Include the appropriate sign and give your answer to three significant figures.
51. (a) PMT It is suggested that the precision of the experiment would be improved by using a thermometer accurate to 0.1 oc. Explain why this suggestion is incorrect. Suggest a simple practical change to the method that would make the experiment more accurate. (Total 10 marks) The following data were collected to use in a Born-Haber cycle for silver fluoride, AgF. enthalpy of atomisation of silver first ionisation energy of silver enthalpy of atomisation of fluorine enthalpy of formation of silver fluoride lattice energy of silver fluoride Value /kJ mol- +285 +731 +79 -205 112
PMT On the following outline of a Born-Haber cycle, complete boxes A and B by adding the formula and state symbol for the appropriate species. Wiite the name of the enthalpy change at C. Ag (g) AgF(s) c. Box (b) Box Ag(s) + hF2(g) AHIatt (theoretical) is the lattice energy calculated assuming the crystal lattice is completely ionic. AHIatt (experimental) is the lattice energy determined experimentally using the Born- Haber cycle. Values for the silver halides are listed below. Formula ofhalide AgF AgCl AgBr Agl (theoretical) / kJ mol- _920 _833 _816 -778 AHIatt (experimental) kJ mol- -958 -905 -891 -889 AHIatt (theoretical) nunus Al-flatt (experimental) kJ mol- 38 72 75 113
(i) (iii) PMT Explain why the theoretical lattice energies become less exothermic from AgF to Agl. Explain why the values ofthe theoretical and experimental lattice energies are different. Explain why the difference between the theoretical and experimental lattice energies increases from AgF to Agl. (Total 10 marks) 114
52. PMT The Hess cycle below can be used to find the enthalpy change, AHr, for the reaction between hydrogen sulphide and sulphur dioxide, using standard enthalpy changes of formation. (i) (ii) (iii) Complete the cycle by filling in the empty box. What is meant by the standard enthalpy change Of formation, Aflt•e, of a compound? Use the cycle and the data below to calculate the enthalpy change ofthe reaction, Ah-Ir. S02 (g) H2S (g) 1120 (l) / k] mol- -296.8 -20.6 -285.8 (Total 7 marks) 115
PMT Which of the equations shown below represents the reaction for which AH is the standard enthalpy change of formation, Afff298, for ethanol, C2H50H. Ethanol melts at 156 K and boils at 352 K. 53. 54. A B c D 2C(g) + 6H(g) + O(g) C2H50H(g) 2C(s) + 3H2(g) + 02(g) -s C2H50H(l) 2C(s) + 3H2(g) + O(g) C2H50H(g) 2C(s) + 3H2(g) + '/a02(g) C2H50H(1) (Total I mark) Airbags, used as safety features in cars, contain sodium azide, NaN3. An ailå)ag requires a large volume of gas to be produced in a few milliseconds. The gas is produced in this reaction: 2NaNg(s) 2Na(s) + 3N2(g) AH is positive When the airbag is tillly inflated, 50 dm3 of nitrogen gas is produced. (a) Calculate the number of molecules in 50 dmg of nitrogen gas under these conditions. [The Avogadro constant = 6.02 x 1023 mol-I. The molar volume ofnitrogen gas under the conditions in the airbag is 24 dm mol- l. 116
(b) (c) (d) PMT Calculate the mass of sodium azide, NaNg, that would produce 50 dmg of nitrogen gas. What will happen to the temperature in the ailå)ag when the reaction occurs? The airbag must be strong enough not to burst in an accident. An airbag which has burst in an accident is hazardous if the sodium azide in it has decomposed. Explain why this is so. (Total 8 marks) 55. A student investigated a reaction which could be used to warm up coffee in self-heating cans. Mg(s) + —+ + Cu(s) In the self heating cans, the bottom has a companment containiæ coppeflll) nitrate solutiom When a button on the bottom of the can is pressed, the magnesium powder is released into the compartment where it reacts with the copper(ll) nitrate solution. 117
(a) PMT A student investigated the enthalpy change for this reaction by measuring 50.0 cm of 0.300 mol dm- copper(ll) nitrate solution into a 100 cm3 beaker and adding lg (an excess) of magnesium powder. The results are shown below. Temperature of copper(ll) nitrate solution at start Temperature of mixture after reaction = 22 oc (i) (iii) Calculate the energy change which took place. The specific heat capacity of the solution is 4.20 J K Which is the correct value for the energy change in joules? (1) How many moles of copper(ll) nitrate were used in the experiment? Calculate the enthalpy change for the reaction. You should include a sign and units m your answer. 118
(b) PMT Suggest two changes you would make to the equipment used in order to improve the accuracy of the result. The ionic equation for the reaction is shown below: Mg(s) + Cu2+(aq) + Cu(s) AH= —532 kJ mol- Would the following affect the value of the experimental result? Explain your answer, stating the effect, if any, on the value ofthe enthalpy change obtained. (i) The student used 2 g rather than lg of magnesium. The heat losses that occurred from the student's beaker. 119
(c) PMT The temperature in the self-heating can needs to increase by 60 oc to produce a hot drink. Suggest a change you could make to the mixture in the experiment in (a) to produce a greater temperature rise. You are not expected to do a calculation. (Total I marks) 56. The following data can be used in a Born-Haber cycle for copper(ll) bromide, CuBr2. Enthalpy change of atomisation of bromine Enthalpy change of atomisation of copper, First ionisation energy of copper, Second ionisation energy of copper, Ema [Cu(g)] Electron affinity of bromine, EatHBr(g)] Enthalpy change of formation of CuBr2(s), AM*f [CuBr2(s)] +111.9kJmorl +338.3 k] mori +746.0 kJ mori +1958.0kJ mori -342.6 kJ mori -141.8kJ (a) On the following outline of a Born-Haber cycle complete the boxes A, B, and C by putting in the formula and state symbol for the appropriate species and writing the name of the enthalpy change D. + 2BV(g) cu(s) + Bh(l) CuBr2(s)
(b) (c) PMT Use the data to calculate a value for the lattice energy of copper(ll) bromide. Give a sign and units in your answer. When the lattice energy of copper(ll) bromide is calculated from ionic radii and charges, the result is a value numerically about 10% less than the one obtained from the Born- Haber cycle. (i) What does this suggest about the nature of the bonding in copper(ll) bromide? Draw a diagram to show how the smaller copper ion alters the shape of the larger bromide ion. (Total 8 marks)

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