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Dubai

PRESENTATION OF CHEMICAL ENERGETIC

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Published in: Chemistry
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This presentation describes chemical energetics in the following parts: i. Endothermic reactions ii. Exothermic reactions iii. Conservation of energy iv. Enthalpy and Enthalpy changes v. Standard enthalpy changes

Athumani R / Dubai

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  1. CHEMICAL ENERGETICS PREPARED BY: ATHUMANI KAWAMBWA PHARMACIST (MUHAS)
  2. LEARNING OUTLINE Endothermic reactions Exothermic reactions Conservation of energy Enthalpy, and enthalpy changes Standard enthalpy changes
  3. • All chemical reactions involve change. In flame, we can see the changes caused by very fast reactions between the chemicals in the burning materials and oxygen from the atmosphere. There are new substances, new colors and change the state, but the obvious changes in these reactions are the transfer of energy as light and heating of the surroundings. • All life on the earth surface depends on transfer of energy in the chemical reactions.
  4. Plants need the energy from the sun for production of carbohydrates by photosynthesis. Animals gain energy from oxidation of their food chemicals. ENERGY TRANSFER Most chemical reactions release energy to the surroundings. These reactions are described as Exothermic We recognize exothermic reactions most easily by detecting the rise in temperature of the surrounding and the reaction mixture.
  5. Examples of exothermic reactions are: i. Oxidation reactions such as: combustion of fuels, respiration in plants and animals which involves oxidation of carbohydrates. ii. Reaction of acids with metals iii. Reaction of water with calcium carbonate (quicklime)
  6. Some chemical reactions occur only while energy is transformed to them from external source. These reactions which require heat input are called Endothermic The energy input may come from the flame, electricity, sunlight or surroundings. Examples of endothermic reactions include: i. Decomposition of limestone by heating ii. Photosynthesis, energy is supplied to the reaction in the cells by sunlight:
  7. ii. Decomposition of limestone by heating CaC03 (g) + energy ca0(s) + C02 (g) Photosynthesis C02 (g)+ 1--12 energy—•carbohydrate in leaves + 02(g) iii. Dissolving Ammonia chloride in water. When the pack is kneaded, water and ammonia crystals mix. Ammonia chloride crystal dissolve in water, energy is transferred from the surroundings, cooling the injury.
  8. QUIZ Classify the following processes as exothermic or endothermic Evaporation Crystallization Making magnesium oxide from magnesium and air. Making copper oxide from copper carbonate
  9. ENTHALPY AND ENTHALPY CHANGES Measurements of energy transfer during chemical reactions must be made under controlled conditions. A special name is given to the energy exchange with the surroundings when it takes place at the constant pressure. This name is Enthalpy change Enthalpy Is the total energy content of the reacting materials. It is given the symbol H.
  10. Enthalpy can not be measured as such, but it is possible to measure the enthalpy change when energy is transferred to or from a reaction system and changes from one enthalpy to another. Enthalpy change is given the symbol AH A is the upper case of Greek letter 5, pronounced "delta", and it is often used in mathematics as symbol for change. Therefore: AH = H -H products reactants As AH is the measure of energy transferred to or from known amount of reactants, the units are kilojoules per mole (kJ/mol)
  11. The enthalpy changes can be illustrated by enthalpy profiles. l. ENTHALPY CHANGE FOR EXOTHERMIC REACTION activation energy energy Of reactants direction of re Jeased energy of produce reaction
  12. The enthalpy change for exothermic reaction is always given negative value, as a energy is lost from the system to the surrounding AH = -x kJ/mol For example, when methane burns: (g) + 202 (g) co + 2H20 (l) AH = -890.3 kj/mot—- This means that when one mole of methane burns completely in oxygen, 890.3 kilojoules of energy are transferred to the surroundings.
  13. ENTHALPY CHANGE PROFILE FOR THE BURNING OF CH4(g) + 202(g) METHANE activation energy (ED C02(g) 2H20(0 progress Of reaction
  14. ll. ENTHALPY CHANGE FOR ENDOTHERMIC REACTION activation energy Of products ener g Y a bed energy Of reactants direction Of reaction
  15. The enthalpy change for exothermic reaction is always given negative value, as a energy is lost from the system to the surrounding AH = + kJ/mol For example, on heating calcium carbonate: AH = + 572 kj/mol cac03 C02 (g) + ca0(s) This means that when one mole of calcium carbonate is heated 572 kilojoules of energy are needed to break calcium carbonate to calcium oxide and carbon dioxide.
  16. ENTHALPY CHANGE PROFILE FOR HEATING OF CALCIUM CARBONATE Energy taken in from surroundings AH + ve CaO(s) + c02(g) AH = +178 kJ mol-t CaCOJ(s) Progress of reaction
  17. STANDARD ENTHALPY CHANGES When we compare the enthalpy changes of various reactions we must use standard conditions, such as known temperatures, pressures, amounts and concentrations of reactants or products. This allows us to compare the standard enthalpy changes for reactions. The standard enthalpy changes takes place under standard condition: ii. iii. iv. Pressure of 100 kilopascals (100 kPa) Temperature of 298 K Any solutions have concentration of 1.0 mol dm-3 The reactants and products must be in the same physical states (liquid, solid, gas) that are normal for these conditions.
  18. The complete symbol for a standard enthalpy change of reaction can be written as, AH r,298, the meaning of the symbols being: 0 means standard and assume pressure of 100 kPa. r is the general symbol for reaction and is changed to f for formation reactions or c for combustion reactions. • 298 means all reactants and products are in their physical states at the temperature of 2298 K. Example: carbon dioxide is a gas at 298 K but water is a liquid. Note that as the values for standard enthalpy changes are usually quoted at 298 K, it is common practice to omit 298 from the symbol.
  19. STANDARD ENTAHLPY CHANGE OF REACTION AH? The standard enthalpy change of reaction is the enthalpy change when amount of reactants react together under standard conditions to give their products in their standard states. It is necessary to make clear which reaction equation we are using when we quote a standard enthalpy change of reaction. For example: Equation for the reaction between hydrogen and oxygen can be written in two different ways and there are different values for AHr9 in each case.
  20. Equation i: Equation ii: 2H20 (1) H20 (l) AHP 572 kJ/mol AHr0 = - 286 kJ/mol Note that the value of AHr0 in (ii) is half of AH? in
  21. STANDARD ENTHALPY CHANGE OF FORMATION AH? Standard enthalpy change of formation, is the enthalpy change when one mole of a compound is formed from its elements under standard conditions. Both elements and their compounds are in the same states. For example: Water is formed from both equations (i) and (ii) in the last slide but only one in equation (ii) is one mole of water formed. • Thus the equation (ii) shows that the value of AH? (H20) = - 286kJ/mol
  22. QUIZ 1. Write balanced equations for the formation of: (i). Ethane (C2H6) (ii) Aluminium oxide (A1203) Use data book to add values for AHr0 in each case. 2. Draw enthalpy profile for the enthalpy change of formation of ethane. Label your diagram fully.
  23. STANDARD ENTHALPY CHANGE OF COMBUSTION The standard enthalpy change of combustion, is the enthalpy change when one mole of an element or compound reacts completely with oxygen under standard conditions. For example: The standard enthalpy change of combustion of hydrogen is given by equation (ii) of the past slide. 1-12 (g) + 72 02 (g) H20 (l) AHF 286 kJ/mol
  24. THE STANDARD ENTHALPY CHANGE OF FORMATION OF WATER 2(g) 2 AH? = - 286 kJ/moI H20(l)
  25. In practice it is not possible to achieve complete combustion under standard conditions. Measurements are taken under experimental conditions, then the values for the enthalpy change is determined and is corrected to standard conditions through calculations.
  26. THE STANDARD ENTHALPY CHANGE FOR COMBUSTION OF SULPHUR DIOXIDE AH? = - 296.8 kJ/mol so
  27. EXERCISE 1. Which of the labels AHP, AH? or AH? could be used for enthalpy change of the profile in the past slide? 2. What are the reaction equations for the combustion of: i). Octane (C8H18) (ii). Ethanol (C2H60) including the values for AH? (use a data book) 3. Why is the AHf of water is the same as AH? of hydrogen?
  28. OTHER STANDARD ENTHALPY CHANGES Other standard enthalpy changes are defined in a similar manner i. Standard enthalpy change of hydration The standard enthalpy change of hydration is the enthalpy change when one mole of gaseous ion dissolves in water to give infinitely dilute solution. ii. The standard enthalpy change of solution The standard enthalpy change of solution is the enthalpy change when one mole of solute dissolves in a solvent to give an infinitely dilute solution.
  29. iii. The standard enthalpy change of neutralization The standard enthalpy change of neutralization is the enthalpy change when one mole of H + ion from an acid is completely neutralize by an alkali to give one mole of water. iv. The standard enthalpy change of atomization The standard enthalpy change of atomization of an element is the enthalpy change when one mole of gaseous atom is formed from one mole of element in tis standard conditions.
  30. BOND MAKING, BOND BREAKING AND ENTHALPY CHANGE A typical combustion reaction, such as combustion of methane, is: (g) + 202 (g) C02 (g) + 2H20 (l) AH = -890.3 kj/mol On drawing molecules to show bonds Methane + Oxygen carbon dioxide + water
  31. For this reaction to occur, some bonds must break and other form: Bond breaking 4 x C-H and 2 x Bond making The basis of understanding energy transfers during chemical reactions is fairly simple rule: When bonds break energy is absorbed (endothermic reaction), and when bonds are formed energy is released (exothermic reaction)
  32. If energy released by formation of some bonds is greater than energy absorbed by the breaking of other bonds, there will be a surplus of energy transferred to the surroundings. The overall reaction will be exothermic If energy released by the bonds formation is less than energy absorbed by bonds breaking then, overall energy change must be transferred from the surrounding to the reaction mixture. The reaction will endothermic reaction. In a case of combustion of methane, after all the bond breaking and formation, the surplus energy transferred to the surroundings is 890.3 kJ/mol for each one mole of methane.
  33. BOND ENTHALPY Chemists found that it is useful to measure the amount of energy needed to break a covalent bond, as this indicates the strength of the bond. They call it the bond enthalpy. The values are always quoted as bond enthalpy per mole (of bond broken) Consider the example of oxygen gas (02), the bond enthalpy of oxygen is the enthalpy change for the process: 02 (g) 20 (g) AH = + 498 kJ/mol
  34. The symbol E is often used for bond enthalpy per mole. It is related to a particular bonds as E(X—Y), where X—Y is a molecule. Thus E(X—Y) is the same as AH for the dissociation process. SOME COMMON BONDS ENTHALPIES BOND c-c E(X-Y) (kJ/mol) +436 +347
  35. BOND SOME COMMON BONDS ENTHALPIES ?-? ?(?—?) (ld/mol) +612 +413 +498 +464 +805
  36. ii. The values quoted in the table above for bond enthalpies per mole satisfy the following four conditions: They are all positive, as the change during the breaking of bonds are endothermic. The same quantity of energy will be released in exothermic process when the bonds form. They are compared for bonds in gaseous compounds only. iii. They are very difficult to measure directly. They are usually calculate using data from measurements of enthalpy changes of combustion of several compounds.
  37. Conditions continue iv. They are average values. The actual value of the bond enthalpy for a particular bond depends upon which molecule the bond is in. For example: The C— C bond has slightly different strengths in ethane (C2H6) and in propane (C3H8), as it is affected by other atoms and bonds in the molecules. The bond enthalpy quoted in data books for C —C is an average of the values from many different molecules.
  38. GET PREPARED FOR THE NEXT SLIDES DO NOT FORGET TO WRITE YOUR POSITIVE REVIEW FOR MR. ATHUMANI NEXT: MEASURING ENERGY TRANSFER AND ENTHALPY CHANGES

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