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Thermochemical Equations

Home > PowerPoint Slides > Thermochemical Equations
Published in: Chemistry
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Dear Students, In this ppt you will find out why to include energy in chemical equations, how to write a thermo chemical equation? Enthalpy heat of combustion, Molar enthalpy heat of vaporization , Molar enthalpy heat of fusion and so on.

Wardah M / Dubai

9 years of teaching experience

Qualification: BSc Physics, BDS , Masters in Biochemistry

Teaches: Biology, Chemistry, Biochemistry, Molecular Biology, EMSAT, IGCSE/AS/AL, Science, Environmental Scinece, General Science, English, Maths, Islamic, Mathematics, Physics

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  1. Thermochemical Equations
  2. Focus Question Why do you include energy changes in chemical equations?
  3. New Vocabulary thermochemical equation enthalpy (heat) of combustion molar enthalpy (heat) of vaporization molar enthalpy (heat) of fusion
  4. Review Vocabulary combustion reaction: a chemical reaction that occurs when a substance reacts with oxygen, releasing energy in the form of heat and light
  5. Writing Thermochemical Equations A thermochemical equation is a balanced chemical equation that includes the physical states of all reactants and products and the energy change. The energy change is usually expressed as the change in enthalpy, AH.
  6. Writing Thermochemical Equations The enthalpy (heat) of combustion (AH ) comb of a substance is the enthalpy change for the complete burning of one mole of the substance.
  7. Writing Thermochemical Equations Standard enthalpies of combustion for several substances are given in Table 3 on the next slide. Standard enthalpy changes have the symbol AHO. The zero superscript indicates the changes were determined under standard conditions.
  8. Writing Thermochemical Equations Table 3 Standard Enthalpies of Combustion Substance Sucrose (table sugar) Octane (a component of gasoline) Glucose (a simple sugar found in fruit) Propane (a gaseous fuel) Methane (a gaseous fuel) Formula C12H22011(s) C8H18(l) C6H1206(s) C3H8(g) CH4(g) (kJ/mol) AHO comb -5644 -5471 -2808 -2219 -891
  9. Changes of State Molar enthalpy (heat) of vaporization (AHvap) is the heat required to vaporize one mole of a liquid. Molar enthalpy (heat) of fusion (AHfus) is the heat required to melt one mole of a solid. Table4 Standard Enthalpies of Vaporization and Fusion Substance Water Ethanol Methanol Acetic acid Ammonia Formula (kJ/mol) (kJ/mol) H 20 C2H 50H CH30H CH 3C00H 40.7 386 35.2 23.4 23.3 6.01 4.94 3.22 11.7 5.66
  10. Changes of State Phase Changes for Water AH 4Hsolid = —6 01 vap = +40.7 kJ AHcond HOI = +6.01 kJ 20(s = —40.7 kJ
  11. THE ENERGY RELEASED IN A REACTION IN-CLASS EXAMPLE Use with Examole Problem 4. Problem A bomb calorimeter is useful for measuring the energy released in combustion reactions. The reaction is carried out in a constant-volume bomb with a high pressure of oxygen. How much heat is evolved when 54.0 g glucose (C6H1206) is burned according to this equation? C6H1206(s) + 6C02(g) + 6H20(l) AH comb -2808 ICJ KNOWN mass of glucose = 54.0 g C6H1206 -2808 k] comb UNKNOWN Response ANALYZE THE PROBLEM You are given a mass of glucose, the equation for the combustion of glucose, and AH comb. You must convert grams of glucose to moles of glucose. Because the molar mass of glucose is more than three times the mass of glucose burned, you can predict that the energy evolved will be less than one-third AH comb SOLVE FOR THE UNKNOWN Convert grams of C6H1206 to moles of C6H1206. 1 mol Multiply by the inverse of molar mass, 180.18 g• 1 mol C6H1706 54.0 g C6H1206X 180.18 g C6H1206 -0.300 molCH O 6 12 6
  12. THE ENERGY RELEASED IN A REACTION IN-CLASS EXAMPLE SOLVE FOR THE UNKNOWN (continued) Multiply moles of C6H1206 by the enthalpy of combustion, AH comb' 2808 k] Multiply moles of glucose by 1 mol CH O 6 12 6 2808 k] 0.300 me4-C6Haa06-X 1 842 Id EVALUATE THE ANSWER All values in the calculation have at least three significant figures, so the answer is correctly stated with three digits. As predicted, the released energy is less than one-third AHcomb
  13. • • Combustion Reactions Combustion is the reaction of a fuel with oxygen. Food is the fuel in combustion reactions in biological systems.
  14. Quiz 1. Which of the following is (are) included in a thermochemical equation? physical states of the reactants and products a balanced chemical equation D change in enthalpy All of the above. CORRECT
  15. 2. Quiz Standard enthalpy changes have the symbol AHO. What component of the symbol indicates that the changes were determined with all the reactants and products at standard conditions? the H the A the zero subscript CORRECT Nothing indicates this.
  16. Quiz 3. Which of the following has a positive change in enthalpy? the combustion of glucose B vaporizing water CORRECT condensing water freezing water
  17. Quiz 4. Which of the following is true for enthalpy of combustion? It is positive for glucose. B It is negative for propane. CORRECT It measures the change in temperature that occurs. None of the above.

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