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Chapter 5

Chapter 5. Thermochemistry. Terms I. Thermochemistry Study of Kinetic Energy Energy of Potential Energy Energy of. Units of E. JOULE Calorie vs. calorie 1 Cal = 1 cal is the amount of heat necessary to raise the temperature of 1 g of water 1 o C 1 cal = J.

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Chapter 5

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  1. Chapter 5 Thermochemistry

  2. Terms I • Thermochemistry • Study of • Kinetic Energy • Energy of • Potential Energy • Energy of

  3. Units of E • JOULE • Calorie vs. calorie • 1 Cal = • 1 cal is the amount of heat necessary to raise the temperature of 1 g of water 1oC • 1 cal = J

  4. Terms II • Force • A • Work • Energy used to move an object • W = • Energy • Capacity to do

  5. First Law of Thermodynamics • Energy • Δ E =

  6. Equation II • Δ E = • q = heat in or heat out of system • w = work done on or by the system

  7. Practice • Calculate the change in the internal energy of the system for a process in which the system absorbs 140 J of heat from the surroundings and does 85 J of work on the surroundings.

  8. Endothermic Reaction • Process when a system • Gets • ΔH >

  9. Exothermic Reaction • Process when a system • Gets • ΔH <

  10. State Function • Describes the condition of a system • Does not matter how the system got there • For example…

  11. ENTHALPY • Heat… • Represented by • Is a

  12. Enthalpy Equation • ΔH = Hfinal – Hinitial = qp • P indicates constant pressure

  13. Enthalpy of Reaction • ΔHrxn = H(products) – H(reactants) • 2H2O (g) 2H2 (g) + O2 (g)ΔH = • 2H2 (g) + O2(g) 2H2O(g) ΔH = Return

  14. Rules for Enthalpy and RXNS • Enthalpy is an extensive property • Enthalpy for a reaction • Enthalpy change depends on state of reactants and products

  15. Practice • 2Mg(s) + O2(g) 2MgO(s)ΔH=-1204 kJ • Is this • How much heat is transferred when 2.40 g of Mg are reacted? • How many grams of MgO are produced during an enthalpy change of 96.0 kJ?

  16. Continued • 2Mg(s) + O2(g) 2MgO(s)ΔH=-1204 kJ • How many kJ of heat are absorbed when 7.50 g of MgO(s) are decomposed into magnesium and oxygen at constant pressure?

  17. Calorimetry • The study of • Heat capacity (C): • Temperature change of an object as it • Amount of heat required to • Greater heat capacity means…

  18. Calorimetry • Molar heat capacity (Cmolar) • Specific heat capacity (Cp) • Equation:

  19. Calorimetry • What is the heat required to raise 400.00 g of water by 34.50oC? • What is the heat lost when 200.00 g of iron changes from 115.50 oC to 22.00 oC?

  20. Calorimetry • Large beds of rocks are used in some solar heated homes to store heat. Assume that the specific heat of rock is 0.082 J/g-K. Calculate the quantity of heat absorbed by 50.0 kg of rocks if their temperature increases by 12.0 oC

  21. Flashback… • Law of conservation of energy (First law of thermodynamics):

  22. Calorimetry • You have heated a 55.00 g piece of iron, Cp = 0.385 J/g-K, to 200.00 oC. You then put the iron into water in a calorimeter. There are 300.00 g of water at 22.00 oC. What is the final temperature of the mixture?

  23. Calorimetry • In the calorimetry lab you will be mixing an acid and a base and studying the temperature changes. • You mix 35.00 mL each of 1 M HCl and 1 M NaOH in a calorimeter. The temperature increases from 21.0 to 27.5 oC. What is the enthalpy change for the reaction in kJ/mol HCl?

  24. Hess’s Law

  25. Enthalpy of Formation • Energy change for the formation • Symbolized by Hf • Tables are for standard conditions • 1 atm • 25oC • Appendix C

  26. Enthalpy of Formation • Units for Hof are in • Magnitude depends on state • 2H2(g) + O2(g) 2 H2O(l)Hof = -285.8 • 2H2(g) + O2(g) 2 H2O(g)Hof = -241.8

  27. Enthalpy of Formation • By definition: the standard enthalpy of formation of the most stable form of ANY element is 0 kJ/mol • Hof C(s) = 0 • Hof H2(g) = 0

  28. Enthalpy of RXN • Horxn = Σn Hof (products) - Σm Hof (reactants)

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