Understanding Enthalpies of Reaction in Chemistry

1. Section 5.4 Enthalpies of Reaction

2. Objectives • Continue examining enthalpy as the quantity of heat energy gained or lost by a system in a process under a constant pressure.

3. Key Terms • Enthalpy • Enthalpy of reaction

4. Enthalpies of Reaction The change in enthalpy, H, is the enthalpy of the products minus the enthalpy of the reactants: H = Hproducts− Hreactants

5. Enthalpies of Reaction This quantity, H, is called the enthalpy of reaction, or the heat of reaction.

6. Enthalpy Guidelines • Enthalpy is an extensive property. • H for a reaction in the forward direction is equal in magnitude, but opposite in sign, to H for the reverse reaction. • H for a reaction depends on the state of the products and the state of the reactants.

7. H2 (g) + 1/2O2 (g) DH= ? DH= -285.8 kJ H2O (g) DH= -44.0 kJ H2O (l) What is the value of the unknown DH in the diagram? • +329.5 kJ • -329.5 kJ • +285.8 kJ • -241.8 kJ • +241.8 kJ

8. H2 (g) + 1/2O2 (g) DH= ? DH= -285.8 kJ H2O (g) DH= -44.0 kJ H2O (l) What is the value of the unknown ΔH in the diagram? • +329.5 kJ • -329.5 kJ • +285.8 kJ • -241.8 kJ • +241.8 kJ

9. The enthalpy of reaction for: 2 C(s) + O2(g) 2 CO(g) is _____. Hf° (CO) = 110.5 kJ • 110.5 kJ • +110.5 kJ 221.0 kJ +221.0 kJ

10. Correct Answer: • 110.5 kJ • +110.5 kJ 221.0 kJ +221.0 kJ

11. The coefficients indicate the numbers of moles of reactants and products that give rise to the stated enthalpy change. • The coefficients indicate the whole number mass ratios with which the reactants and products give rise to the stated enthalpy change. • The coefficients indicate the relative amounts of heat absorbed or given off by each reactant and product in the reaction. • The coefficients indicate the number of bonds which must be broken or formed in each reactant and product in the reaction.

12. The coefficients indicate the numbers of moles of reactants and products that give rise to the stated enthalpy change. • The coefficients indicate the whole number mass ratios with which the reactants and products give rise to the stated enthalpy change. • The coefficients indicate the relative amounts of heat absorbed or given off by each reactant and product in the reaction. • The coefficients indicate the number of bonds which must be broken or formed in each reactant and product in the reaction.

13. The energy changes of a system and its surroundings are not related. • The energy lost by a system is proportional to the energy lost by the surroundings. • The energy lost by a system is equal to the energy lost by the surroundings. qsystem = qsurroundings • The energy lost by a system is gained by its surroundings. qsystem = -qsurroundings

14. The energy changes of a system and its surroundings are not related. • The energy lost by a system is proportional to the energy lost by the surroundings. • The energy lost by a system is equal to the energy lost by the surroundings. qsystem = qsurroundings • The energy lost by a system is gained by its surroundings. qsystem = -qsurroundings

15. Give it Some Thought What effect does reversing a reaction have on H for the reaction? • Hnew = –Hold. • Hnew = Hold K • Hnew = Hold • Hnew = 1/Hold.

16. Give it Some Thought What effect does reversing a reaction have on H for the reaction? • Hnew = –Hold. • Hnew = Hold K • Hnew = Hold • Hnew = 1/Hold.