Chapter 10 (part 2) Energy. Copyright Cengage Learning. All rights reserved 1

Section 10.7 Hess s Law In going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps. Copyright Cengage Learning. All rights reserved 2

Section 10.7 Hess s Law N 2 (g) + 2O 2 (g) 2NO 2 (g) ΔH 1 = 68 kj This reaction also can be carried out in two distinct steps, with enthalpy changes designated by ΔH 2 and ΔH 3. N 2 (g) + O 2 (g) 2NO(g) 2NO(g) + O 2 (g) 2NO 2 (g) N 2 (g) + 2O 2 (g) 2NO 2 (g) ΔH 2 = 180 kj ΔH 3 = 112 kj ΔH 2 + ΔH 3 = 68 kj ΔH 1 = ΔH 2 + ΔH 3 = 68 kj Copyright Cengage Learning. All rights reserved 3

Section 10.7 Hess s Law Characteristics of Enthalpy Changes If a reaction is reversed, the sign of ΔH is also reversed. The magnitude of ΔH is directly proportional to the quantities of reactants and products in a reaction. If the coefficients in a balanced reaction are multiplied by an integer, the value of ΔH is multiplied by the same integer. Copyright Cengage Learning. All rights reserved 4

Section 10.7 Hess s Law Example Consider the following data: 1 3 NH ( g) ¾¾ N ( g) + H ( g) DH = 46 kj 3 2 2 2 2 2 H ( g) + O ( g) ¾¾ 2 H O( g) DH = - 484 kj 2 2 2 Calculate ΔH for the reaction 2 N ( g) + 6 H O( g) ¾¾ 3 O ( g) + 4 NH ( g) 2 2 2 3 Copyright Cengage Learning. All rights reserved 5

Section 10.7 Hess s Law Problem-Solving Strategy Work backward from the required reaction, using the reactants and products to decide how to manipulate the other given reactions at your disposal. Reverse any reactions as needed to give the required reactants and products. Multiply reactions to give the correct numbers of reactants and products. Copyright Cengage Learning. All rights reserved 6

Section 10.7 Hess s Law Example Reverse the two reactions: 1 3 N ( g) + H ( g) ¾¾ NH ( g) DH = - 46 kj 2 2 3 2 2 2 H O( g) ¾¾ 2 H ( g) + O ( g) DH = +484 kj 2 2 2 Desired reaction: 2 N ( g) + 6 H O( g) ¾¾ 3 O ( g) + 4 NH ( g) 2 2 2 3 Copyright Cengage Learning. All rights reserved 7

Section 10.7 Hess s Law Example Multiply reactions to give the correct numbers of reactants and products: 1 3 4( N 2( g) + H 2( g) ¾¾ NH 3( g) ) 4( DH = - 46 kj ) 2 2 3( 2 H2O( g) ¾¾ 2 H 2( g) + O 2( g) 3( DH = +484 kj ) Desired reaction: 2 N ( g) + 6 H O( g) ¾¾ 3 O ( g) + 4 NH ( g) 2 2 2 3 Copyright Cengage Learning. All rights reserved 8

Section 10.7 Hess s Law Example Final reactions: 2 N ( g) + 6 H ( g) ¾¾ 4 NH ( g) DH = -184 kj 2 2 3 6 H O( g) ¾¾ 6 H ( g) + 3 O ( g) DH = +1452 kj 2 2 2 Desired reaction: 2 N ( g) + 6 H O( g) ¾¾ 3 O ( g) + 4 NH ( g) 2 2 2 3 ΔH = +1268 kj Copyright Cengage Learning. All rights reserved 9

Section 10.7 Hess s Law Concept Check Calculate ΔH for the reaction: SO 2 + ½O 2 SO 3 ΔH =? Given: (1) S + O 2 SO 2 ΔH = 297 kj (2) 2S + 3O 2 2SO 3 ΔH = 792 kj a) 693 kj b) 101 kj c) 693 kj d) 99 kj Copyright Cengage Learning. All rights reserved 10

Section 10.8 Quality Versus Quantity of Energy When we use energy to do work we degrade its usefulness. While the total amount or quantity of energy in the universe is constant (1st Law) the quality of energy is degraded as it is used. Burning of petroleum: Copyright Cengage Learning. All rights reserved 11

Section 10.9 Energy and Our World Effects of Carbon Dioxide on Climate Atmospheric CO 2 Controlled by water cycle Could increase temperature by 10 o C Copyright Cengage Learning. All rights reserved 17

Section 10.10 Energy as a Driving Force Natural processes occur in the direction that leads to an increase in the disorder of the universe. Example Consider a gas trapped as shown Copyright Cengage Learning. All rights reserved 19

Section 10.10 Energy as a Driving Force Energy Spread In a given process concentrated energy is dispersed widely. This happens in every exothermic process. Copyright Cengage Learning. All rights reserved 22

Section 10.10 Energy as a Driving Force Matter Spread Molecules of a substance spread out to occupy a larger volume. Processes are favored if they involve energy and matter spread. Copyright Cengage Learning. All rights reserved 23

Section 10.10 Energy as a Driving Force Entropy, S Function which keeps track of the tendency for the components of the universe to become disordered. Measure of disorder or randomness Copyright Cengage Learning. All rights reserved 24