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.

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Presentation transcript:

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. Enthalpy is a state function. The idea is to manipulate the equations given in the problem statement so that they add up to the overall reaction. Copyright © Cengage Learning. All rights reserved

There are two ways that the equations can be manipulated: 1. Reverse the reaction. The sign of ΔH is also reversed. 2. Multiply the coefficients in a balanced reaction by an integer. The value of ΔH is multiplied by the same integer. Enthalpy is a state function. The idea is to manipulate the equations given in the problem statement so that they add up to the overall reaction. Copyright © Cengage Learning. All rights reserved

N2(g) + 2O2(g) → 2NO2(g) ΔH1 = 68 kJ This reaction also can be carried out in two distinct steps, with enthalpy changes designated by ΔH2 and ΔH3. N2(g) + O2(g) → 2NO(g) ΔH2 = 180 kJ 2NO(g) + O2(g) → 2NO2(g) ΔH3 = – 112 kJ N2(g) + 2O2(g) → 2NO2(g) ΔH2 + ΔH3 = 68 kJ ΔH1 = ΔH2 + ΔH3 = 68 kJ Copyright © Cengage Learning. All rights reserved

The Principle of Hess’s Law Copyright © Cengage Learning. All rights reserved

Consider the following data: Example Consider the following data: Calculate ΔH for the reaction Copyright © Cengage Learning. All rights reserved

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

Reverse the two reactions: Example Reverse the two reactions: Desired reaction: Copyright © Cengage Learning. All rights reserved

Example Multiply reactions to give the correct numbers of reactants and products: 4( ) 4( ) 3( ) 3( ) Desired reaction: Copyright © Cengage Learning. All rights reserved

Final reactions: Desired reaction: ΔH = +1268 kJ Example Copyright © Cengage Learning. All rights reserved

Example ΔH° = -1429 kJ 2 H2O (g) Copyright © Cengage Learning. All rights reserved