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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 on theme: "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."— Presentation transcript:

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2 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

3 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

4 The Principle of Hess’s Law
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6 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

7 Example Consider the following data: Calculate ΔH for the reaction
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8 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

9 Example Reverse the two reactions: Desired reaction:
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10 Example Multiply reactions to give the correct numbers of reactants and products: 4( ) 4( ) 3( ) 3( ) Desired reaction:

11 Example Final reactions: Desired reaction: ΔH = +1268 kJ
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