   Chapter 21, Problem 112GQ

Chapter
Section
Textbook Problem

Using enthalpy of formation data in Appendix L, determine whether the decomposition of NH4NO3(s) to give N2O(g) and H2O(g) is endothermic or exothermic.

Interpretation Introduction

Interpretation:

To determine the reaction is endothermic or exothermic by using enthaply of formation.

Concept introduction:

Enthalpy of formation of the reaction can be calculated by following expression,

ΔrH°=nΔfH°(products)nΔfH°(reactants)

The standard enthalpy of formation is equals to sum of standard enthalpy of formation of products minus sum of standard enthalpy of reactants.

If the value of ΔrH is negative, then the reaction will be exothermic.

If the value of ΔrH is positive, then the reaction will be endothermic.

Explanation

The value of ΔrH for the reaction is calculated below.

Given:

Refer to Appendix L for the values of standard entropies and enthalpies.

The standard enthalpy of NH4NO3(s) is 365.56 kJ/mol.

The standard enthalpy of N2O(g) is 82.05 kJ/mol

The standard enthalpy of H2O(g) is 241.83 kJ/mol

The balanced chemical equation is:

NH4NO3(s) N2O(g) + 2H2O(g)

The ΔrHo can be calculated by the following expression,

ΔrH°=nΔfH°(products)nΔfH°(reactants)=[[(1 mol N2O(g)/mol-rxn)ΔfH°[N2O(g)

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