   # The enthalpy of combustion of CH 4 ( g ) when H 2 O ( 1 ) is formed is − 891 kJ/mol and the enthalpy of combustion of CH 4 ( g ) when H 2 O( g ) is formed is − 803 kJ/mol. Use these data and Hess’s law to determine the enthalpy of vaporization for water. ### Chemistry: An Atoms First Approach

2nd Edition
Steven S. Zumdahl + 1 other
Publisher: Cengage Learning
ISBN: 9781305079243

#### Solutions

Chapter
Section ### Chemistry: An Atoms First Approach

2nd Edition
Steven S. Zumdahl + 1 other
Publisher: Cengage Learning
ISBN: 9781305079243
Chapter 7, Problem 17Q
Textbook Problem
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## The enthalpy of combustion of CH4(g) when H2O (1) is formed is − 891 kJ/mol and the enthalpy of combustion of CH4(g) when H2O(g) is formed is − 803 kJ/mol. Use these data and Hess’s law to determine the enthalpy of vaporization for water.

Interpretation Introduction

Interpretation:

The enthalpy of vaporization for water should be determined.

Concept Introduction:

Hess's Law:

• The enthalpy change accompanying a chemical reaction is independent of the route by which the chemical reaction occurs.
• Hess's Law is saying that if you convert reactants to products the overall enthalpy change will be exactly the same whether you do it in one or multiple steps.
• Formula:

ΔH(total)ΔH(number of steps)......(1)

Rules:

• The sign of ΔH reversed when the reaction is reversed.
• The quantities of reactants and products are directly proportional to the magnitude of ΔH 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.

Enthalpy of vaporization:

• The enthalpy change for the process of vaporization (liquid gas) is called enthalpy of vaporization.

### Explanation of Solution

Explanation

• Enthalpy of combustion of CH4(g) when H2O (l) is formed is - 891 kJ/mol.
• CH4(g)+2O2(g)CO2(g)+2H2O(l)ΔH=-891KJ......(1)

• Enthalpy of combustion of CH4(g) when H2O(g) is formed is - 803 kJ/mol.
• CH4(g)+2O2(g)CO2(g)+2H2O(g)ΔH=-803KJ......(2)

The enthalpy of combustion of CH4(g) are recorded as shown above.

In the Hess's Law,

H2O(l)+12CO2(g)12CH4(g)+O2(g)ΔH1=-12(-891KJ)

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