Chapter 5, Problem 51PS

### Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074

Chapter
Section

### Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074
Textbook Problem

# Enthalpy changes for the following reactions can be determined experimentally:N2(g) + 3 H2(g) → 2 NH3(g)ΔrH° = –91.8 kJ/mol-rxn4 NH3(g) + 5 O2(g) → 4 NO(g) + 6 H2O(g)ΔrH° = –906.2 kJ/mol-rxnH2(g) + ½ O2(g) → H2O(g)ΔrH° = –241.8 kl/mol-rxnUse these values to determine the enthalpy change for the formation of NO(g) from the elements (an enthalpy change that cannot be measured directly because the reaction is reactant-favored).½ N2(g) + ½ O2(g) → NO(g) ΔrH° = ?

Interpretation Introduction

Interpretation:

The enthalpy change for the formation of NO has to be determined.

Concept Introduction:

If a reaction proceeds in two or more other reaction,ΔrH0 for the overall process is the sum of ΔrH0 values of those reactions-Hess’s law.

Explanation

Divide the equation 2 by 2

â€‚Â 2N2â€‰+â€‰6H2â€‰â†’â€‰â€‰4NH3Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Î”H=183.6kJ

â€‚Â 4NH3â€‰+â€‰5O2â€‰â†’â€‰4NOâ€‰+â€‰6H2O Â Â Â Â Â Â Î”H=-906.2kJ

Reverse the 3rd equation. And multiply by 6

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