Chapter 15.4, Problem 37PP

Interpretation Introduction

Interpretation:

The value of $\Delta {\rm H}$ is to be calculated for the given equation.

Concept introduction:

The change in enthalpy of a reaction can be calculated using Hess Law.

According to it if one equation is the resultant of two different equations then change in enthalpy of the equation is sum of enthalpy change of both reactions.

Answer to Problem 37PP

$\Delta {\rm H}(b)=33.2kJ$

Explanation of Solution

Given data:

$\begin{array}{l}NO+\frac{1}{2}{O}_2\to N{O}_2\Delta {{\rm H}}_{rxn}^0=-58.1kJ\\ (a)\frac{1}{2}{N}_2+\frac{1}{2}{O}_2\to NO\Delta {{\rm H}}_f^0=91.3kJ\\ (b)\frac{1}{2}{N}_2+O_2\to N{O}_2\Delta {{\rm H}}_f^0=?\end{array}$

According to Hess Law,

Adding reverse of first equation to second equation, resultant equation is obtained.

Therefore,

$\begin{array}{c}\Delta {{\rm H}}_{rxn}^0=-\Delta {\rm H}(a)+\Delta {\rm H}(b)\\ -58.1=-91.3+\Delta {\rm H}(b)\\ \Delta {\rm H}(b)=-58.1+91.3\\ \Delta {\rm H}(b)=33.2kJ\end{array}$

Conclusion

Thus, $\Delta {\rm H}(b)=33.2kJ$