Chapter 6.5, Problem 6.9AFP

Interpretation Introduction

Interpretation:

$\Delta H$ of the following reaction is to be determined.

${\text{2NO}}_2\left(g\right)+\frac{1}{2}{\text{O}}_2\left(g\right)\to {\text{N}}_2{\text{O}}_5\left(s\right)$

Concept introduction:

Hess’s law is used to calculate the enthalpy change of an overall reaction that can be derived as a sum of two or more reaction. According to Hess’s law $\Delta H$ of an overall reaction is equal to the sum of the enthalpy change for each individual reaction. $\Delta H_{\text{overall rxn}}=\Delta H_1+\Delta H_2+\mathrm{.......}+\Delta H_{\text{n}}$

Enthalpy is a state function so the value depends upon the initial state and final state not on the path so $\Delta H$ of an overall reaction can be calculated by the addition or subtraction of the individual steps whose $\Delta H$ is known.