Chapter EN, Problem 18PQ

Interpretation Introduction

Interpretation:

$\Delta H^{\text{o}}$ of below reaction has to be determined.

  ${\text{3H}}_2\left(g\right)+{\text{O}}_3\left(g\right)\to 3{\text{H}}_2\text{O}\left(l\right)$

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 that can be shown as follows:

  $\Delta H_{\text{overall rxn}}=\Delta H_1+\Delta H_2+\mathrm{.......}+\Delta H_{\text{n}}$

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.