Chapter 6, Problem 6.74P

Interpretation Introduction

Interpretation:

$\Delta H$ for the vaporization of water, when $241.8\text{ kJ}$ of heat is released when 1 mol of ${\text{H}}_{\text{2}}\text{O}\left(g\right)$ forms from its element is to be determined.

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.