Chapter 5, Problem 20QP

Interpretation Introduction

Interpretation:

“Enthalpy” and “enthalpy of reaction” are to be defined and also the conditions under which heat of reaction equal to the enthalpy change of reaction is to be explained.

Concept introduction:

Enthalpy of a system is the thermodynamic quantity that represents the total heat of a system. It equals the internal energy of the system when work is not done.

Equation for enthalpy is $\text{H=U+PV}$,

where $\text{U}$ is the internal energy of the system, and $\text{P}$ and $\text{V}$ are the pressure and volume of the system, respectively.

For any reaction, the change in enthalpy, called as the enthalpy of reaction $\text{ΔH}$, is defined as the difference between the enthalpies of the products and the reactants. Equation for enthalpy of reaction is $\text{ΔH=H}\left(\text{products}\right)\text{ -H}\left(\text{reactants}\right)$.

The enthalpy of reaction can be positive or negative, depending on the process. For an endothermic process, $\text{ΔH}$ is positive, whereas for an exothermic process, $\text{ΔH}$ is negative.