Chapter 7, Problem 153SAE

Interpretation Introduction

Interpretation:

A concept map to show the use of enthalpy for chemical reactions should be constructed.

Concept introduction:

The relation between internal energy, heat and work done can be represented as follows:

$\Delta U=q+w$

$\begin{array}{l}\text{For a constant pressure;}\\ w=-\text{P}\Delta V\\ \Delta U=q_{\text{p}}-\text{P}\Delta V\\ \Delta U=q_{\text{p}}-\text{P}(V_f-V_i)\\ U_f-U_i=q_{\text{p}}-\text{P}{V}_f+{\text{P}}_{\text{ext}}{V}_i\\ \left(U_f+\text{P}{V}_f\right)-\left(U_i+\text{P}{V}_f\right)=q_{\text{p}}\end{array}$

The left side of this expression is the change in the quantity $U+\text{P}V$ .The quantities U, P, and V are all state functions, $U+\text{P}V$ must also be a state function. This state function is called the enthalpy, H.

$\begin{array}{l}\left(U_f+\text{P}{V}_f\right)-\left(U_i+\text{P}{V}_f\right)=q_{\text{p}}\\ H_f-H_i=q_{\text{p}}\\ \Delta H=q_{\text{p}}\end{array}$

Therefore, for the constant pressure;

$\Delta U=\Delta H-\text{P}\Delta V$