Chapter 15, Problem 71A

Interpretation Introduction

Interpretation:

The condition in which the heat change in a chemical reaction equal to change in enthalpy is to be determined.

Concept introduction:

Change in enthalpy of the system is given by:

$\begin{array}{l}\Delta H=\Delta U+P\Delta V\\ where\\ \Delta Hischangeinenthalpy\\ \Delta Uischangein\mathrm{int}ernalenergy\\ Pispressure\\ \Delta Vischangeinvolume\end{array}$

Answer to Problem 71A

At constant pressure, the heat change in a chemical reaction equal to change in enthalpy.

Explanation of Solution

According to first law of thermodynamics,

$\Delta U=q-P\Delta V$

Change in enthalpy at constant pressure is given by:

$\Delta H=\Delta U+P\Delta V$

Thus $\begin{array}{c}\Delta H=\Delta U+P\Delta V\\ =q-P\Delta V+P\Delta V\\ =q\end{array}$

Conclusion

Thus change in enthalpy is equal to heat change at constant pressure.