Chapter 15.4, Problem 38SSC

Interpretation Introduction

Interpretation:

The meaning of Hess’s Law and its application is to be explained.

Concept introduction:

Enthalpy change of some reactions can’t be calculated directly. Indirect method is used to calculate it. Hess’s law helps to calculate it

Answer to Problem 38SSC

According to it if one equation is the resultant of two different equations then change in enthalpy of the equation is sum of enthalpy change of both reactions.

It is used to calculate enthalpy change in a reaction.

Explanation of Solution

According to Hess’s Law, the enthalpy change in a reaction can be determined by choosing the equations which on multiplication, division or reversal of equation gives the resultant equation.

For example enthalpy change for formation of sulphuric acid from sulphur trioxide and water can be calculated as:

$\Delta {{\rm H}}_{rxn}=\Delta {{\rm H}}_f(H_{2}S{O}_4)-\Delta {{\rm H}}_f(S{O}_3)-\Delta {{\rm H}}_f(H_{2}O)$

Conclusion

Thus, Hess’s Law is applied to calculate enthalpy change in a reaction.