Chapter 15.2, Problem 17SSC

Interpretation Introduction

Interpretation:

The reason why the heat enthalpy of exothermic reaction has a negative value is to be justified.

Concept introduction:

Heat enthalpy is the amount of energy either released or produced per mole in a chemical reaction. It can be measured using a calorimeter in an isolated system with constant pressure. The heat enthalpy for a chemical reaction is the difference between the heat enthalpy of products and the reactants.

Answer to Problem 17SSC

In an exothermic reaction, where heat is released as a by-product of the reaction, the enthalpy of formation of products is lower than that of the reactants. As a result, the heat enthalpy has a negative value for an exothermic reaction.

Explanation of Solution

The graph shows the energy released as an exothermic reaction between two reactants, A and B takes place to form C.

So based on the graph, the heat enthalpy, $\Delta H$, of the reaction is,

$\begin{array}{l}\Delta H=\Delta H_C-\Delta H_A-\Delta H_B\\ \Delta H<0\end{array}$

Conclusion

An exothermic reaction has a negative value for heat enthalpy, $\Delta H$.