Chemistry, Third edition

216 13 · ENERGY CHANGES IN CHEMICAL REACTIONS

More about enthalpy changes

The enthalpy change Hof a reaction

reactants products

is defined by the equation

H(sum of enthalpies of products) (sum of enthalpies of reactants)

Using the mathematical symbol for sum, and the letters R and P for reactants and

products, we can write this as follows:

HHPHR (13.1)

Suppose that heat is given out during a chemical reaction (this is the case for the

burning of fuels). Such a reaction is said to be exothermic, and the container holding

the reaction mixture will be warm or hot to the touch. This loss of energy shows that

the reactant molecules were at a higher level of enthalpy than the product molecules.

Therefore, for exothermic reactions, H(as defined by equation (13.1)) is negative.

This is represented in Fig. 13.1(a), which shows the total enthalpy of the substances

before the reaction starts and after reaction is complete. In this sense, the xaxis

(labelled ‘progress of reaction’) may be thought of as a time axis.

Figure 13.1(b) shows the change of enthalpy for a reaction in which heat is taken

in (an endothermic reaction). The container holding the reaction mixture will be

cool or cold to the touch. In this case, the reactant molecules are at a lower level of

enthalpy than the product molecules. Therefore, for endothermic reactions His

positive.

Once the enthalpy change of a reaction is known, we may write it down on the

right-hand side of the chemical equation to which it applies. For example, the

reaction between ethene and hydrogen is exothermic with H 137 kJ per mol of

reacted ethene at 25 °C. We would write this as

CH 2 ==CH 2 (g)H 2 (g) CH 3 CH 3 (g) H 137 kJ mol^1

The combination of a chemical equation and its Hvalue is known as a thermo-

chemical equation.

One application of reactions that take in or give out heat is in cooling packs (Box

13.1).

Fig. 13.1Enthalpy diagrams for (a) exothermic and (b) endothermic reactions.