PHYSICAL CHEMISTRY IN BRIEF

CHAP. 5: THERMOCHEMISTRY [CONTENTS] 130

Example

The chemical reaction

CH 4 +CO 2 = 2CO+ 2H 2 (5.8)

is the linear combination of the reactions

C+ 2H 2 = CH 4 , (5.9)

C+

1

2

O 2 = CO, (5.10)

C+O 2 = CO 2. (5.11)

We obtain equation (5.8) by subtracting equations (5.9) and (5.11) from twice equation (5.10).

5.1.2 Hess’s law

If a chemical reactionRis a linear combination of reactionsR 1 ,R 2 ,... , its reaction enthalpy
is the same linear combination of the reaction enthalpies of the reactionsR 1 ,R 2 ,...

Note:Hess’s law is a consequence of the fact that enthalpy is a state function. An analogy

of Hess’s law also holds for other thermodynamic quantities of reaction, e.g. the reaction

entropy or the Gibbs energy.

Example

At temperatureT= 298 K and pressurep= 101.325 kPa, the reaction enthalpy of the reaction

(5.9) equals−74.852 kJ/mol, that of the reaction (5.10) equals−110.529 kJ/mol, and that of

the reaction (5.11) equals−393.522 kJ/mol. Calculate the reaction enthalpy of the reaction

(5.8).

Solution

Based on the result of the preceding example, the reaction enthalpy is

2 ×(− 110 .529) + (−1)×(− 74 .852) + (−1)×(− 393 .522) = 247. 316 kJ mol−^1.