Enthalpy Change Problem

Estimate the change in enthalpy, ΔH, for the following reaction:

H2 (g) + Cl2 (g) → 2 HCl (g)

Solution

To work this problem, think of the reaction in terms of simple steps:

Step 1 The reactant molecules, H2 and Cl2, break down into their atoms

H2(g) → 2 H(g)
Cl2(g) → 2 Cl(g)

Step 2 These atoms combine to form HCl molecules

2 H (g) + 2 Cl (g) → 2 HCl (g)

In the first step, the H-H and Cl-Cl bonds are broken. In both cases, one mole of bonds is broken. When we look up the single bond energies for the H-H and Cl-Cl bonds, we find them to be +436 kJ/mol and + 243 kJ/mol, therefore for the first step of the reaction:

ΔH1 = +(436 kJ + 243 kJ) = +679 kJ

Bond breaking requires energy, so we expect the value for ΔH to be positive for this step.