| Bond Energies & Enthalpies | |
|
Review
You may wish to review the Laws of Thermochemistry and Endothermic and Exothermic Reactions before you begin. A table of single bond energies is available to help you.
Problem
Estimate the change in enthalpy, DH, 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:
H2(g) --> 2 H(g)
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:
DH1 = +(436 kJ + 243 kJ) = +679 kJ
Bond breaking requires energy, so we expect the value for DH to be positive for this step.
In the second step of the reaction, two moles are H-Cl bonds are formed. Bond breaking liberates energy, so we expect the DH for this portion of the reaction to have a negative value. Using the table, the single bond energy for one mole of H-Cl bonds is found to be 431 kJ:
DH2 = -2(431 kJ) = -862 kJ
By applying Hess's Law, DH = DH1 + DH2
DH = +679 kJ - 862 kJ
DH = -183 kJ
Answer
DH = -183 kJ
Cl2(g) --> 2 Cl(g)
