Calculating the Energy Released by a Reaction

Example: The Energy Released by the Combustion of Methane

Complete Example: The Combustion of Methane

Methane CH4 is the main constituent of natural gas. Its complete combustion is written:

CH4 + 2 O2 -> CO2 + 2 H2O

Let's first check that the equation is balanced: carbon 1=1; hydrogen 4=2×2=4; oxygen, on the left 2×2=4, on the right 2 (in CO2) + 2×1 (in 2 H2O) = 4. The equation is indeed balanced.

Counting the Bonds

Bonds broken (in the reactants):

CH4 has 4 C-H bonds: 4 × 413 = 1652 kJ
2 O2 have 2 O=O bonds: 2 × 498 = 996 kJ
Total broken: 1652 + 996 = 2648 kJ

Bonds formed (in the products):

CO2 has 2 C=O bonds: 2 × 799 = 1598 kJ
2 H2O have 4 O-H bonds (2 per molecule): 4 × 463 = 1852 kJ
Total formed: 1598 + 1852 = 3450 kJ

The Final Calculation

ΔrH = 2648 - 3450 = -802 kJ/mol

The result is negative: the combustion of methane is indeed exothermic, it releases about 802 kJ for each mole of methane burned. This is the energy that heats a pot on a gas stove, or drives a gas-fired power plant.

Why This Number Matters

This value, called the standard enthalpy of combustion, allows fuels to be compared with one another: for equal mass, some release more energy than others. It is also used to size a heating system, or to assess the CO2 emissions associated with a given amount of energy produced.

Common Mistake