How is stoichiometry related to enthalpy changes?
The enthalpy change for a given reaction depends on the stoichiometry of the reaction.
For example, 2 mol of carbon monoxide react with 2 mol of nitrogen(II) oxide according to the balanced equation.
2CO(g) + 2 NO(g) → 2 CO₂(g) + N₂(g); ΔH = -746 kJ
The process is exothermic, releasing 746 kJ of energy. This amounts to 373 kJ of energy per mole of CO.
Consider now the reaction of 0.250 mol carbon monoxide. We can calculate the amount of energy released from the enthalpy of reaction
(ΔH = -373 kJ/mol CO).
Since enthalpy H is an extensive variable, it depends on the amount of substance present.
Therefore, a change in enthalpy ΔH also depends on the amount of substance that reacts.
The reaction of 0.250 mol of CO with NO, for example, results in the release of
0.250 mol CO × (373 kJ/1 mol CO) = 93.3 kJ
Now consider the reaction
N₂ + O₂ → 2NO; ΔH = +180.6 kJ
a) What is the enthalpy change for the formation of one mole of nitrogen(II) oxide?
Here we use the conversion factor 180.6 kJ/2 mol NO.
ΔH = 1 mol NO × (180.6 kJ/2 mol NO) = 95.30 kJ
b) What is the enthalpy change for the reaction of 100.0 g of nitrogen with excess oxygen?
Here, we need to convert grams of N₂ to moles of N₂ and use the conversion factor 180.6 kJ/1 mol N₂.
180.6 kJ × (1 mol N₂/28.01 g N₂) × (180.6 kJ/1 mol N₂) = 1164 kJ