| ΔrH = | qp |
| n |
Consider the reaction in which SO3(g) is formed from its elements.
| Direct pathway: or Indirect pathway: | S(s) + 3/2 O2(g)  SO3(g) SO2(g)  |
The enthalpy change for a direct pathway equals the enthalpy change for the indirect pathway if the products and reactants of both pathways are the same. The enthalpy change for the indirect pathway can be calculated from enthalpy changes for reactions in this pathway by
- determining how the equations in the indirect pathway combine to give the equation in the direct pathway
- combining the enthalpy changes for the reactions in the same way
In example the equations for the reactions in the indirect pathway add to give the equation in the direct pathway.
Therefore their enthalpy changes also add to give the overall enthalpy change.
Therefore their enthalpy changes also add to give the overall enthalpy change.
| (1) | S(s) + O2(g) | | |
| plus | ΔrH° = Δ1H° | ||
| (2) | | SO3(g) | |
| equals | ΔrH° = Δ2H° | ||
| (3) | S(s) + 3/2 O2(g) | | SO3(g) |
| ΔrH° = Δ3H° = Δ1H° + Δ2H° | |||
Having completed this activity you should be able to deduce how equations and their enthalpy changes in an indirect pathway can be combined in cases where the combination involves
- no change to either reaction
- one or more of the equations being reversed
- one or more of the equations having the coefficients multiplied by a common factor