Hess's Law
Hess's law states that if a reaction is carried out in multiple steps, the reaction enthalpy is equal to the sum of the reaction enthalpies of each state.
A chemical reaction can occur in multiple steps and through different pathways. The exact path taken for a reaction or the order of the steps does not affect the reaction enthalpy. Enthalpy is a state function, which means it is independent of the path taken.
Hess's law, named for Swiss-Russian chemist Germain Hess (1802–50), states that in a multistep reaction, the reaction enthalpy is equal to the sum of the reaction enthalpies for each individual step. Hess's law means that enthalpies of reaction can be added to each other.
Consider the reaction for the combustion of methane:Hess's law also allows scientists to calculate enthalpies for reactions that do not normally occur.
Step-By-Step Example
Calculation of Enthalpy Change for the Formation of Carbon Monoxide
The reaction does not occur under normal conditions.
Carbon and carbon monoxide both undergo combustion with oxygen to form carbon dioxide. These reactions occur, and their reaction enthalpies are measured:Step 1
The goal is to construct reaction 3 using reactions 1 and 2. Check the products. Reaction 3 has CO(g) as a product. Reaction 2 has CO(g) as a reactant. The first step is to reverse reaction 2 so that carbon monoxide becomes a product, as in reaction 3. Note that this reverses the sign of as well.
Step 2
Now add reaction 1 and reaction 4.
Solution
The term CO2(g) appears on both sides of the reaction arrow, so it can be eliminated. There is 1 mol of O2(g) on the reactant side and half a mole of O2(g) on the product side. We can eliminate half a mole of O2(g) from both sides as well. The remainder is the net reaction.