Hess’s law is named after Germain Hess, a Russian chemist, who published it in 1840. It is based on the principle of conservation of energy, and states that:
The total enthalpy change in a chemical reaction is sum of enthalpy changes of all steps from the reactants to the products regardless of the route taken.
For example, there are two possible chemical routes (indicated by red arrows in the diagram below) for the combustion of methane to form carbon dioxide and water:
On closer scrutiny, ΔH1o is composed of the following standard enthalpy changes of formation:
Similarly, ΔH2o is composed of the following standard enthalpy changes of formation:
The choice of the indices R and P will be apparent shortly. Hence,
where vR and vP are the stoichiometric coefficients of the products of the respective standard enthalpy change of formation reactions.
In general, for a reaction:
where R denotes the reactants and P denotes the products. Hess’ law states that:
Eq 6 is a very useful formula for calculating standard enthalpy changes.
Question
a) Calculate the standard enthalpy change of formation of CO2(g) given:
b) Deduce the relationship between ΔHsolo, ΔHhydo and ΔHlatto using MgCl2 as an example.
Answer
a)
Using eq6, the standard enthalpy change of C(graphite) + O2(g) → CO2(g) is:
b)
where vi is the stoichiometric coefficient of the reactant of the respective standard enthalpy change of hydration reaction. If ΔHlatto is defined as the change in enthalpy when one mole of an ionic solid is formed from its gaseous ions that are initially infinitely apart under standard conditions, the relation becomes: