Thermodynamics: Hess's Law and ΔH

Hess's Law: In going from a set of reactants to a set of products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps. If two or more chemical reactions sum to a third reaction, then the enthalpy change of the third reaction is the sum of the enthalpy reactions of the step-wise reactions.

There are two common experiments used to illustrate Hess's Law. In one the target is the reaction of hydrochloric acid with solid sodium hydroxide.

HCl(aq) + NaOH(s) arrow H2O(l) + NaCl(aq)

In the other the target reaction is the combustion of magnesium:

Mg(s) + 1/2 O2(g) arrow MgO(s)

Exercise. In each case above, your job is to use the Test Reactions page of the applet to find a set of reactions that sum to the target reaction. Once you have found such a set, then use the Enthalpy Diagrams portion of the applet to construct a diagram showing the enthalpy difference between the reactants and products of this set of reactions.