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