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)  H2O(l) +
NaCl(aq)
In
the other the target reaction is the combustion
of magnesium:
Mg(s)
+ 1/2 O2(g) 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.
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