|Hess's Law of Heat Summation|
|When thermochemical equations are added to give some new equation,
their values of Ho are also added to give the Ho of
the new equation.
|The enthalpy change for a reaction is a state function. Its value
is determined only by the enthalpies of the initial and final states of the
chemical system, and not by the path taken by the reactant as they form the
products. To appreciate the significance of this, let us consider again the
combustion of carbon.
| C(s) + O2(g)
-------> CO2(g) Ho = -393.5 kJ
|This is only one possible way to make CO2.
|The second pathway to CO2 involves two steps. The first
is the combination of carbon with just enough oxygen to form carbon monoxide.
Then, in the second step, this CO is burned in additional oxygen to produce
CO2. Both steps are exothermic, and their thermochemical equations
C(s) + ½O2(g) ------> CO(g) Ho = -110.5 kJ
CO(g) + ½O2(g) ------> CO2(g) Ho = -283.0 kJ
|Note please that if we add the amount of heat liberated in the first
step to the amount released in the second, the total is the same as the heat
given off by the one-step reaction that was described first.
(-110.5 kJ) + (-283.0) = -393.5 kJ