A Hess law practice problem provided by the University of South Carolina is to find the ?H for the reaction PCl5(g) ? PCl3(g) + Cl2(g), given these reactions and ?H values: P4(s) + 6Cl2(g) ? 4PCl3(g) where ?H = -2439 kJ and 4PCl5(g) ? P4(s) + 10Cl 2(g) where ?H = 3438 kJ. This sample problem requires following two ?H values to come up with the correct answer.
Hess's Law is the principle of the conservation of energy in the chemistry field. Chemist Henri Hess theorized that the heat in chemical processes is the same, even if the process takes several steps to complete. In other words, the amount of energy that is absorbed or evolves from a chemical reaction is determined by the state of the reactants themselves, not the steps it takes to get to the final product.
In problems such as the example from the University of South Carolina, the total energy in the correct answer is 249.8 kJ. This answer comes from the Hess principle that the sum of the energy changes equals the total energy in the entire reaction.
Hess's Law is usually used to calculate reaction energies that are difficult to measure. As long as a chemist has data from related reactions, the new reaction is simple to calculate without taking measurements.