AS-Level Chemistry Chapter 6 Chemical Energetics Hess' Law

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Hess' Law

Hess' law and Hess' cycle

1) Hess' law states that the total enthalpy change in a chemical reaction is independent of the route which the reaction takes place as long as the initial and final conditions are the same.

2) For example, the enthalpy change of reaction from A to C is the same whether the reaction takes place in one stage(A to C) or two stages(A to B then to C).

3) This is because the enthalpy change of a chemical reaction depends only on the difference between enthalpy of the products and the reactants. It does not depend on how the reaction took place.

4) Hess' law is useful to calculate enthalpy changes that cannot be measured directly from experiments(theoretical reactions that will not occur under standard conditions).

5) Steps to construct a Hess' cycle:

i. Write down the equation for the enthalpy change that you are asked to find and write ΔH on top of the arrow.
ii. Use the information that you have been given to complete a cycle.
iii. Find the direct route and indirect route. The routes should not go against any
arrows. (Equations may be reversed when necessary, but do not forget to reverse the sign as well.)
iv. Apply Hess' law and form an equation to solve for ΔH.