6.3    Hess’ Law

Hess’ law and Hess’ cycle

• 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
• 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).

• 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

• Hess’ law is useful to calculate enthalpy changes that cannot be measured directly from experiments(theoretical reactions that will not occur under standard conditions).
• Steps to construct a Hess’ cycle:
• • Write down the equation for the enthalpy change that you are asked to find and write ΔH on top of the arrow
  • Use the information that you have been given to complete a cycle
  • Find the direct route and indirect The routes should not go against any arrows. (Equations may be reversed when necessary, but do not forget to reverse the sign as well.)
  • Apply Hess’ law and form an equation to solve for ΔH.

• Examples: