Topic 5.2: Hess's Law

Question 1

Hess’s law statement

Answer 1

Total enthalpy change in a chemical reaction depends only upon the initial and final states and is independent of the reaction pathway.

Question 2

Why does Hess’s law work?

Answer 2

a) Enthalpy is a property of a substance under a certain set of conditions
b) The route that is taken between A and B cannot affect the amount of enthalpy A or B has
c) Thus, the difference in enthalpy between A and B is constant

Question 3

Rules for Hess’s law (4)

Answer 3

a) The magnitude of ΔH is proportional to the amount of reactant consumed in the process.
b) The enthalpy change for a reaction is equal in magnitude, but opposite in sign, to ∆H for the reverse reaction
c) The enthalpy change for a reaction depends on the states of the reactants and products