Hess's Law (DP IB Chemistry): Revision Note

Hess's Law

• In 1840, the Russian chemist Germain Hess formulated a law which went on to be known as Hess’s Law

• This went on to form the basis of one of the laws of thermodynamics. The first law of thermodynamics relates to the Law of Conservation of Energy

• It is sometimes expressed in the following form:

Energy cannot be created or destroyed, it can only change form

• This means that in a closed system, the total amount of energy present is always constant

• Hess’s law can be used to calculate the standard enthalpy change of a reaction from known standard enthalpy changes

• Hess’s Law states that:

"The total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same."

• This means that whether the reaction takes place in one or two steps, the total enthalpy change of the reaction will still be the same

Diagram to show Hess's Law

• Hess’ Law is used to calculate enthalpy changes which can’t be found experimentally using calorimetry, eg:

3C (s) + 4H₂ (g) → C₃H₈(g)

• ΔH_f (propane) can’t be found experimentally as hydrogen and carbon don’t react under standard conditions

Calculating ΔH_r from ΔH_f using Hess’s Law energy cycles

Diagram to show Hess's Law 

• According to Hess’s Law, the total enthalpy change is the same regardless of the pathway taken, because energy is conserved

• There are two possible routes to form the products from the elements:

1. Direct formation (one-step route):

• Elements → Products

• Enthalpy change: ΔH₂

2. Indirect formation (two-step route via reactants):

• Elements → Reactants → Products

• Enthalpy change: ΔH₁ + ΔH_r

By Hess’s Law:

ΔH₂ = ΔH₁ + ΔH_r

• Rearranged to solve for the enthalpy change of reaction (ΔHᵣ):

ΔH_r = ΔH₂−ΔH₁