Hess’s Law (Cambridge (CIE) AS Chemistry)

Hess Cycles

• 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

Illustration of Hess's Law

According to Hess’ Law, the enthalpy change of the direct route, going from reactants (A+B) to product (C), is equal to the enthalpy change of the indirect routes

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

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

• The products can be directly formed from the elements = ΔH₂ OR The products can be indirectly formed from the elements = ΔH₁ + ΔH_r

Applying Hess's Law

The enthalpy change from elements to products (direct route) is equal to the enthalpy change of elements forming reactants and then products (indirect route)

• Equation

ΔH₂ = ΔH₁ + ΔH_r

Therefore,

ΔH_r = ΔH₂ – ΔH₁ 

Calculating ΔH_f  from ΔH_c using Hess’s Law energy cycles

• The combustion products can be formed directly from elements to combustion products = ΔH₁ OR The combustion products can be formed indirectly from elements to compounds to combustion products = ΔH_f + ΔH₂

 Hess's Law and combustion enthalpies 

The enthalpy change going from elements to products (direct route) is equal to the enthalpy change of elements forming reactants and then products (indirect route)

• Equation

ΔH₁ = ΔH_f + ΔH₂

Therefore,

ΔH_f  = ΔH₁ – ΔH₂

Calculating average bond energies using Hess's cycles

• Bond energies cannot be found directly so enthalpy cycles are used to find the average bond energy

• This can be done using enthalpy changes of atomisation and combustion or formation

• The enthalpy change of atomisation (ΔH^θ_at ) is the enthalpy change when one mole of gaseous atoms is formed from its elements under standard conditions.

  • E.g. ΔH^θ_at [H₂] relates to the equation:

½H₂ (g) → H (g)