Calculate Enthalpy Changes Using ΔHc⦵ (HL) (HL IB Chemistry)

• The standard enthalpy change of combustion is

The enthalpy change that occurs when one mole of the substance burns completely under standard conditions

• We can use enthalpy of combustion to find an unknown enthalpy change using a Hess cycle
• In this type of cycle, the combustion products are always placed at the bottom of the diagram and the arrows should be pointing downwards

Energy cycle including combustion products 

The combustion products of both reactants and products should be placed at the bottom of the cycle 

• The general expression for ΔH^Ꝋ  is therefore:

ΔH^Ꝋ  = ∑ΔH_c^Ꝋ(reactants) - ∑ΔH_c^Ꝋ(products)

Using Hess's Law to solve ΔH^θ_f problems

• Standard enthalpy changes of formation, ΔH^θ_f can also be used to calculate standard enthalpy changes of reactions, ΔH^θ
• The overall equation will be:

ΔH^θ_f (reactants) + ΔH^θ = ΔH^θ_f (products)

• Which rearranges to:

ΔH^θ = ΔH^θ_f (products) - ΔH^θ_f (reactants)

• Be careful to count up all the atoms you need to use, and make sure they are written as they occur in the elements in their standard state

Diagram to show the Hess's Law cycle for calculating ΔH^θ from ΔH^θ_f  data

Elements should be in their standard state and balanced when written in a Hess's Law cycle

Using Hess's Law to solve ΔH^θ_c problems

• Standard enthalpy changes of combustion, ΔH^θ_c can also be used to calculate standard enthalpy changes of reactions, ΔH^θ
• The overall equation will be:

ΔH^θ_c (products) + ΔH^θ = ΔH^θ_c (reactants)

• Which rearranges to:

ΔH^θ = ΔH^θ_c (reactants) - ΔH^θ_c (products)

Diagram to show the Hess's Law cycle for calculating ΔH^θ from ΔH^θ_c  data

Hess's Law says that the enthalpy changes on the two routes are the same