Hess' Law (AQA A Level Chemistry)

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

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

Answer

Step 1: Write the balanced equation at the top

Step 2: Draw the cycle with the elements at the bottom

Step 3: Draw in all arrows, making sure they go in the correct directions. Write the standard enthalpy of formations

Step 4: Apply Hess’s Law

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_at^ꝋ ) is the enthalpy change when one mole of gaseous atoms is formed from its elements under standard conditions.
  • Eg. ΔH_at^ꝋ [H₂] relates to the equation:

½ H₂(g) → H(g)