Hess's Law!

Hess’s Law

“The overall enthalpy change of a reaction is independent of the route taken.”

Example question:

Given that Δ H ^θ _combustion of water is -286 KJ mol^-1

Given that Δ H ^θ _combustion of Carbon dioxide is -394 KJ mol^-1

Given that Δ H ^θ _combustion of Methanol is -726 KJ mol^-1

Calculate the Δ H ^θ _formation of Methanol.

Step 1 – Balance the equation.

C + H₂ >> C₈H₁₈

C + 2 H₂ >> C₈H₁₈

Step 2 – Make the route diagram and label the Δ H’s

Step 3 – realise the route.

In this case, we want Δ H4+ Δ H3 which is equal to Δ H1 + Δ H2

This is because, regardless of what route we take, we end up getting the same product, and therefore the enthalpy change will be the same.

Step 4 – Substitute the values

Δ H1 + Δ H2 = Δ H3 + Δ H4

-394 + (2x-286) = Δ H1 + -726

-966 = Δ H3 – 726

- make the wanted delta H the subject of the formulae, and add the unit. -

Δ H3 = -966 + 726

Δ H3 = -240 Kj Mol ^-1