## 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 + H

_{2}>> C_{8}H_{18}C +

**2**H_{2}>> C_{8}H_{18}_{}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}
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