Hess' Law
Chemistry: AQA A Level, topic 1.4.5
Hess' Law
Hess' Law provides a useful way of calculating enthalpy changes.
Hess' Law
- The enthalpy change of a reaction is independent of the pathway.
- This means that the enthalpy change of the reaction is the same for if the reaction is direct or indirect.
What does this mean?
- Hess' Law is useful for obtaining enthalpy changes that aren't directly observable.
- These are often obscure reactions.
- With the help of some easy reactions that involve some of these compounds, we can derive the enthalpy changes we're interested in.
The break down
- This means that the following will all have the same enthalpy change:
- A → B → C → D → E
- A → C → E
- A → E
- Overall, they all just convert A to E and so the overall enthalpy change will be identical for all of them.
Hess Cycles
We can use Hess' Law to derive Hess Cycles which will help us with calculations. We can create cycles using enthalpies of formation or combustion.
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Hess cycles
- Above shows how we can convert the principle of Hess' Law into an actual equation.
- We can set up an equation using the pathways:
- ΔH1 + ΔH2 = ΔH3
- We can then subtitute in the known values and rearrange to find the unknown values.
Using enthalpies of formation
- The first type of Hess cycle is those that use enthalpies of formation to calculate an overall reaction's enthalpy change.
- We set up a cycle, as shown, which includes two enthalpies of formation arrows to create the cycle.
Using enthalpies of formation
- To do this we must know all the enthalpies of formation for the reactants and the products.
- The enthalpy of formation for any element is zero.
Using enthalpies of combustion
- We can use a similar method to set up a cycle using enthalpies of combustion.
- Below ΔH2 and ΔH3 represent enthalpies of combustion.
- This can be spotted by the combustion products: CO2 + H2O
Test yourself on Hess' Law
If the products were CO + H2O, what would the reactants be for the enthalpy of formation equation?
Which of the following does NOT have an enthalpy of formation of zero?