1.4.3

# Hess' Law

Test yourself

## 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.

### 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