**What is Hess’s Law?**

To put it simply, Hess’s Law states that

- The net change in energy (enthalpy/ entropy/ gibb’s energy) of a chemical reaction will always be the same no matter what type of route the reactants take to reach the final product
- An example: The net distance from Auckland to Dunedin is 1000km on maps. There are many ways in which you can travel from Auckland to Dunedin
- You can take a flight
- You can drive, take a ferry and drive again
- You can sail (if you are brave enough)

- It does not matter which route or how many route you take, as long as you start from Auckland and end up in Dunedin, the net distance will still be 1000km

**When Do We Use Hess’s Law?**

In exam, you use the Hess’s Law when the question

- wants you to calculate the change in energy (enthalpy/ entropy/ gibb’s energy)
- and gives you more than 2 chemical equations
- For example:

Note: in most exam questions, they will give you the main equation. If not then you have to create one. I will elaborate more on this in future posts.

**How Do We Use Hess’s Law?**

So when a question requires you to use Hess’s law, follow these 3 simple steps

Determine/ find out the overall equation

Manipulate the other equations to **get rid of intermediates**. Intermediates are species that do not exist in the overall equation.

- You can manipulate equations by multiply or divide or reverse or add the equations. Make sure you track the changes to enthalpy/gibb’s.

Add the equations and the enthalpy/ gibb’s energy together

To cancel out or get rid of the intermediates, they need to be on the **opposite sides** of the equation (ie, as reactant in one equation and as product in the other equation). Remember your goal is to calculate enthalpy/ gibb’s, so make sure you keep track of the changes and add them at the end!

That is Hess’s Law in a nutshell. The diagrams are taken from my notes. Download here