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