AS-Level Energy and Enthalpy
There is often more than one way to turn a substance into a product.
The energy change that occurs when a substance is changed into a product is the same, regardless of the route taken. This is called Hess’ Law.
Hess’ Cycles are ways of drawing out the energy changes that occur when a substance is turned into a product via more than one route.
Hess' Law (and Hess Cycles)
Very often in chemistry there is more than one route (way) to turn a substance into a product.
When a certain substance reacts to form a new product, a given amount of energy is either released or taken in by the substance.
This amount of energy is the same regardless of how we get to our end product. This is called Hess’ Law.
Imagine having to get to the other side of a mountain – you can either walk over the mountain (which is hard work) or walk around the mountain (less work but it takes longer), either way the energy required to get from one side to the other is the same.
Note – this is an analogy! It’s not exact and obviously with human effort there would be a discrepancy in energy required, but the general idea is the same.
This means if we know the enthalpy changes for individual stages within a reaction process, we can calculate overall enthalpy changes or we can calculate enthalpy changes for a specific reaction.
If we combust (react with oxygen) the reactants (carbon and hydrogen) and the products (propane), we can form a cycle. Remember Hess’ law states that the enthalpy change of a reaction is the same, regardless of which route we take to form the products.
In this case, we are producing carbon dioxide and water from carbon and hydrogen, but with two different routes. Hess’s law tells us the overall enthalpy change for both ways will be the same.
If we know how much energy change occurs when we combust carbon, hydrogen and propane, we can calculate the enthalpy changes for our known steps.
It is now just a question of using our original cycle to re-arrange an equation, to find our unknown enthalpy change for the production of propane from carbon and hydrogen.