Enthalpy changes are the result of heat transfers during chemical reactions.
We are familiar with the idea of exothermic and endothermic reactions from GCSE. Here, I start with clarifying the key language before moving on to look at determining enthalpy changes experimentally as well as theoretically, sharing foolproof methods for drawing out enthalpy cycles and using them to calculate an enthalpy change indirectly.
There is a tendency when teaching thermochemistry (the chemistry of energy changes) to leap straight in and assume that the basics covered at GCSE are still fresh in everyone’s mind, and that students have a genuine understanding of the chemistry behind exothermic and endothermic reactions simply because they can sort of sketch an energy level diagram.
In my experience, teachers, let alone students, struggle to explain how heat transfers in reactions can be used for useful work . . . so don’t skip the first post which is written specifically to get you up to speed and iron out any misconceptions.
Getting up to speed in thermochemistry
Thermochemistry is all about the energy changes that happen alongside chemical reactions.
Chemical reactions either release heat (exothermic) or absorb heat (endothermic) – in many instances, the heat can be used to do useful work such as in a battery or engine.
How to calculate the enthalpy change of combustion of a fuel
You can calculate an experimental value for the enthalpy change of combustion for a fuel very simply – this is a required practical so you need to be able to describe both the method and how you would use the data in your calculations.
How to calculate the enthalpy change of neutralisation
Determining either the enthalpy change of neutralisation, or the enthalpy change for a classic displacement reaction (they are pretty much the same experiment) is also a required practical.
Enthalpy cycles, Hess cycles, thermochemical cycles
Hess’s Law states that the change in enthalpy depends only on the final and initial conditions of the system and is independent of the route taken to get from A to B, which means we can use it to indirectly calculate the enthalpy change of a chemical reaction.
Using bond enthalpies to calculate an enthalpy change of reaction
Reactions are either endothermic or exothermic – the heat transferred during a chemical reaction is either moving from the system to the surroundings or vice versa and is always the consequence of breaking bonds in the reactants and making new bonds in the products.