Enthalpy Changes
Chemistry: AQA A Level, topic 1.4.1
Enthalpy Changes
For a given process or reaction, we can measure the enthalpy change.
Definition of enthalpy change
- An enthalpy change is a measure of the heat given out or taken in during a process.
- When objects are heated, they use energy to expand.
- Enthalpy takes into account the energy used in the expansion.
Enthalpy vs energy changes
- We use enthalpy instead of energy because we cannot easily measure energy changes. This is because objects expand when heated.
- To measure an energy change, we would have to fix the volume of the object.
- Enthalpy is much easier to use as it allows for expansion.
- Enthalpy changes are instead measured under constant pressure.
- The atmosphere is at a constant pressure, so we require no extra equipment.
Endothermic vs exothermic reactions
- An exothermic reaction is one which gives out heat energy.
- An exothermic reaction has a negative enthalpy change.
- An endothermic reaction is one which takes in heat energy.
- An endothermic reaction has a positive enthalpy change.
Combustion is exothermic
- Combustion is an exothermic process as it gives out heat!
- E.g. Burning methane:
- CH4 + 2O2 → CO2 + 2H2O
- ΔH = −882.00kJmol-1
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Thermal decomposition is endothermic
- Thermal decomposition is an endothermic process.
- E.g. The thermal decomposition of calcium carbonate:
- CaCO3 → CaO + CO2
- ΔH = +178.30kJmol-1
Test yourself on Enthalpy Changes
Types of enthalpy changes
Which of the following is true of exothermic reactions?