AS-Level Energy and Enthalpy

  • The energy change that occurs during a reaction can be found by measuring how the temperature of the surroundings changes during a reaction, this is called calorimetry.

  • Specific heat capacity refers to the energy required to raise the temperature of 1kg of a substance by 1°C.

  • The temperature change that a reaction forces on a substance can be used with the specific heat capacity to find the energy that has been exchanged.

  • Change in energy = mass x specific heat capacity x change in temperature

    • Q = mc∆T

  • Energy is often lost to the surroundings from the substance being used to find the temperature change (for example water).

  • To increase the accuracy of calorimetry, heat loss must be minimised (extra insulation can help).




To calculate the enthalpy change that occurs in a reaction, we need to know how much heat energy has either been released to a reaction’s surroundings, or how much energy has been taken in from the surroundings. The process of calculating of enthalpy changes from experimental temperature data is called calorimetry.


In order to do this, we measure the temperature of a reaction’s surroundings before and then after a reaction. From this temperature change, we can then calculate the enthalpy change of the reaction.


We use the equation:

  • Q = mc∆T



  • Q = heat energy change

  • m = mass of surroundings

  • c = specific heat capacity of surroundings

  • ∆T = temperature change of surroundings



Different substances require different amounts of energy to heat up by a certain amount. For example, warming 10g of water by 10°C requires a different amount of energy to warming 10g of solid copper by 10°C.  In order to calculate the energy change occurring in a reaction, we have to account for the differing amounts of energy taken to heat different substances. This is why we use specific heat capacity: the amount of energy it takes to heat 1g of a substance by 10°C.


The mass is just simply how much substance is being heated (or cooled) by a reaction.