Measuring enthalpy changes
- Calorimetry is the measurement enthalpy changes in chemical reactions
- A simple calorimeter can be made from a polystyrene drinking cup, a vacuum flask or metal can
A polystyrene cup can act as a calorimeter to find enthalpy changes in a chemical reaction
- The energy needed to increase the temperature of 1 g of a substance by 1 oC is called the specific heat capacity (c ) of the liquid
- The specific heat capacity of water is 4.18 J g-1 K-1
- The energy transferred as heat can be calculated by:
Equation for calculating energy transferred in a calorimeter
Specific heat capacity calculations
In a calorimetry experiment 2.50 g of methane is burnt in excess oxygen.
30% of the energy released during the combustion is absorbed by 500 g of water, the temperature of which rises from 25 °C to 68 °C.
The specific heat capacity of water is 4.18 J g-1 K−1
What is the total energy released per gram of methane burnt?
Aqueous solutions of acid, alkalis and salts are assumed to be largely water so you can just use the m and c values of water when calculating the energy transferred.
To calculate any changes in enthalpy per mole of a reactant or product the following relationship can be used:
When there is a rise in temperature, the value for ΔH becomes negative suggesting that the reaction is exothermic
- This means that your value should be negative for an exothermic reaction, e.g. combustion
When the temperature falls, the value for ΔH becomes positive suggesting that the reaction is endothermic
- This means that your value should be positive for an endothermic reaction