Measurement of energy change
It is difficult to know the exact quantity of chemical potential energy possessed by the components of a system, but by reference to the graph below, we can see that the change in chemical energy from reactants to products MUST be equal to the change in heat energy (this only applies to systems at constant pressure, which is the usual case in a laboratory).
In order to calculate the change in chemical energy, we simply have to measure the change in heat energy of a system.
There are two basic types of methodology for measuring the heat of a chemical process.
Direct measurement requires knowledge of the specific heat capacity of the material being heated up, or the heat capacity of the whole system being heated. Once this is known it is simply a matter of recording starting and finishing temperatures.
If the reaction is not instantanous, then it may be necessary to plot a graph of temperature change against time and carry out a back extrapolation to account for heat losses.
Most thermochemical experiments involve a change in temperature of a solution, in which case the solution mass, specific heat capacity and temperature change can be used to calculate the energy change. It is usually assumed that all aqueous solutions have the same specific heat capacity as water.
Indirect measurements involve collecting the heat being given off during the reaction in another medium, such as a beaker of water. This is subject to many inaccuracies and is covered in Section 4.21 - Combustion Enthalpy.
Change in heat energy
If the specific heat capacity of a substance is known then the heat energy change may be calculated using: E = m x c x ΔT, where:
- m= mass in kg of the substance being heated up
- c = specific heat capacity
- ΔT = change in temperature
Example: If 50cm3 sodium hydroxide 1.0M is allowed to react with 50cm3 hydrochloric acid 1.0M the temperature increases from 22ºC to 28.5ºC. Calculate the energy released on formation of 1 mole of water.
The energy released in this specific reaction is given by:
m= mass in kg of the system being heated up = 50g + 50g =100g = 0.1kg
E = 0.1 x 4.18 x 6.5
∴ moles of NaOH = moles of water
Moles of NaOH = molarity x volume (litres)
∴ 2.72 kJ energy is released by the formation of 0.05 moles water
∴ the formation of 1 mole of water will release 2.72/0.05 kJ = 54.4 kJ mol-1