4.11      describe simple calorimetry experiments for reactions such as combustion, displacement, dissolving and neutralisation in which heat energy changes can be calculated from measured temperature changes

Reactions such as displacements and neutralisation or the dissolving of solids can be done in a polystyrene cup with a lid. The initial temperature of the liquid(s) is measured. The reaction is then started and the mixture is stirred with a thermometer. The maximum (or minimum) temperature reached is noted.

The heat energy change (q) can be calculated using the relationship below: q = m c ∆T

NOTE: You will not be expected to remember this relationship.  It will be given to you if you need to use it in the exam.

q = m c ∆T

m = mass of liquid in the “cup” (aqueous solutions are taken to weigh 1 g per cm3)

c = specific heat capacity of water = 4.2 J/g/°C

∆T = temperature change  =  final temperature - initial temperature

e.g.      1.24 g of Mg was added to 25 cm3 of 1.0 mol/dm3 hydrochloric acid. The temperature rose from 18.5 °C to 30.8 °C. What was the energy change?

q          = 25 g x 4.2 J/g/°C x (30.8 - 18.5) °C

= 1292 J  =  1.292 kJ

(This can be converted into kJ/mol if you divide it by the number of moles of the substance reacting that is not present in excess. See the next example. This step is extension material and is not required for the Core paper 1.  It could appear on Paper 2.)

To measure the energy change of combustion, we can heat water in a copper calorimeter as shown below. We need to know the mass of water heated and the temperature rise of the water. If we want to be able to work out the energy of combustion in kJ/mol, we also need the mass of fuel burned.

e.g. 200 cm3 of water is heated by burning ethanol in a spirit burner.  The initial mass of the burner is

145.34 g and the final mass is 144.82 g.  The initial temperature of the water is 20.5 °C, and the final temperature is 26.8 °C.  How much heat energy is given out in this reaction?

q        = m c ∆T

q           = 200 g x 4.2 J/g/°C x (26.8 - 20.5) °C

= 5292 J   =  5.292 kJ