The apparatus used for such an experiment is a calorimeter. The first step is to weigh the burner at the start. It is important to keep the cover over the wick until it is time to ignite the methanol. Secondly, the temperature of the water in the calorimeter is measured. With a slow current the air is drawn through the copper spiral, the wick is lit and the water in the calorimeter gently stirred. When a temperature rise of
10-20

C has been achieved, the cover is placed over the burner. Finally, the maximum temperature of the water is recorded and the burner reweighed.
The problem that remains is to discover the heat capacity of the apparatus, which contains water, copper and glass, each with its own heat capacity.
The easiest method of getting around this problem is as follows. We could place an electric heater in the water (once it has returned to room temperature). The heater would be connected to a joule meter, which measures the number of joules of energy supplied to the calorimeter. We would keep the heater on until the apparatus has reached the same temperature as in the experiment wit the alcohol. The joule meter would tell us how much energy was needed to produce the same temperature rise.
Results table
Starting temperature
Final temperature
Starting mass of burner + methanol
Final mass of burner + methanol
19.4

C
34.6

C
28.44g
27.42g
Mass of methanol burned
Starting reading on joule meter
Final reading on joule meter
1.02g
7030J
29 020J
Difference 21 990 J
The joule meter readings tells us that it takes 21 990 J to increase the temperature of the calorimeter from 19.4

C to 34.6

C. Therefore we know that 1.02 g of methanol released 21 990 J when burned.
Given that the mass of 1 mole of methanol is 32 g, we can say that:
energy released when 1 mol of methanol is burned = 21 990 J x 32g/1.02g = 689 882J
Thus, the heat of combustion methanol is nearly –690 kJ mol -1 .
The result obtained from a much more accurate experiment is that the standard heat of combustion is
–
726.3 kJ mol -1 .
Questions
Clearly there is a considerable error in this experiment. Here is a list of possible sources of error. For each one, say, with a reasons, whether you think the error would tend to increase (=closer to -726.3), decrease, or make no difference to the measured value for the heat of combustion.
1. The cover is left off the burner before it is weighed for the first time.
2. The cover is left off the burner after it is weighed for the first time.
3. The cover is left off the burner after it is used in the experiment, but before it is weighed for the last time.
4. The water is not stirred properly
5. The thermometer is placed so that its bulb touches the bottom of the glass calorimeter.
6. The amount of air drawn through the copper spiral is too low. (when this occurs you often see soot appear on the glass).
7. The flow of air through the copper spiral is very fast.