Using Calorimetry

 
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Aim

To determine the molar heat of combustion of ethanol.

Apparatus

A reaction calorimeter such as the one pictured below was used in this experiment.


The reaction calorimeter used in this procedure.

Experimental

The water vessel was charged with water (105.00 g) and left to come to the ambient temperature for 15 min. The temperature of the water was measured (Tinit) using a thermometer.

Ethanol (1.00 g) was added to the reaction vessel and ignited using a match. The temperature of the water in the water vessel was monitored using the thermometer and the maximum temperature (Tmax) attained was recorded.

Results

T /°C
Tinit21
Tmax89

change in T68

Discussion

The molecular weight of ethanol is 46.0 g/mol, thus the amount of ethanol added was 0.0217 mol. This was able to procduce a temperature change in the water of 68 °C (which is also a temperature change of 68 K).

The specific heat of water is the amount of heat required to raise the temperature of 1 g of water by 1 K at atmospheric pressure. This value is pretty constant across the range of temperatures at which water is liquid, and is 4.184 J K-1 g-1. The amount of energy produced may thus be calculated:

energy produced = 4.184 J K-1 g-1 × 68 K × 105 g = 29.87 kJ
= 29.87 kJ

The molar heat of combustion is thus determined by dividing the amount of heat produced in the reaction by the number of moles of ethanol that was reacted:

molar heat of combustion for ethanol = 29.87 kJ &247; 0.0217 mol
= 1374 kJ/mol
= 1.4 × 103 kJ/mol (2 sig. fig.)

Conclusion

The molar heat of combustion for ethanol may be calculated from a simple calorimetric expriment. In the experiment reported here, the molar heat of combustion was found to be 1.4 × 103 kJ/mol.

This value happens to be in reasonable agreement with the literature value for this property of 1409.4 kJ/mol; however, the reproducibility of this experiment has not been assessed here, hence the precision of the obtained result cannot be discussed further.

Note that this assumes all the energy goes into heating up the water. Some, of course, goes into heating the calorimeter, and if we were really serious about this, we'd take that into account as well, by calibrating our calorimeter. Then there are the losses to the atmosphere...)