Calorimetry answer 2

The heat capacity of a combustion calorimeter was measured by burning some ethanol in it until a temperature rise of 10.0 ºC was achieved. The mass of the burner fell from 13.539 g to 12.121 g. The calorimeter was now heated up using a hexan-1-ol burner. The mass of the burner fell from 12.829 g to 11.717 g and the temperature rose from 24.6 ºC to 33.7 ºC. Calculate the enthalpy of combustion of hexan-1-ol.

For a challenge, try to answer the next question. What effect would evaporation of alcohols before and after combustion have on your result?

DH_c (ethanol) = - 1367.3 kJ mol^-1

Mass of ethanol burnt = 13.539 - 12.121 = 1.418 g

C₂H₅OH = 46

\ number of moles of ethanol burnt = 1.418/46 = 0.0308 mol

DH_c = - 1367.3 kJ mol^-1

\energy released = 0.0308 × 1367.3 = 42.1 kJ

Temperature rise = 10.0 K

\ heat capacity of calorimeter = 42.1/10.0 = 4.21 kJ K^-1

Temperature rise in hexan-1-ol experiment = 33.7 - 24.6 = 9.1 ºC

\ energy released = 4.21 × 9.1 = 38.4 kJ

Mass of hexan-1-ol = 12.829 - 11.717 = 1.112 g

C₆H₁₃OH = 102

\ number of moles of hexan-1-ol burnt = 1.112/102 = 0.0109 mol

\ enthalpy of combustion of hexan-1-ol = - 38.4/0.0109 = - 3500 kJ mol^-1

This is less than the accepted value of - 3983.8 kJ mol^-1, largely due to incomplete combustion of the hexan-1-ol.

Evaporation occurs with both alcohols between the weighings and combustion. In both cases this will mean that our measured masses are greater than the masses which actually burnt. In the case of the ethanol, the greater mass will give a greater energy value in the calculation, and will therefore suggest that the calorimeter has a greater heat capacity than it actually has. This will lead to a larger final answer. In the case of the hexan-1-ol, the greater mass will give a larger number of moles which when divided into the heat released will give a smaller final answer. These two effects will, therefore, cancel out to some extent. However, as the smaller alcohol, ethanol, is more volatile we would expect more evaporation. If this is the bigger of the two effects we would expect our final answer to have been increased by the effect of evaporation. Clearly, looking at the final answer, any such effect has been far outweighed by the incomplete combustion of the hexan-1-ol.