Enthalpy of combustion
In combustion reactions, some substances will release more energy than others.
Enthalpies of combustion can be used to compare which fuels or substances release the most energy when they are burned. They can be calculated using a bomb calorimeter.
A simplified version of this can be set up in the lab with a spirit burner and a metal can as shown below.
Several measurements must be taken:
- the starting temperature of the water
- the final temperature of the water
- the mass of the burner before the experiment
- the mass of the burner after heating
- the temperature change
Fuel is burned and the temperature increase measured. The mass of fuel corresponding to the temperature increase can be used to calculate the enthalpy change of the reaction, which in turn can be used to calculate the enthalpy of combustion of that fuel.
The enthalpy of combustion of a substance is defined as the heat energy given out when one mole of a substance burns completely in oxygen.
Combustion reactions are exothermic so the value for the enthalpy change (\(\Delta H\)) is always negative.
Question
Ethanol (C2H5OH) was placed in a spirit burner and used to heat 200 cm3 of water in a copper can. When the temperature of the water had increased by 5藲C, the mass of the burner and ethanol had decreased by 0.36 g.
Calculate the enthalpy of combustion of ethanol.
Firstly, we know that 0.36 g of ethanol burned, so we can convert this into a number of moles.
Secondly, we can use \(\Delta H=cm\Delta T\) to calculate the enthalpy change in the experiment described in the question (ie when 0.008 moles of ethanol is burned).
\(\Delta H=cm\Delta T\) = 4.18 x 0.2 x 5
= 4.18 kJ
Lastly, we can use the enthalpy change for the experiment in the question to calculate the enthalpy of combustion (ie when one mole of ethanol is burned).
\(0.008\,\,moles\,\,ethanol = 4.18kJ\)
\(1\,\,mole\,\,ethanol = \frac{1}{0.008} \times 4.18\)
\(= -522.5\,\, kJmol^{-1}\)
Remember that the value obtained for the enthalpy of combustion must be negative as combustion reactions are always exothermic (energy is released).