Production of Materials > 3. Renewable Ethanol >
Define the molar heat of combustion of a compound and calculate the value for ethanol from first-hand data
- Molar Heat of Combustion: The heat liberated when one mole of a certain compound undergoes complete combustion with oxygen at a constant pressure of exactly one atmosphere (100 kPa) and at 25°C with the final products being carbon dioxide gas and liquid water.
- Heats of combustion are quoted as positive numbers while the enthalpy changes of combustion reactions (ΔH) are quoted as negative numbers, as combustion reactions are always exothermic.
- Heats of combustion are typically stated in kilojoules per mole (kJ/mol. or kJ mol.-1).
- The accepted value for the molar heat of combustion of ethanol is 1360 kJ mol.-1.
- The following steps allow the calculation of an experimental value for the molar heat of combustion of ethanol:
- Measure and record the mass of a burner containing ethanol.
- Measure 100 mL of water into a beaker and measure the temperature of the water.
- Place the beaker of water directly above the burner and light it.
- Allow the burner to heat the water for one minute, then extinguish it.
- Immediately measure and record the mass of the burner and the temperature of the water.
- Calculate the change in mass and the change in temperature.
- Substitute the experimental results into the formula below to determine the enthalpy change:
ΔH = -mCΔT
where:
- ΔH = enthalpy change in joules
- m = mass of water
- C = thermal capacity (4.2 for water)
- ΔT = change in temperature in degrees Celsius
- Determine the number of moles of ethanol combusted (number of moles = mass/FM), and divide the enthalpy change in kilojoules by this number to determine the experimental value of the molar heat of combustion of ethanol in kilojoules per mole.
- Due to heat loss to surroundings, the experimental value of the molar heat of combustion of ethanol will be significantly lower than the accepted value.
- If the same experiment was conducted for another alkanol, such as methanol, and the difference between the experimental value and accepted value was found, this difference could be used to calibrate the experimental results for ethanol and produce a more accurate experimental result.
- The process described above can be applied to any alkanol, and can be modified slightly in order to find the heat of combustion in kilojoules per gram instead of kilojoules per mole.