Energetics

The method to determine the enthalpy of combustion of a fuel such as ethanol or methanol is as follows:

- accurately measure 100cm^3 of water into a calorimeter/beaker. (100cm^3 of water is the same as 100g of water as the density of water is 1g/cm^3).

- weigh a spirit burner containing the liquid to be burnt.

- measure the initial temperature of the water using a thermometer (T1).

- use the spirit burner to heat the water.

-stop heating when there is a reasonable temperature rise (15˚c). Stir and measure the final temperature (T2) of the water using a thermometer.

-reweigh the spirit burner.

- calculate the temperature change, and the heat energy change.

- calculate the mass of fuel used, and then the moles of fuel used.

- calculate the energy change per mole of fuel used.

The value for the enthalpy of combustion determined experimentally is frequently lower than the value found in data books. Reasons for this are:

- heat losses to the surroundings from the spirit burner, wick and calorimeter.

- loss of fuel from the wick or burner, by evaporation.

- loss of water by evaporation.

- incomplete combustion of fuel.

- heat used to raise the temperature of the calorimeter.

- the reaction is unlikely to occur under standard condiitons.