The enthalpies of combustion

Standard enthalpy change of combustion is the energy change that occurs when 1 mol of fuel is completely burnt in oxygen under standard conditions. However, it is impossible for substances to be burnt under standard conditions. The enthalpies of combustion are different depending on the number of carbons and hydrogens in each substance. For example, the enthalpy combustion for methane is -890 kJ mol-1, however the enthalpy change of combustion for ethane or propane will be greater because more bonds are involved and therefore more bonds are broken and made.

All combustion reactions are exothermic because energy is given out to make bonds. Exothermic reaction Before you start doing calculations for enthalpy change of combustion you firstly need to balance the equation.

For example:

Methane CH4 (g) + 2O2 (g) 2H2O (g) + CO2 (g)

Also it is important to note when making a combustion reaction balanced equation that the state symbols for all the substances are gaseous even water. Moreover, it is only in a combustion reaction that you use halves in an oxygen molecule.

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An example for octane is: C8H12 (g) + 12.5O2 (g) 8CO2 (g) + 9H2O (g)

Method for the enthalpy of combustion for various alcohols

1. Set up a tri pod and place tiles around it.
2. Fill up a metal calorimeter with 200 cm3 of water and place on the tri pod. Also, place a thermometer on the container.
3. Record the reading on the thermometer of the temperature of the water.
4. Weigh the spirit burner containing the alcohol you are going to use and again record the reading.
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5. Place the spirit burner containing the alcohol under the tri pod and quickly as possible take the lid off and light a match on it.
6. Wait till you will see a 15 degree increase on the thermometer.
7. When you see a 15 degrees increase, place the lid back on the alcohol and stir the water well with the thermometer until you see there is no change in temperature.
8. Weigh the alcohol and record the reading. Also record the reading of the temperature after it has increased.
9. Repeat steps 1-8 for all different alcohols. Results obtained from experiment.

While I was carrying the experiment I realised that a black substance began to form underneath the colorimeter (container where the water was put in and heated up) which was carbon, meaning there was incomplete combustion. This therefore effected the results I obtained because the combustion experiment was not complete. Evaluation Experimental errors In any experiment, errors are made which are unavoidable. We can work out the errors of the equipments that were used in the experiment to find out which has the most effect.

Procedural errors

A thermometer was placed on the colorimeter and when reached to 500c and recorded the highest temperature by siring until there was no increase in temperature. * Keep lid on spirit burner to endure the alcohol did not evaporate to obtain a accurate measurement of the mass of alcohol combusted.  Draft exclusion made using insulating tiles. This is so less heat and temperate near the spirit burner that is lighted is relatively kept constant and not effected by air coming from window or heat from radiator.