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.
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.
An example for octane is: C8H12 (g) + 12.5O2 (g) 8CO2 (g) + 9H2O (g)
Method for the enthalpy of combustion for various alcohols
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.
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.
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