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# 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 grea

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Introduction

Introduction on enthalpy 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. Figure 1: 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. ...read more.

Middle

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 Methanol Ethanol propan-1-ol Butan-1-ol Pentan-1-ol Enthalpy change of combustion (kJmol-1) 379.80 1071.23 1332.62 1715.82 2110.04 Enthalpy change of combustion = (1/moles of alcohol burned) x enthalpy released during the ex. True results of alcohols that should be obtained Methanol Ethanol propan-1-ol Butan-1-ol Pentan-1-ol Enthalpy change of combustion (kJmol-1) -890.30 1559.70 2219.70 2876.50 3509.10 From the results I obtained I can make a conclusion that the results were not very close to the true, this is because the experiment could not done under standard conditions like the other and therefore because of that some heat was lost to the surrounding. 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) ...read more.

Conclusion

propan-1-ol Butan-1-ol Pentan-1-ol Final mass of spirit burner / g 197.41 188.16 180.09 212.08 189.08 Percentage error / % 0.003 0.003 0.003 0.002 0.003 Looking at the results above for the percentage errors, I can conclude that the balance did not effect the results I obtained as it gave me a small percentage error that was less than 1%. This ensures me that the measurements of the spirit burners that I obtained using the weighing balance where very accurate. 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. ...read more.

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