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Investigating the Combustion of Alcohols

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Introduction

CHEMISTRY: INVESTIGATING THE COMBUSTION OF ALCOHOLS PLANNING Aim: �To investigate the trend in enthalpy change of combustion of a homologous series - the alcohols: methanol, ethanol, propan-1-ol, butan-1-ol, pentan-1-ol by burning in a combustion calorimeter. Name Methanol Ethanol Propan-1-ol Butan-1-ol Pentan-1-ol Structural Formula CH3OH CH3CH2OH CH3CH2CH2OH CH3CH2CH2CH2OH CH3CH2CH2CH2CH2OH Displayed Formula �To find out if the -CH2- group between successive alcohols makes a specific contribution to the enthalpy change of combustion between each alcohol. One way of attempting to test these aims would be to burn the alcohols and measure the enthalpy change per mole of each. Introduction Alcohols are used as fuels and when fuels burn they give out energy. The amount of energy given out by burning one mole of a fuel in excess oxygen is known as the enthalpy change of combustion. The enthalpy change of combustion of a substance, ?HC is defined as the 'enthalpy change that occurs when one mole of substance undergoes complete combustion' (i.e. in a plentiful supply of oxygen) as quoted in the Nuffield Advanced Chemistry Students' Book. In simpler terms, the chemical molecules involved in combustion reactions have energy stored in their bonds. This energy is known as enthalpy and is given the symbol 'H'. The movement of heat energy that takes place during the combustion reaction, from reactants to surroundings, is known as enthalpy change, ?H. The enthalpy change for a combustion reaction is shown overleaf. Diagram: Enthalpy Level diagram for an exothermic reaction: Combustion of methanol. For an exothermic reaction, the ?H is negative because energy has been lost from the chemical reactants to the surroundings. The products are at a lower energy level than the reactants. The enthalpy change is shown alongside the equation. For a compound containing carbon, complete combustion means the conversion of the whole of the carbon in the compound, in this case methanol, to carbon dioxide as shown in the following equation: During combustion, the different bonds between the atoms in the alcohol molecule are broken. ...read more.

Middle

Note: To change the units from Joules (J) to Kilojoules (kJ), divide the answer to the above equation by 1000. Energy transferred (J) = mass of water (g) � specific heat capacity � temperature change Q (J) = M/g � -4.2 J g-1 K-1 � ?T �C Q (J) = 75 � -4.2 � 25 = -7875 J The number of moles of methanol burned is calculated by the following steps: Mass of 1 mol of methanol, CH3OH = 12 + (4�1) + 16 = 32g Number of moles of methanol burned = mass burned mass of 1 mol =1.06g = 0.031 mol 32 Enthalpy Change of Combustion of Methanol = Energy transferred ?HC Number of moles = -7875 J 0.031mol = -254032.2581 J = -254032.2581 / 1000 = - 254.0 kJmol-1 So in this experiment, I have calculated that the enthalpy change for the combustion of methanol is -254.0 kJmol-1. The theoretical value for the standard enthalpy change of combustion of methanol is -726 kJmol-1. Clearly, this value is much greater than that obtained in this experiment. This difference is due to the large heat losses that occur in this experiment. For example, heat losses to the surroundings are quite large even though heat shields are used. Also heat energy from the flame heats up the copper container even though it is a good conductor of heat. Other methods by which experimental errors might be reduced are discussed in the evaluation section. 2) Ethanol - C2H5OH is calculated in the same way as methanol Mass /g Number of Repeats 1 2 3 Original Mass/g 118.34 Original Mass/g 117.40 Original Mass/g 116.45 Final Mass/g 117.40 Final Mass/g 116.45 Final Mass/g 115.49 Difference in Mass/g 0.94 Difference in Mass/g 0.95 Difference in Mass/g 0.96 Average loss in mass of ethanol = 0.94 + 0.95 + 0.96 = 0.95 g 3 This means that in three trials, the average mass of ethanol burned was 0.95g. ...read more.

Conclusion

This is because if the flame is broader at the base, much of the alcohol is not completely burned and a deposit of carbon may be seen on the sides (which was evident during the experiment). This also prevents efficient energy transfer. * When the hot spirit burner is extinguished, there is considerable evaporation. To reduce this effect, a metal cap could be placed over the wick to extinguish the flame and to trap any alcohol vapour so there is no inaccuracy in the mass of the alcohol when it is re-weighed at the end. * Use a fume cupboard instead of heat proof mats as heat shields. I think that the fume cupboard would greatly reduce the effect of draught and heat loss. * Although the copper combustion calorimeter used in this experiment is insulated, some heat will be lost from it. A bomb calorimeter could be used as an alternative. The apparatus is specially designed to avoid heat losses by completely surrounding the 'bomb' with water. Diagram: A Bomb Calorimeter * The experiment could be caried out in an oxygen rich area to prevent incomplete combustion. For the investigation to be furthered, some factors could be altered to see their affect on the enthalpy changes of the alcohols. These altered factors could be the same as to those used in this investigation: the distance from the wick and metal calorimeter, the amount of water in the metal calorimeter and lastly the temperature rise in which the water in the calorimeter should rise by. All other factors not being measured should be carefully controlled while one factor is being studied. One other way to further the investigation could be to look into the enthalpy changes for other homologous series such as alkanes and alkenes. Once results from the enthalpy changes for any of these other compounds have been established, they can be compared to those of the results from the combustion of alcohols and any similar trends can be noted down easily. ...read more.

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