Comparing the enthalpy change of combustion of alcohols down a homologous series.

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Comparing the enthalpy change of combustion of alcohols down a homologous series


The aim of this investigation is to discover what effect of increasing the length of the carbon chain in an alcohol has on the energy produced. To investigate this, the enthalpy of combustion will be calculated, by burning the fuels and heating a measured volume of water.

Preliminary Investigation

A preliminary investigation was carried out in order to show any weaknesses in the method that is used for this investigation.

Diagram of Apparatus


The apparatus was assembled as shown in the diagram above. 100cm3 of cold water was measured out into a calorimeter. The temperature of the water was recorded. The spirit burner was weighed and recorded, then placed under the calorimeter. The wick was then lit and left until the water had reached a temperature 15ºC higher than the original recorded temperature. When this temperature was reached the flame was put out and the spirit burner reweighed and the final weight recorded.



Combustion occurs due to oxidation; in this case the oxidation is of alcohols. When combustion of alcohols occurs an exothermic reaction takes place. An exothermic reaction releases energy in the form of mainly heat but also light. Exothermic reactions mean that the energy produced in forming new bonds exceeds the energy required to break the bonds originally. Combustion of alcohols produces water and carbon dioxide. With this knowledge the standard enthalpy of combustion can be calculated for each fuel used. In this investigation the first five alcohols of the group are being investigated. These are methanol, ethanol, propanol, butanol and pentanol. In an alcohol there is an    O-H functional group. The size of the alcohol increases due to an additional CH2 on the chain

Measurements of the energy transferred during chemical reaction must be made under controlled conditions. Enthalpy is the total energy content of the reacting materials, in this case the alcohols, it is given the symbol H. Enthalpy itself cannot be measured, but it is possible to measure enthalpy changes, when energy is transferred to or from a reaction system and changes from one enthalpy to another. The standard enthalpy of combustion is the enthalpy change when one mole of an element or compound reacts completely with oxygen, under standard conditions.

The law of conservation of energy states that energy can neither be created nor destroyed.

Hess’s law of constant heat summation states that the energy change in converting reactants, A + B, to products X + Y, is the same regardless of the route by which chemicals change occurs provided the initial and final conditions are the same. Ref. Chemistry                                                                                                               in context

This is shown below

                A + B                                                X + Y

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A + B + X + Y

The average bond enthalpy is the average energy needed to break one mole of covalent bonds. It is given the symbol E and is measured in Kjmol-1. These values are average because the exact value depends on the compound the bonds are in.

The use of the above enthalpy cycles will help to predict the results of the investigation.

The first alcohol of the group will be used as a worked example.


CH3OH(l) + 1½O2 (g)                                 CO2 (g) + ...

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