Enthalpy Changes of Combustion of Different Alcohols

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Comparing the Enthalpy Changes of Combustion of Different Alcohols


Combustion of alcohols is exothermic; energy is given out. Salter’s Chemical storylines says, “ Different fuels have different enthalpy changes”. I will investigate how the carbon chain length of the alcohol affects its enthalpy of combustion.

Preliminary Work

Chemistry for you page 184 says, “breaking bonds requires energy, it is endothermic “. It also says “ making new bonds gives out energy it is exothermic”. The difference between the two determines whether the reaction is exothermic or endothermic. It also determines how much the total enthalpy of combustion is this can be shown in energy level diagrams.

Diagram 1 is an energy level diagram of an exothermic reaction. It shows that more energy is given out making bonds than is used breaking them. Combustion is an exothermic reaction.

Diagram 2 is an energy level diagram of an endothermic reaction. It shows that more energy is used breaking bonds than is used to make them.

Number and Range

I plan to use five alcohols in this experiment. The alcohols I will use are; methanol, ethanol, propanol, butanol and octanol.

 It is important that I use five alcohols so that there are enough results to spot any patterns. The experiments are to be done three times so that an average can be taken this will make the results more reliable because any anomalous results will have less affect on the overall findings.

The first four of the alcohols listed above have carbon chains with 1,2,3 and 4

Carbons. I chose to use these alcohols with an increasing number of carbon atoms in their carbon chains. The reason for this is I can investigate whether the change in the enthalpy of combustion increases by the same amount for every extra carbon in the carbon chain. I also decided to use octanol it has

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a carbon chain length of eight so it has twice as many carbons as butanol. So I can see if the enthalpy of combustion is proportional to the number of carbons in the alcohol. I would expect the enthalpy value of octanol to be roughly twice that of butanol.


The variables in this investigation are:

  • Volume of water used
  • Height of cooper can above flame
  • Type of can
  • Temperature the water is heated to
  • Draft
  • Type of alcohol

Fair Test

To make this investigation a fair test all of the variables except the ...

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