Comparing the enthalpy change of combustion

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Sophie Catt

Comparing the enthalpy changes of combustion of different alcohols

The aim of this experiment is to investigate how the length of an alcohol molecule and the position of its OH group affect its enthalpy change of combustion. This will be done using the comparison method, using a known value for the enthalpy change of combustion for one fuel (here, propan-1-ol) and replicating the experiment for each alcohol, then accurately comparing the results.

The 4 alcohols that will be compared to propan-1-ol (CH3CH2CH2OH) are:

  • methanol (CH3OH)
  • ethanol (CH3CH2OH)
  • propan-2-ol (CH3CH(OH)CH3)
  • butan-1-ol (CH3CH2CH2CH2OH)

The propan-2-ol differs from propan-1-ol because its OH group is positioned on the 2nd carbon atom in the chain.

Combustion reactions involve breaking the intra-molecular bonds (bonds between atoms, within the molecule) of a fuel, then making new bonds with oxygen. The formula for the combustion reaction is:

Alcohol + Oxygen  Carbon Dioxide + Water

e.g. CH3OH + 1½O2  CO2 + 2H2O

 

Breaking bonds takes energy in from the surroundings, and making bonds gives energy back out. The enthalpy change is the net energy transfer of the reaction; it is negative if the reaction is exothermic, like combustion, and positive if the reaction is endothermic.

Variables

The independent variable in this experiment is the alcohol used. The dependant variable is the enthalpy change of combustion.

The variables to be controlled are the equipment, the mass of water, and the initial temperature of water. If the mass of water was greater for one experiment, it would require more energy to heat it by 10K, so more alcohol would be used up, so the value for the enthalpy change would be smaller. The temperature of the water may affect the enthalpy change, as if the water was at a higher temperature to begin with, heat would be lost from it to the surroundings more quickly, so more alcohol would be required to heat it by 10K, so the enthalpy change would be too small. Keeping the equipment the same controls the amount of energy lost to the surroundings and equipment itself, so it is the same energy loss in each experiment.

Prediction

In a larger molecule, there are more bonds to be broken and made, so the enthalpy change of combustion will increase in comparison with a shorter molecule. Each extra carbon in the molecule introduces one more C-C bond, 2 more C-H bonds, and 2 more O=O bonds to be broken, and 2 extra C=O bonds and 2 more O-H bonds to be made.

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The extra enthalpy change per extra carbon atom in the molecule is 1920-2538=-618. This shows how as the molecule gets longer, the enthalpy change of combustion will increase.

Both of the propanol isomers have the same bonds to break and make, but the position of the OH group affects the stability of the molecule due to dipoles. When the OH group is at the end of the molecule, it is more unstable than when it is bonded to the 2nd carbon atom. An unstable molecule has more energy contained in its bonds than a stable molecule, so will ...

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