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To find a relationship between the molecular size of different alcohols and the associated energy change involved in the reaction.

Extracts from this document...


Steven John 11c                


The aim of the investigation was to find a relationship between the molecular size of different alcohols and the associated energy change involved in the reaction.

A preliminary experiment had been carried out already and errors were noted accordingly and were adjusted in this experiment where possible.

This investigation looked into the energy changes when different alcohols were combusted, this is the reaction of the alcohol with oxygen in the surrounding air. The alcohols are a homologous series of compounds that contain –OH as the functional group (a group of atoms in a structure that determines the characteristic reactions of a compound). For this experiment, only methanol, ethanol, propanol, butanol and pentanol will be used.


Molecular formula, CnH2n+1OH

Boiling point (ºC)
















The structures of alcohols

Here’s where the alcohol structures go, sorry I couldn’t send them!

In general,

alcohol + oxygen  carbon dioxide + water

E.g.                 ethanol + oxygen  carbon dioxide + water

                        C2H5OH(l) + 3O2(g) 2CO2(g) +3H2O(l)


The larger the molecular size, i.e. the more carbon atoms it has, the larger the amount of energy given out.

Some chemical reactions are capable of releasing a lot of energy; usually it is through heat. The combustion of alcohols is a good example, but it is known that they do give off different amounts of heat.

When an alcohol is combusted, it reacts with the oxygen in the air and the products are carbon dioxide and water:

alcohol + oxygen carbon dioxide + water

During this reaction, as with all others, bonds are first broken and then new bonds are made. In alcohol, hydrogen atoms are covalently bonded to hydrogen atoms.

...read more.


Specific heat capacities: it is understood that different materials are able to store different amounts of energy in a period of time, hence we use the same can throughout the experiment.

Energy released: to allow suitable and accurate comparison between results of the five different alcohols, and to allow an accurate relationship to be produced, we will convert all taken results into a value of total energy released.


To produce results with the maximum accuracy suitable, we repeated the experiment twice in order that we could find an average between the results and hence allowing us to identify and eliminate the anomalous results.

We used the most accurate measuring equipment available, including digital balance to 2 decimal places and a thermometer to the nearest degree.

We also learnt from the causes of errors in the preliminary and adjusted our method accordingly; i.e. the use of draft excludes and taking into accounts the energy stored in the can.

We kept the variable as constant as possible by using the same equipment throughout, based on the concept that different materials have different specific heat capacities and so store different amounts of energy; even if energy is lost we can either account for it or know that it is a constant amount each time, hence still allowing us to make an accurate comparison between results.

We also took all measurements and calculations to 2 decimal places where possible, which we thought to be a suitable accuracy for school laboratory experiments and for finding an accurate relationship. This also was suited to the equipment we chose to use.


Below is a lost op apparatus used in the experiment:

  • Measuring cylinder – to measure out amount of water to be heated 100ml.
  • Fuels (methanol, ethanol, propanol, butanol and pentanol) – used to heat water in can. Stored in spirit burners and weighed with the lid.
  • Beverage can – used to store water, either made of aluminium or steel (allows conduction of heat to the water; has only a small hole to prevent heat loss).
  • Draft excluder – to prevent heat loss from naked flame to environment.
  • Thermometer – to check temperature of the water.
  • Heatproof mat – to conduct experiment upon.
  • Clamp and retort stand – used to hold can above flame.
  • Ruler – used to measure distance between bottom of can and tip of flame.
  • Electronic balance – used to measure mass change of fuel, i.e. amount combusted.
...read more.


The evidence is reliable in showing the sort of trend that would be produced. The anomalous results were also very small and still show the trend clearly and so the results are accurate. The difference in the actual result and the predicted results can also be fully accounted for. The actual results are also more realistic in terms of energy transfer as it takes into account the energy loss. The obtained evidence is sufficient to support a firm conclusion that as the molecular size of the alcohol increases so does the amount of energy released. This is because the results show this trend very clearly and are similar to the predicted results. The anomalies are also not far from the line of best fit and so support the trend making them reliable. Even though the actual results differ from those that were predicted, it can be explained by the fact that energy is lost to the environment.

Further work for this investigation would include testing to see the rate at which energy is produced; how long it takes for each alcohol to heat the water by a certain amount. My prediction would be that the alcohols with the larger molecules would take less time because they have more bonds and so more energy is released in a certain amount of time, and so it would heat the water faster. Additional evidence for the conclusion could also be obtained by continuing the experiment with more alcohols with more carbons and so allowing us to gain a more detailed trend in the relationship. Also by replacing the calorimeter with a thermocouple would allow us to see a more accurate trend and find other factors apart from heat loss that may cause anomalous results.

...read more.

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