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Investigating the heat of combustion of a series of Alcohols

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Introduction

Investigating the heat of combustion of a series of Alcohols

Background

Many countries in the world burn alcohols as a source of fuel. They do this because it is a good clean source of energy and heat.

In this experiment I will be trying to find out the amount of energy produced when different alcohols are burned.

I will be using five different alcohols for the experiment.  These are Ethanol, Propanol, Butanol, Pentanol and Hexanol.  Each one has a different number of carbon atoms arranged in the form of a chain.

Prediction

I predict that as the length of the chain of the alcohol increases then so will the energy given off during combustion.  More energy will be given out in the form of heat as shown in the calculations below.  This also means that much more heat will be lost to the environment.

This is usually recorded as the heat of combustion of a molecule.  A bond energy is the energy taken to make or break a bond between two atoms.

Ethanol

C2H5OH = 46g

Ethanol        +        Oxygen                Water                +        Carbon Dioxide

C2H5OH        +        3O2                        3H2O                +        2CO2

               Bond

     Energies

C – H = 5 X 435 = 2175                        H – O = 6 X 464 = 2784

C – O = 1 X 336 = 336                        C – O = 8 X 336 = 2688

O – H = 1 X 464 = 464

C – C = 1 X 346 = 346

O = O = 3 X 497 = 1491

Total Energy in = 4812 kJ                        Total Energy out = 5472 kJ

Energy Released = 4812 – 5472 = -660 kJ

When a bond breaks, energy is taken in so that it has enough energy to break the bond.

...read more.

Middle

O = O = 15 X 497 = 7455

Total Energy in = 21393 kJ                        Total Energy out = 24576 kJ

Energy Released = 21393 – 24576 = -3183 kJ

-3183 = -1591.5 kJ

    2

Hexanol

C6H13OH = 102g

Hexanol        +        Oxygen                Water                +        Carbon Dioxide

C6H13OH        +        9O2                        7H2O                +        6CO2

C – H = 13 X 435 = 5655                        H – O = 14 X 464 = 6496

C – O = 1 X 336 = 336                        C – O = 24 X 336 = 8064

O – H = 1 X 464 = 464

C – C = 5 X 346 = 1730

O = O = 9 X 497 = 4473

Total Energy in = 12658 kJ                        Total Energy out = 14560 kJ

Energy Released = 12658 – 14560 = -1902 kJ

These results are negative because there is more energy given off in the reaction than is taken in.  Therefore energy is lost to the environment.  This creates a negative amount of energy left in the substance at the end of the experiment. The results on the graph are positive because I changed it from the negative to a positive.  This was possible because I assumed that theoretically all of the energy from the burning alcohol is being transferred to the water.  This makes it a lot easier to compare the theoretical results with the practical results.

As preliminary work to this experiment I had already done the experiment once with Ethanol.  I did, therefore, know how to do the experiment safely and properly.

Method

Equipment needed: 0.5g of Ethanol, Butanol, Propanol, Pentanol, Hexanol; Lighted Splint; Crucible; Clamp stand; Clasp; Thermometer; Metal Can; water.

  1. Set up the experiment as shown in the diagram below.
  1. Measure out 0.5g of Ethanol and place this into a crucible.
...read more.

Conclusion

Weight of one mole of alcohol (g)

Predicted heat loss when burnt (kJ/mole)

Actual heat lost when burnt (kJ/mole)

Ethanol

2

46

660

386.4

Propanol

3

60

970.5

504

Butanol

4

74

1281

621.6

Pentanol

5

88

1591.5

813.12

Hexanol

6

102

1902

942.48

The table shows that there is a definite link between the predicted and actual amount of heat lost by the burning of the alcohols.

Evaluation

The experiment went very well, as there were very few anomalous results.

The experiment could be easily improved because during the experiment, a lot of heat was lost to the environment.  This meant that the water didn’t heat up as much as it should have done.  This is the reason why the results for the experiment are shown as giving off less energy than predicted.

This heat loss could be minimised through the use of a bomb calorimeter.  This would trap all of the heat produced inside a container, which is inside a water bath.  The heat is then passed through a copper coil, which is also suspended in the water.  This creates the maximum amount of contact with the water possible so more heat is transferred.  The apparatus also contains a stirrer so that the water heated up by the experiment doesn’t stay around the container and copper coil.  This also helps to maximise the contact between the water and the energy from the experiment.

The evidence shown in the experiment shows that my prediction that the bond energies needed and given will increase as the length of the chain of carbons increases.  The conclusions and the graph I made from the experiment also support my prediction.

Bibliography

  1. Nuffield Co-ordinated Sciences - Chemistry

...read more.

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