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Energy of different alcohols.

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Introduction

Energy of different alcohols

The measurements recorded were:

  • Volume of water in the can
  • Starting temperature of water
  • Final temperature of water
  • Change in temperature of water
  • Start mass of alcohol and spirit burner
  • Final mass of alcohol and spirit burner
  • Change in mass of alcohol and spirit burner.

Using the mass of water used (1 cm3 water has a mass 1g), temperature rise of the water and specific heat capacity of water (4.2 J raises the temperature of 1g of water by 1°C) the energy transferred from burning the fuel to the water was calculated.  This was then converted energy released per mole of fuel (kJ/mol) using the mass of the fuel used and the formula mass of the alcohol.  These are the figures given in Table 1.  The figures from five groups were averaged.

Table 2 Formula masses of the alcohols used

Alcohol

Formula mass

Methanol

32

Ethanol

46

Propanol

60

Butanol

74

Pentanol

88

Hexanol

102

As the alcohol molecule being burnt gets bigger the energy released when it is burnt goes up (Graph 1).

The line of best fit on graph 2 shows that the number of carbon atoms in the alcohol is proportional to the energy released on its combustion.

Breaking and making bonds

To break bonds energy is needed.

...read more.

Middle

3OH + 1.5O2                    CO2 + 2H2O

Ethanol

C2H5OH + 3O2                    2CO2 + 3H2O

Propanol

C3H7OH + 4.5O2                  3CO2 + 4H2O

Butanol

C4H9OH + 6O2                     4CO2 + 5H2O

Pentanol

C5H11OH + 7.5O2                  5CO2 + 6H2O

Hexanol

C6H13OH + 9O2                   6CO2 + 7H2O

Table 4 Theoretical energy released when alcohols burnt

Calculated from bond energies

Alcohol

Energy (kJ/mol)

Methanol

658

Ethanol

1276

Propanol

1844

Butanol

2515

Pentanol

3130

Hexanol

3748

Table 4 shows that the theoretical energy released from alcohol combustion increases as the size of the alcohol molecule increases.  This is because the bigger molecules have more bonds to be broken and more bonds are made in the products of the reaction (water and carbon dioxide).  The extra bonds that are broken in the alcohol with more carbon in it need more energy but the products release even more energy as they formed therefore the overall energy released by the reaction (combustion) goes up.  The results of the experiment showed the same pattern but the results were much lower due to inefficient energy transfer to the water (Graph 3).  Table 5 shows that a

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Conclusion

  • The wick varied in length.

The wick could have been adjusted to the same length before each alcohol was burnt.

  • Heat from the flame could have escaped out to the sides of the spirit burner.

Draught excluders could have been put up around the spirit burner and can.

  • The can was made of copper which meant that it absorbed and transferred heat well to the water but it also lost heat to the air.

The can could have been insulated at the sides.  The can already had a lid.

Although the results are low they do show a pattern of increasing energy transfer to water with increasing carbon content of the alcohol, suggesting some reliability.  If the results were completely unreliable they would not show any pattern.  The trend in a decreasing percentage energy transfer to the water compared to the theoretical with increasing size of the alcohol molecule may have been due to the alcohol burning faster causing increased heat loss and decreased heat transfer.

The results were good enough to come to a firm conclusion but further evidence could be provided with further reading from this experiment and from another experiments.  The liquid in the can could have been changed to something else with a different heat capacity.

...read more.

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