Experimental work Question 4

Paper 3 Section A Experimental work Question 4

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A student determined the enthalpy of combustion of ethanol using a spirit lamp to heat up a known mass of water as shown in the diagram below:

A mass of 0.553 g of ethanol was combusted. This was measured by filling the spirit lamp with ethanol, and weighing it before and after the experiment, Other than when the ethanol was being combusted a cap was placed on the spirit lamp. The temperature of the 250 g of water increased by 10.4 oC during the experiment.

(a) State why it is necessary to use a cap on the spirit lamp apart from when the ethanol is being combusted. [1]

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(b) Show that the enthalpy of combustion of ethanol determined from the experiment is equal to – 906 kJ mol−1 if the specific heat capacity of water is taken as 4.18 kJ kg−1 K−1. [2]

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(c) Use Section 13 of the data booklet to determine the experimental error. [1]

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(d) In his write-up of the experiment the student listed the following reasons which contributed to the experimental error.

  • There was incomplete combustion as the ethanol was being combusted in air, not oxygen.
  • There was considerable heat loss from the apparatus during the combustion.
  • The ethanol may not have been completely pure.
  • The water used in the beaker was tap water, not distilled water.
  • All of the 0.553 g of ethanol was assumed to have combusted.
  • Only one result was obtained - The experiment should have been repeated several times and an average taken.

Suggest two more reasons that contributed to the experimental error. [2]
 

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