AS LEVEL CHEMISTRY
TOPIC 4 – ENERGETICS
ASSESSED HOMEWORK
Answer all questions
Max 80 marks
Name ……………………………………………………………..
Mark ……../80 ……....% Grade ………
,1. The alcohol 2-methylpropan-2-ol, (CH3)3COH, reacts to form esters that are used as
flavourings by the food industry. The alcohol can be oxidised to produce carbon dioxide and
water.
A student carried out an experiment on a pure sample of 2-methylpropan-2-ol to determine its
enthalpy of combustion. A sample of the alcohol was placed into a spirit burner and positioned
under a beaker containing 50 cm 3 of water. The spirit burner was ignited and allowed to burn for
several minutes before it was extinguished.
The results for the experiment are shown in Table 1.
Table 1
Initial temperature of the water / °C 18.1
Final temperature of the water / °C 45.4
Initial mass of spirit burner and alcohol / g 208.80
Final mass of spirit burner and alcohol / g 208.58
(a) Use the results from Table 1 to calculate a value for the heat energy released from the
combustion of this sample of 2-methylpropan-2-ol.
The specific heat capacity of water is 4.18 J K –1 g–1.
Show your working.
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(2)
(b) Calculate the amount, in moles, of 2-methylpropan-2-ol burned in the experiment.
Hence calculate a value, in kJ mol–1, for the enthalpy of combustion of
2-methylpropan-2-ol.
Show your working.
(If you were unable to calculate an answer to part (a), you should assume that the heat
energy released was 5580 J. This is not the correct value.)
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(3)
, (c) An equation for the combustion of 2-methylpropan-2-ol is
(CH3)3COH(I) + 6O 2(g) 4CO2(g) + 5H2O(I)
Table 2 contains some standard enthalpy of formation data.
Table 2
(CH3)3COH(I) O2(g) CO2(g) H2O(I)
∆Hf / kJ mol–1 –360 0 –393 –286
Use the data from Table 2 to calculate a value for the standard enthalpy of combustion of
2-methylpropan-2-ol. Show your working.
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(3)
(d) An accurate value for the enthalpy of combustion of 2-methylpropan-2-ol in which water is
formed as a gas is –2422 kJ mol–1.
Use this value and your answer from part (b) to calculate the overall percentage error in
the student’s experimental value for the enthalpy of combustion of 2-methylpropan-2-ol.
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........................................................................................................................
(1)
(e) Suggest one improvement that would reduce errors due to heat loss in the student’s
experiment.
........................................................................................................................
........................................................................................................................
........................................................................................................................
(1)
TOPIC 4 – ENERGETICS
ASSESSED HOMEWORK
Answer all questions
Max 80 marks
Name ……………………………………………………………..
Mark ……../80 ……....% Grade ………
,1. The alcohol 2-methylpropan-2-ol, (CH3)3COH, reacts to form esters that are used as
flavourings by the food industry. The alcohol can be oxidised to produce carbon dioxide and
water.
A student carried out an experiment on a pure sample of 2-methylpropan-2-ol to determine its
enthalpy of combustion. A sample of the alcohol was placed into a spirit burner and positioned
under a beaker containing 50 cm 3 of water. The spirit burner was ignited and allowed to burn for
several minutes before it was extinguished.
The results for the experiment are shown in Table 1.
Table 1
Initial temperature of the water / °C 18.1
Final temperature of the water / °C 45.4
Initial mass of spirit burner and alcohol / g 208.80
Final mass of spirit burner and alcohol / g 208.58
(a) Use the results from Table 1 to calculate a value for the heat energy released from the
combustion of this sample of 2-methylpropan-2-ol.
The specific heat capacity of water is 4.18 J K –1 g–1.
Show your working.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(2)
(b) Calculate the amount, in moles, of 2-methylpropan-2-ol burned in the experiment.
Hence calculate a value, in kJ mol–1, for the enthalpy of combustion of
2-methylpropan-2-ol.
Show your working.
(If you were unable to calculate an answer to part (a), you should assume that the heat
energy released was 5580 J. This is not the correct value.)
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(3)
, (c) An equation for the combustion of 2-methylpropan-2-ol is
(CH3)3COH(I) + 6O 2(g) 4CO2(g) + 5H2O(I)
Table 2 contains some standard enthalpy of formation data.
Table 2
(CH3)3COH(I) O2(g) CO2(g) H2O(I)
∆Hf / kJ mol–1 –360 0 –393 –286
Use the data from Table 2 to calculate a value for the standard enthalpy of combustion of
2-methylpropan-2-ol. Show your working.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(3)
(d) An accurate value for the enthalpy of combustion of 2-methylpropan-2-ol in which water is
formed as a gas is –2422 kJ mol–1.
Use this value and your answer from part (b) to calculate the overall percentage error in
the student’s experimental value for the enthalpy of combustion of 2-methylpropan-2-ol.
........................................................................................................................
........................................................................................................................
........................................................................................................................
(1)
(e) Suggest one improvement that would reduce errors due to heat loss in the student’s
experiment.
........................................................................................................................
........................................................................................................................
........................................................................................................................
(1)
