Enthalpy2 due 16-12-16
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NAME:
Questions
Q1.
Calculate the enthalpy change, in kJ mol−1, for the reaction
H2(g) + ½O2(g) → H2O(g)
DATA:
A
−243
B
−6
C
+6
D
+221
(Total for question = 1 mark)
1
Enthalpy2 due 16-12-16
/25
%
NAME:
Q2.
Fractional distillation is used in industry to obtain alkanes from crude oil.
(a) (i) The enthalpy change of combustion of a liquid hydrocarbon, pentane, was determined in an
experiment.
A sample of pentane was burned in a spirit burner and the energy produced used to heat water in a
calorimeter.
The results of the experiment are as follows:
Heat energy transferred (J) = mass of water × temperature change × 4.18
Calculate the enthalpy change of combustion of pentane. Include a sign and units in your answer.
(3)
(ii) Give one reason, other than heat loss, why the enthalpy change determined in this experiment
differs substantially from the Data Booklet value.
(1)
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Enthalpy2 due 16-12-16
/25
%
NAME:
(iii) Suggest a reason why this experiment would be too hazardous to carry out in a school laboratory.
(1)
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(b) (i) Write an equation for the complete combustion of pentane. State symbols are not required.
(1)
(ii) Identify the type and number of bonds broken and formed during the combustion of a molecule of
pentane.
(2)
(iii) Explain why the enthalpy change of combustion of pentane is exothermic.
(1)
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(Total for question = 9 marks)
3
Enthalpy2 due 16-12-16
/25
%
NAME:
Q3.
The equation for the complete combustion of butanone, C2H5COCH3, is
C2H5COCH3(l) + 5½O2(g) → 4CO2(g) + 4H2O(l)
ΔH
= −2440 kJ mol−1
From the above data, the standard enthalpy change of formation of butanone, in kJ mol−1, is
A
−280
B
+280
C
−1760
D
+1760
(Total for Question = 1 mark)
4
Enthalpy2 due 16-12-16
/25
%
NAME:
Q4.
This question is about methanol and the energy changes that accompany some of its
reactions.
(a) Complete the diagram (using dots and crosses) to show the bonding in methanol,
CH3OH. You should show outer electrons only.
(2)
(b) The Hess cycle below can be used to calculate the standard enthalpy change of
combustion of methanol, using standard enthalpy changes of formation.
(i) Complete the cycle by filling in the empty box.
(2)
*(ii) Define the term standard enthalpy change of formation of a compound,
making clear the meaning of standard in this context.
(3)
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Enthalpy2 due 16-12-16
/25
%
NAME:
(iii) Use your cycle and the data below to calculate the standard enthalpy change of
combustion of methanol, ΔHc .
(2)
(c) An experiment was carried out, using the apparatus below, to estimate the standard
enthalpy change of combustion of methanol.
After burning the methanol for a few minutes, the temperature of water in the beaker
had risen by 20.7 °C and the mass of methanol burnt was 0.848 g.
(i) Calculate the amount of energy transferred to the water.
Energy transferred (J) = mass of water × 4.18 × temperature change
(1)
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Enthalpy2 due 16-12-16
/25
%
NAME:
(ii) Calculate the number of moles of methanol, CH3OH, burnt during the
experiment.
(1)
(iii) Use your answers to (c)(i) and (ii) to calculate the experimental value for the
standard enthalpy change of combustion. Include a sign and units in your
answer, which should be given to three significant figures.
(1)
(iv) Compare your answers to (b)(iii) and (c)(iii) and give TWO reasons to explain
any differences.
(2)
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(Total for question = 14 marks)
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