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CHEMISTRY
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A-level
CHEMISTRY
Paper 3
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Section A box
Answer all questions in this section.
0 1 A value for enthalpy of solution can be determined in two ways:
• from a cycle, using lattice enthalpy and enthalpies of hydration
• from the results of a calorimetry experiment.
0 1 . 1 Define the term enthalpy of lattice dissociation.
[2 marks]
0 1 . 2 The enthalpy of solution for ammonium nitrate is the enthalpy change for the reaction
shown.
NH4NO3(s) + aq → NH4
+
(aq) + NO3
−
(aq) ∆H = +26 kJ mol−1
Table 1
NH4
+
(g) NO3
−
(g)
Enthalpy of hydration hydH/ kJ mol 1 −307 −314
Draw a suitably labelled cycle and use it, with data from Table 1, to calculate the
enthalpy of lattice dissociation for ammonium nitrate.
[3 marks]
Enthalpy of lattice dissociation kJ mol−1
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box 0 1 . 3 A student does an experiment to determine a value for the enthalpy of solution for
ammonium nitrate.
The student uses this method.
• Measure 25.0 cm3 of distilled water in a measuring cylinder.
• Pour the water into a beaker.
• Record the temperature of the water in the beaker.
• Add 4.00 g of solid NH4NO3 to the water in the beaker.
• Stir the solution and record the lowest temperature reached.
Table 2 shows the student’s results.
Table 2
Initial temperature / °C 20.2
Lowest temperature / °C 12.2
Calculate the enthalpy of solution, in kJ mol−1, for ammonium nitrate in this
experiment.
Assume that the specific heat capacity of the solution, c = 4.18 JK−1 g−1
Assume that the density of the solution = 1.00 gcm−3
[3 marks]
Enthalpy of solution kJ mol−1
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box 0 1 . 4 The uncertainty in each of the temperature readings from the thermometer used in
this experiment is ±0.1°C
Calculate the percentage uncertainty in the temperature change in this experiment.
[1 mark]
Percentage uncertainty
0 1 . 5 Suggest a change to the student’s method, using the same apparatus, that would
reduce the percentage uncertainty in the temperature change.
Give a reason for your answer.
[2 marks]
Change
Reason
0 1 . 6 Another student obtained a value of +15 kJ mol−1 using the same method.
Suggest the main reason for the difference between this experimental value for the
enthalpy of solution and the correct value of +26 kJ mol−1
[1 mark]
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Do not write
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box 0 1 . 7 Table 3 shows some entropy data at 298 K
Table 3
Entropy S/ JK 1mol 1
NH4NO3(s) 151
NH4
+
(aq) 113
NO3
−
(aq) 146
Calculate a value for the Gibbs free-energy change (∆G), at 298 K, for the reaction
when ammonium nitrate dissolves in water.
NH4NO3(s) + aq → NH4
+
(aq) + NO3
−
(aq) ∆H = +26 kJ mol−1
Use data from Table 3 and the value of ∆H from the equation.
Assume for the solvent, water, that the entropy change, ∆S = 0
Explain what the calculated value of ∆G indicates about the feasibility of this reaction
at 298 K
[4 marks]
∆G kJ mol−1
Explanation
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box 0 1 . 8 Ammonium nitrate decomposes as shown.
NH4NO3(s) → N2(g) + 1
2 O2(g) + 2H2O(g) ∆H = +123 kJ mol−1
The entropy change (∆S) for this reaction is +144 J K–1 mol−1
Calculate the temperature at which this reaction becomes feasible.
[2 marks]
Temperature K
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box 0 2 Tschermigite is a hydrated, water-soluble mineral, with relative formula mass of 453.2
The formula of tschermigite can be represented as M.xH2O, where M represents all
the ions present.
Table 4 shows its composition by mass.
Table 4
Element % by mass
N 3.09
H 6.18
Al 5.96
S 14.16
O 70.61
In an analysis, it is found that the mineral contains the ions NH4
+, Al
3+ and SO4
2−
Calculate the empirical formula of tschermigite and the value of x in M.xH2O
Describe the tests, with their results, including ionic equations, that would confirm the
identities of the ions present.
[6 marks]
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box 0 3 Under suitable conditions, 2-bromobutane reacts with sodium hydroxide to produce a
mixture of five products, A, B, C, D and E.
Products A, B and C are alkenes.
A is a structural isomer of B and C.
A does not exhibit stereoisomerism.
B and C are a pair of stereoisomers.
Products D and E are alcohols.
D and E are a pair of enantiomers.
0 3 . 1 Give the names of the two concurrent mechanisms responsible for the formation of
the alkenes and the alcohols.
[2 marks]
Mechanism to form alkenes
Mechanism to form alcohols
0 3 . 2 Define the term stereoisomers.
[2 marks]
0 3 . 3 Deduce the name of isomer A.
Explain why A does not exhibit stereoisomerism.
[2 marks]
Name
Explanation
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Do not write
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box 0 3 . 4 Outline the mechanism for the reaction of 2-bromobutane with sodium hydroxide to
form alkene A.
[3 marks]
0 3 . 5 Deduce the name of isomer B and the name of isomer C.
Explain the origin of the stereoisomerism in B and C.
[2 marks]
Names
Explanation
0 3 . 6 Draw 3D representations of enantiomers D and E to show how their structures are
related.
[2 marks]
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Do not write
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box 0 3 . 7 A student compares the rates of hydrolysis of 1-chlorobutane, 1-bromobutane and
1-iodobutane.
The suggested method is:
• add equal volumes of the three halogenoalkanes to separate test tubes
• add equal volumes of aqueous silver nitrate to each test tube
• record the time taken for a precipitate to appear in each test tube.
State and explain the order in which precipitates appear.
[2 marks]
Order in which precipitates appear
Explanation
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box 0 4 Hydrogen peroxide solution decomposes to form water and oxygen.
2H2O2(aq) → 2H2O(l) + O2(g)
The reaction is catalysed by manganese(IV) oxide.
A student determines the order of this reaction with respect to hydrogen peroxide.
The student uses a continuous monitoring method in the experiment.
The student places hydrogen peroxide solution in a conical flask with the catalyst and
uses a gas syringe to collect the oxygen formed. The student records the volume of
oxygen every 10 seconds for 100 seconds.
0 4 . 1 Explain why the reaction is fastest at the start.
[2 marks]
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box 0 4 . 2 The graph in Figure 1 shows how the concentration of hydrogen peroxide changes
with time in this experiment.
Figure 1
Tangents to the curve in Figure 1 can be used to determine rates of reaction.
Draw a tangent to the curve when the concentration of hydrogen peroxide solution is
0.05 mol dm–3
Use your tangent to calculate the gradient of the curve at this point.
[2 marks]
Gradient mol dm–3 s–1
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Do not write
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box 0 4 . 3 The concentration of hydrogen peroxide solution at time t during the experiment can
be calculated using this expression.
[ ] [ ] max
22 22 initial
max
HO HO t
t
V V
V
− =
[H O2 2 ]t = concentration of hydrogen peroxide solution at time t / mol dm–3
[ 2 2 ]initial H O = concentration of hydrogen peroxide solution at the start / mol dm–3
Vmax = total volume of oxygen gas collected during the whole experiment / cm3
Vt = volume of oxygen gas collected at time t / cm3
In this experiment,Vmax = 100 cm3
Use Figure 1 and the expression to calculate [H O
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