Edexcel A-Level Chemistry AS Paper 2, June 2017: Question 7
18 marks · Hard difficulty · Practical Techniques and Data Analysis
Investigate the hydrolysis of halogenoalkanes, including rates of reaction, mechanisms, and graphical analysis of concentration-time data.
Practise this questionQuestion
Question text
7 This question is about halogenoalkanes.
The tables show some relevant data.
Bond Bond enthalpy/kJ mol–1 Atom Electronegativity
C F 467 C 2.5
C Cl 346 F 4.0
C Br 290 Cl 3.0
C I 228 Br 2.8
I 2.5
(a) In an experiment, 1 cm3 of ethanol and 5 cm3 of 0.1 mol dm–3 silver nitrate were
placed in each of three test tubes X, Y and Z. The test tubes and their contents
were placed in a water bath at 50°C for five minutes.
Two drops of 1-chlorobutane were then added to test tube X and the tube was
shaken to mix the contents. The time taken for a precipitate to appear was
measured.
The experiment was repeated using two drops of 1-bromobutane in test tube Y
and two drops of 1-iodobutane in test tube Z.
(i) The time taken for a precipitate to appear increases in the order
(1)
A X, Y, Z
B Z, Y, X
C Y, X, Z
D Z, X, Y
(ii) Give a reason for the addition of ethanol to each test tube.
(1)
(iii) Give a reason why the test tubes were left in the water bath for five minutes
before adding the halogenoalkanes.
(1)
16 (iv) The precipitates form as a result of reactions between aqueous silver ions and
aqueous halide ions.*P49857A01624*
Explain why halide ions are present in the mixture containing a
halogenoalkane which has only covalent bonds.
(2)
(v) Write the ionic equation, including state symbols, for the reaction involving
the silver nitrate in test tube X.
(1)
(b) 1-bromo-2-methylpropane was mixed with a large excess of potassium hydroxide solution.
The 1-bromo-2-methylpropane is hydrolysed during the reaction and its
concentration decreases as the reaction proceeds. Samples of the reaction
mixture were analysed at time intervals to determine the remaining concentration
of 1-bromo-2-methylpropane.
Time/s [1-bromo-2-methylpropane]/mol dm–3
0 0.1000
50 0.0500
100 0.0250
200 0.0063
300 0.0016
(i) Draw a graph of [1-bromo-2-methylpropane] against time.
(3) 17
*P49857A01724*
(ii) Use your graph to calculate a value for the rate of reaction at 100 s.
18 Include units in your answer.
*P49857A01824* (3)
(c) (i) Which term best describes the role of the OH– ion in the reaction in (b)?
(1)
A catalyst
B electrophile
C free radical
D nucleophile
(ii) Draw a diagram to show the mechanism for the hydrolysis of
1-bromo-2-methylpropane by the hydroxide ion.
Include any appropriate lone pairs and dipoles.
(4)
(iii) The hydrolysis reaction described in part (b) may also be classified as
(1)
A addition
B elimination
C redox
D substitution
(Total for Question 7 = 18 marks)
Mark scheme
Show the mark scheme
Question
Answer Mark
Number
7(a)(i) 7(a)(i). The only correct answer is B (1)
A is not correct because X,Y,Z is chloro/bromo/iodo, and would be for increasing rate not
time taken
C is not correct because Y,X,Z is bromo/chloro/iodo, ie incorrect for rate or time taken
D is not correct because Z,X,Y is iodo/chloro/bromo, also incorrect for either rate or time
taken
Question Acceptable Answer Additional Guidance Mark
Number
7(a)(ii) to increase the solubility of / dissolves the Do not award just ‘as a good (1)
halogenoalkane /reactants / so that reactants are solvent’
miscible
Allow cosolvent / as a (good)
solvent for both reactants
Ignore ‘stop formation of layers’
Ignore ‘to allow mixing’
Comment
Water, aqueous silver nitrate and
just silver nitrate are all acceptable
alternatives for the other reactant
Question Acceptable Answer Additional Guidance Mark
Number
7(a)(iii) to allow the solutions to equilibrate / reach the same Do not award to keep temperature (1)
temperature /reach 50oC/reach the required constant
temperature Ignore references to reaction rates
Ignore reference to fair test
Question
Acceptable Answer Additional Guidance Mark
Number
7(a)(iv) an explanation that makes reference to the following (2)
points:
(the halogenoalkane is) hydrolysed by water (1) reward recognition of reaction with
water
Do not award reaction with OH-
C- Hal bond breaks (heterolytically producing ions) Comment
(1) Must be clear that the C-Hal bond is
breaking. Allow statements like ‘the
halogen ion / halide breaks off’
Question Acceptable Answer Additional Guidance Mark
Number
7(a)(v) Ignore previous workings. Mark the (1)
Ag+(aq) + Cl (aq) AgCl(s) final equation.
Do not award uncancelled
spectator ions
Question
Acceptable Answer Additional Guidance Mark
Number
7(b)(i) (1)
Graph: both axes labelled and graph
Do not award 1-bromo-2-methylpropane without [ ]
covering at least half the grid in -3
Do not award just ‘concentration/mol dm ’
both directions. (1) -3
Allow ‘concentration of 1-bromo-2-methylpropane/mol dm ’
Units required on both axes
Accept / between label and mol dm-3 or (mol dm-3)
Non-linear scale on either axis loses M1 and M2 but can get
M3 for a smooth curve based on their points
Reversed axes loses M1 only
points plotted correctly Accuracy ½small square
(1) Do not award dot-to-dot lines
smooth line of best fit
(1)
Question
Acceptable Answer Additional Guidance Mark
Number
line drawn as tangent to curve at time 100 s. (1) (3)
7(b)(ii)
gradient= (-)3.3 x10-4
(allow range (-)2.5 x10-4 to (-)4.5 x10-4) (1) ignore missing negative sign.
Allow any SF except 1
Do not award answers that use only the one
point at 100s
Example 0.0250/100 = 2.5 x 10-4
Do not award for gradient of a straight line
graph
Do not award for gradient as a fraction
mol dm-3 s-1 (1) Allow mol dm-3 /s
Question Answer
Mark
Number
7(c)(i) 1. The only correct answer is D (1)
-
A is not correct because the OH ion is consumed, therefore not acting as a catalyst
-
B is not correct because the OH ion has negative charge and will not act as an electrophile
-
C is not correct because the OH ion does not have a single unpaired electron therefore not a free
radical
Question Acceptable Answer Additional Guidance Mark
Number
7(c)(ii) (4)
..
SN2 mechanism
M1, M2 and M4 still available for SN1
mechanism
correct structure of 1-bromo-2-methylpropane (1) TE for any other halogenoalkane, M2, M3 and
M4 still available
dipole on C – Br bond, i.e. + and - (1)
lone pair shown on OH and curly arrow from Lone pair must be located (anywhere) on the
lone pair on OH to correct carbon (1) O atom of the hydroxide ion
curly arrow from C-Br bond to Br and correct
products (1)
Question Acceptable Answer Mark
Number
7(c)(iii) 7(c)(ii). The only correct answer is D 1
A is not correct because addition involves the joining together of two molecules to make a bigger one
B is not correct because elimination involves the loss of a small molecule during the reaction
C is not correct because there are no changes in oxidation number
(Total for Question 7 = 18 mark)
How to answer it
Reactivity, Rates, and Mechanisms of Halogenoalkanes
What this question tests
This comprehensive question assesses core AS topics surrounding halogenoalkanes: carbon-halogen bond enthalpies, hydrolysis practical techniques, rate determination from concentration-time graphs, nucleophilic substitution definitions, and curly arrow reaction mechanisms.
Part (a): Halogenoalkane Hydrolysis Rates and Practical Techniques
(i) Order of precipitation time
✅ Correct Answer
B: Z, Y, X (Iodobutane reacts fastest, taking the least time; chlorobutane reacts slowest, taking the longest time).
💡 Key Knowledge
Rate of hydrolysis depends directly on carbon-halogen bond enthalpy. C-I has the lowest bond enthalpy (228 kJ mol⁻¹) and breaks most easily, whereas C-Cl has the highest bond enthalpy (346 kJ mol⁻¹) and breaks slowest.
❌ Common Errors
Students often confuse rate with time. Z,Y,X represents increasing time taken because iodine reacts the fastest!
(ii) Role of ethanol
✅ Correct Answer
To act as a common/mutual solvent so that the halogenoalkane and aqueous silver nitrate can mix (are miscible).
❌ Common Errors
Do not just write "as a good solvent" without specifying that it dissolves both reactants or makes them miscible. Avoid saying "to stop layers forming".
(iii) Pre-heating in a water bath
✅ Correct Answer
To allow the solutions to equilibrate / reach the required temperature (50 °C) before mixing.
❌ Common Errors
Do not state "to keep temperature constant" or reference fair testing. It is strictly about reaching the specified reaction temperature.
(iv) Halide ions from covalent compounds
✅ Correct Answer
1. The halogenoalkane is hydrolysed by water.
2. The C-Hal bond breaks heterolytically, producing free ions (halide ions).
🧠 Exam Technique
You must explicitly state that the C-Hal bond undergoes heterolytic fission to produce halide ions.
(v) Ionic equation for test tube X
✅ Correct Answer
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
❌ Common Errors
Including uncancelled spectator ions or missing state symbols will lose the mark. Ensure state symbols are present: (aq) and (s).
Part (b): Graphical Determination of Reaction Rates
(i) Drawing the concentration-time graph
✅ Correct Answer
A smooth curve starting at (0, 0.1000) decreasing smoothly through all plotted points, with properly labelled axes including units ( mol dm⁻³ ).
🧠 Exam Technique
Ensure axes cover at least half the available grid in both directions. Never use dot-to-dot straight lines; draw a smooth curve of best fit.
(ii) Calculating rate at 100 s
📐 Step-by-Step Calculation
- Draw a precise tangent touching the curve strictly at t = 100 s .
- Construct a large triangle on your tangent line to calculate the gradient.
- Divide change in concentration ( Δy ) by change in time ( Δx ).
- Expected gradient range: -2.5 × 10⁻⁴ to -4.5 × 10⁻⁴ .
Correct Answer with Units: -3.3 × 10⁻⁴ mol dm⁻³ s⁻¹ (allow missing negative sign, but units are mandatory).
Part (c): Mechanisms and Terminology
(i) Role of the hydroxide ion
✅ Correct Answer
D: nucleophile
💡 Key Knowledge
A nucleophile is an electron-pair donor. OH⁻ donates an electron pair to an electron-deficient carbon atom.
(ii) Mechanism for hydrolysis of 1-bromo-2-methylpropane
✅ Correct Answer
Diagram showing:
- Structure of 1-bromo-2-methylpropane with correct δ+ on C and δ- on Br.
- Lone pair and full negative charge explicitly on the oxygen of OH⁻ .
- Curly arrow starting from the OH⁻ lone pair to the δ+ carbon.
- Curly arrow starting from the C-Br bond going directly onto the bromine atom.
🧠 Exam Technique
Arrowheads must be precise! The nucleophile arrow points to the carbon atom, and the bond-breaking arrow points completely onto the halogen atom.
(iii) Classification of the reaction
✅ Correct Answer
D: substitution
💡 Key Knowledge
The bromine atom is replaced by a hydroxyl group ( -OH ), defining it clearly as a nucleophilic substitution reaction.
Topics
Organic Chemistry · Core Practicals · Topic 6: Organic Chemistry I · Core Practical 4: Investigate the hydrolysis of halogenoalkanes
Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.