Edexcel A-Level Chemistry AS Paper 2, November 2021: Question 6
14 marks · Medium difficulty · Short Open Response
Calculate the rate of reaction at 50 s from a concentration-time graph, identify the mechanism of halogenoalkane hydrolysis, determine the relative rates of hydrolysis for different halogenoalkanes, state reagents for elimination, draw a polymer repeat unit, and draw the electrophilic addition mechanism for hydrogen bromide to 2-methylpropene.
Practise this questionQuestion
Question text
6 This question is about halogenoalkanes.
2‑chloro‑2‑methylpropane can be hydrolysed by water.
The equation for this reaction is
(CH )3CCl + H O → (CH )3COH + H+ + Cl−
32 3
The graph shows how the concentration of 2‑chloro‑2‑methylpropane changes with
time during an investigation of this reaction.
0.07
0.06
0.05
0.04
[(CH3)3CCl]
/ mol dm–3
0.03
0.02
0.01
0 50 100 150 200
Time / s
(a) Calculate the rate of reaction at 50s. Show your working on the graph.
Include units with your final answer.
(3)
Rate of reaction at 50 s = …
(b) What is the classification of the mechanism for the hydrolysis of
2‑chloro‑2‑methylpropane by water?
(1)
A addition
B elimination
C free radical substitution
D nucleophilic substitution
(c) The letters X, Y and Z refer to three different halogenoalkanes:
X 1‑bromobutane
12 Y 2‑bromobutane
Z *P67084A01228*2‑bromo‑2‑methylpropane
1 cm3 of each of these halogenoalkanes was added to separate test tubes
containing 5 cm3 of ethanol and 5 cm3 of aqueous silver nitrate solution in a
water bath at 50°C.
(i) State the visible change in the reaction of an ethanol/silver nitrate solution
with halogenoalkane X.
Include the formula of the compound responsible for this observation.
(2)
(ii) The three halogenoalkanes were placed in order of decreasing rate of reaction.
Which is the correct sequence?
(1)
A X, Z, Y
B Z, X, Y
C Z, Y, X
D X, Y, Z
(d) Under different conditions, 2‑chloro‑2‑methylpropane can react to produce
2‑methylpropene, (CH3)2C CH2.
(i) State the reagent and conditions needed for this reaction.
(2)
(ii) Draw the displayed formula for the repeat unit of a polymer that is made by
the polymerisation of 2‑methylpropene, (CH3)2C CH2.
(1)
(iii) Draw a mechanism for the addition of hydrogen bromide, HBr, to 2‑methylpropene*P67084A01328*
to form 2‑bromo‑2‑methylpropane.
Include curly arrows, and any relevant dipoles and lone pairs.
(4)
(Total for Question 6 = 14 marks)
Mark scheme
Show the mark scheme
Question Answer Additional Guidance Mark
Number
6(a) (3)
•tangent drawn on graph at 50 s (1)
Example of calculation
vertical axis 0.055 mol dm-3
•calculation of rate (1)
horizontal axis 110 s
rate = 0.055 ÷ 110 = (-)5.0 x 10-4
Allow answers in the range (-)4.0 – (-)6.0 x 10-4
Ignore missing negative sign
mol dm-3 s-1
•units (1)
mol dm-3 /s
mol dm-3 per s
Question
Answer Mark
Number
6(b) The only correct answer is D (nucleophilic substitution) (1)
A is not correct because there is more than one product
B is not correct because substitution occurs, not elimination
C is not correct because free radicals are not involved in this reaction
Number
6(c)(i) an answer that makes reference to the following points: (2)
• cream precipitate/precipitation (1) Allow off-white / very pale yellow for cream
Do not allow just yellow
Allow ppt / ppte / solid / crystals for precipitate
• AgBr (1) Ignore silver bromide
Ignore state (if shown)
Question
Answer Mark
Number
6(c)(ii) The only correct answer is C (Z, Y, X) (1)
A is not correct because hydrolysis of primary halogenoalkane (X) is the slowest
B is not correct because hydrolysis of primary halogenoalkane (X) is the slowest
D is not correct because hydrolysis of primary halogenoalkane (X) is the slowest
Question Answer Additional Guidance
Number Mark
6(d)(i) An answer that makes reference to the following points: Mark independently (2)
• potassium hydroxide / KOH (1) Allow sodium hydroxide / NaOH
• alcohol / ethanol and reflux (1) Allow just ‘heat’ in place of reflux
Do not award aqueous ethanol
Number Mark
6(d)(ii) Example of repeat unit (1)
• repeat unit
Allow non-displayed methyl groups (-CH3)
Ignore connectivity of the methyl group
Allow n outside brackets
Ignore missing brackets / round brackets
Number Mark
6(d)(iii) Example of mechanism (4)
• curly arrow from double bond to H of HBr
and
correct structure of 2-methylpropene (1)
• curly arrow from H-Br bond to Br atom
and
correct dipole on HBr molecule (1)
• intermediate with + on correct Carbon
and
Br− (1)
• lone pair on Br− incorrect structure of 2-methylpropene loses M1 only
and + on incorrect carbon loses M3 only
curly arrow from lone pair to C+ (1)
(Total for Question 6 = 14 marks)
How to answer it
Halogenoalkanes Study Guide
What this question tests
This comprehensive question assesses core topics from AS Chemistry regarding halogenoalkanes: rate of reaction graphical analysis, mechanisms of nucleophilic substitution vs. elimination, hydrolysis trends using silver nitrate, addition polymerisation, and curly-arrow electrophilic addition reaction mechanisms.
Calculating Rate of Reaction from a Graph
📐 Step-by-Step Calculation
- Step 1: Draw a precise tangent to the curve at exactly t = 50 s .
- Step 2: Choose large, clear points on your tangent line to calculate the gradient ( change in y / change in x ).
- Step 3: Substitute values. Example: 0.055 ÷ 110 = 5.0 × 10⁻⁴ .
✅ Correct Answer & Units
Acceptable rate range: 4.0 × 10⁻⁴ to 6.0 × 10⁻⁴
Required Units: mol dm⁻³ s⁻¹ (or mol dm⁻³ / s )
❌ Common Errors & Examiner Commentary
Students frequently lose marks by drawing short, inaccurate tangents or forgetting to include proper units for the rate. A negative sign on the rate value is ignored by examiners, but missing the unit entirely loses the final mark.
Mechanism Classification
✅ Correct Answer
D - nucleophilic substitution
💡 Key Knowledge
Hydrolysis of a halogenoalkane by water replaces the halogen atom with an -OH group. Water acts as a nucleophile, donating a lone pair.
Hydrolysis and Reactivity of Halogenoalkanes
✅ Part (i): Observations & Formula (2 Marks)
- Observation: Cream precipitate (allow off-white / very pale yellow).
- Formula: AgBr (Silver bromide).
✅ Part (ii): Decreasing Reactivity Sequence (1 Mark)
C - Z, Y, X (2-bromo-2-methylpropane > 2-bromobutane > 1-bromobutane)
🧠 Exam Technique & Understanding
Tertiary halogenoalkanes ( Z ) hydrolyse the fastest via an S_N1 mechanism due to stable tertiary carbocation intermediates, whereas primary halogenoalkanes ( X ) react slowest via S_N2 . Always watch out for whether the question asks for increasing or decreasing order!
Elimination, Polymers, and Electrophilic Addition
✅ Part (i): Elimination Reagents & Conditions (2 Marks)
- Reagent: Potassium hydroxide ( KOH ) or Sodium hydroxide ( NaOH ).
- Conditions: Ethanol / alcoholic solvent and reflux (allow just "heat"). Do not use aqueous conditions.
✅ Part (ii): Polymer Repeat Unit (1 Mark)
Draw the repeat unit of poly(2-methylpropene) with open bonds breaking out of square brackets:
[ -C(CH₃)₂-CH₂- ]
✅ Part (iii): Mechanism for Addition of HBr to 2-methylpropene (4 Marks)
- Mark 1: Curly arrow starting from the C=C double bond pointing to the H atom of HBr ; correct structure of 2-methylpropene.
- Mark 2: Curly arrow breaking the H-Br bond, and correct partial charges ( δ+ on H, δ- on Br) shown on the HBr molecule.
- Mark 3: Intermediate carbocation structure featuring a + charge on the correct tertiary carbon atom, alongside the bromide ion ( Br⁻ ).
- Mark 4: Lone pair explicitly shown on the Br⁻ ion with a curly arrow pointing directly to the positively charged carbon ( C⁺ ).
❌ Common Pitfalls in Mechanisms
Ensure curly arrows start precisely from bonds or lone pairs, not floating in empty space. The positive charge in the intermediate carbocation must be positioned on the correct tertiary carbon atom, otherwise you lose the mark for the intermediate.
Topics
Organic Chemistry · Core Practicals · Physical Chemistry · Topic 6: Organic Chemistry I · Topic 9: Kinetics I · Core Practical 4: Investigate the hydrolysis of halogenoalkanes
Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.