AQA A-Level Chemistry Paper 2, 2022: Question 1

21 marks · Easy difficulty · State/Explain/Numerical

Reaction of Butanone with Iodine This question examines the reaction between butanone and iodine, requiring the displayed formula and name of the product. It includes rate calculations, such as determining the rate constant (k) and the initial rate when concentrations are halved. Graph interpretation for temperature effects on rate, deducing time for specific reactions, activation energy calculation using the Arrhenius equation, and outlining the mechanism for a reaction with KCN are also included.

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Question

Question text

01 An acidified solution of butanone reacts with iodine as shown.

CH3CH2COCH3 + I2 → CH3CH2COCH2I + HI

01.1 Draw the displayed formula for CH3CH2COCH2I

Give the name of CH3CH2COCH2I

[2 marks]

Displayed formula

Name

01.2 The rate equation for the reaction is

rate = k[CH CH COCH ][H+]

32 3

Table 1 shows the initial concentrations used in an experiment.

Table 1

CH CH COCH I H+

32 3 2

Initial concentration / mol dm−3 4.35 0.00500 0.825

The initial rate of reaction in this experiment is 1.45 ×10−4 mol dm−3 s−1

Calculate the value of the rate constant, k, for the reaction and give its units.

[3 marks]

Units

01.3 Calculate the initial rate of reaction when all of the initial concentrations are halved.

[1 mark]

4 −3 −1

Initial rate of reaction mol dm s

01.4 An experiment was done to measure the time, t, taken for a solution of iodine to react

completely when added to an excess of an acidified solution of butanone.

Suggest an observation used to judge when all the iodine had reacted.

[1 mark]

The experiment was repeated at different temperatures.

Figure 1 shows how varied with temperature for these experiments.

Figure 1

01.5 Describe and explain the shape of the graph in Figure 1.

[3 marks]

01.6 Deduce the time taken for the reaction at 35 oC

[1 mark]

Time s

01.7 For a different reaction, Table 2 shows the value of the rate constant at different

temperatures.

Table 2

Experiment Temperature / K Rate constant / s−1

1 T = 303 k = 1.55 ×10−5

2 T = 333 k = 1.70 ×10−4

This equation can be used to calculate the activation energy, Ea

k1 Ea 1 1

In � � = � – �

k2 R T2 T1

Calculate the value, in kJ mol−1, of the activation energy, E

The gas constant, R = 8.31 J K−1 mol−1

[5 marks]

7 –1

Ea kJ mol

01.8 Name and outline the mechanism for the reaction of butanone with KCN followed by

dilute acid.

[5 marks]

*06* Name of mechanism

Outline of mechanism

Mark scheme

Show the mark scheme

Question Answers Additional Comments/Guidelines Mark

H H H M1

Apply list principle for more than one structure

H C C C C I given

01.1

H H O H

Allow 1-iodo-2-butanone M2

1-iodobutan(-2-)one

(2 x AO1)

Rate = k Rearranged expression Or with numbers M1

[CH CH COCH ] [H+]

32 3

01.2 –5 –3 M2

k = 4.(04) × 10 or 0.00004(04) If upside down = 24752 mol dm s

If multiply = 5.20 x 10–4 mol3 dm–9 s–1

–1 3 –1 M3

mol dm s

(3 x AO1)

3.6(25) × 10–5 (mol dm–3 s–1) Allow 3.59 x 10–5 to 3.63 x 10–5

01.3

(AO1)

Brown colour removed Goes colourless

11 Allow (orange) brown to colourless 1

01.4

Allow purple to colourless (AO3)

As T increases rate (1/t ) increases OR time for completion decreases M1

Exponentially Or rate increases more and more as temp

OR increases i.e. description of exponential increase

By a greater/ increasing factor

01.5

Many more particles have E ≥ Ea NOT just higher collision frequency

NOT just more successful collisions

(2 x AO1,

1 x AO2)

Time = 1/ = 33 s 1

01.6 0.03

(AO2)

ln (1.55 x 10–5 / 1.70 x 10–4) = Ea ( 1/ – 1/ ) Insertion of correct values

/R 333 303 M1

–2.39 = Ea (–2.97 × 10–4) Evaluate LHS and fraction on RHS M2

12 M3

2.39 × 8.31/ –4 = E Re-arrange for E

2.97 × 10 a a

66937 Evaluate M4

–1 –1 M5

66.9 kJ mol convert to kJ mol

01.7 (5 x AO2)

If only k1 and k2 reversed this gives a negative

answer for Ea Lose M1 and M5

If AE in M2 allow ECF

Allow ECF from M4 to M5 for a correct unit

conversion

Allow range 66.3 – 67.1

Nucleophilic Addition ALLOW negative charge anywhere on cyanide M1

But attacking lone pair must be on C

Do not award M3 without attempt of M2 M3

Allow M2 for attack to a positive carbon following

breaking of C=O M5

(1 x AO1,

01.8 Penalise covalent KCN in M2 4 x AO2)

M3 ignore partial charges unless wrong

Penalise M3 for incorrect connection between CN

and C

NB Allow fully displayed or other structural

formulae

How to answer it

Learning Guide for Rates of Reaction and Mechanisms

Part (a): Structure and Naming

What to do: Draw the displayed formula for the product and name it correctly using IUPAC rules.

Common errors:

Failing to use a number prefix for the position of the iodine substituent (e.g., "iodobutanone" instead of "1-iodobutan-2-one").

Why it's wrong: Students may overlook the importance of indicating the position of the substituent, which is essential for IUPAC naming.

Teacher Tip: Remember that every substituent needs a number to show its position on the chain.

Part (b): Rate Equation and Rate Constant

What to do: Rearrange the rate equation to calculate the rate constant k, ensuring correct substitution of values.

Rate = k [CH₃CH₂COCH₃][I₂][H⁺]

Common errors:

Rearranging the equation incorrectly.
Forgetting to include the correct units of the rate constant.

Teacher Tip: Ensure you write out the values substituted into the equation and simplify step by step to avoid errors.

Part (c): Initial Rate

What to do: Calculate the new initial rate by halving all concentrations and using the proportionality of the rate equation.

Common errors:

Using the rate constant method instead of directly using proportionality (e.g., multiplying the rate by 1/4).

Teacher Tip: Recognise that halving each concentration in a second-order reaction reduces the rate by a factor of four.

Part (d): Judging Completion

What to do: Identify the colour change that indicates all iodine has reacted (e.g., brown to colourless).

Common errors:

Failing to mention the colourless end point of the reaction.

Why it's wrong: Students may describe iodine’s initial colour but forget to state what happens when iodine is used up.

Part (e): Temperature and Rate Relationship

What to do: Explain that as temperature increases, more particles have energy ≥ activation energy, leading to an exponential rate increase.

Common errors:

Only mentioning "more successful collisions" without linking to activation energy.
Failing to describe the exponential nature of the increase.

Why it's wrong: Students may oversimplify collision theory and neglect the quantitative relationship with activation energy.

Teacher Tip: Always connect the increase in rate to the proportion of particles with energy ≥ activation energy.

Part (g): Activation Energy

What to do: Use the Arrhenius equation to calculate activation energy (E_a) from experimental data.

ln(k₁/k₂) = E_a/R (1/T₂ - 1/T₁)

Common errors:

Incorrect substitution of k₁ and k₂.
Failing to convert the final energy into kJ mol^-1.

Teacher Tip: Clearly write each step and convert all values into consistent units to avoid simple calculation errors.

Part (h): Reaction Mechanism

What to do: Draw the full nucleophilic addition mechanism, ensuring curly arrows are placed correctly.

Common errors:

Incorrect placement of curly arrows, such as starting from the wrong atom.
Incorrect structure for the intermediate, e.g., forgetting charges or lone pairs.

Why it's wrong: Misplacing curly arrows often stems from misunderstanding the nucleophilic attack and bond formation.

Teacher Tip: Practise drawing mechanisms systematically, focusing on correct arrow placement and charges.

Final Notes

This topic combines conceptual understanding, mathematical skills, and detailed mechanisms. Ensure consistent practice to master each type of question.

Topics

Physical Chemistry · Organic Chemistry · 3.1.9 Rate Equations · 3.3.1 Introduction to Organic Chemistry

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 2, 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.