DP IB Chemistry: HL

Topic Questions

Syllabus Edition

First teaching 2014

Last exams 2024

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16.1 Rate Expression & Reaction Mechanism

1a2 marks

Outline two ways a rate of a reaction can be expressed and state the units for rate of reaction.

1b2 marks

Explain what is meant by the order of a reaction and how it may be determined.

1c1 mark

Carbon monoxide and chlorine react together to make phosgene, COCl2. The equation for the reaction is given below:

CO (g) + Cl2 (g) → COCl2 ( g)

A possible rate equation for the reaction is:

rate = k[CO (g)]2[Cl2 (g)]½

What is the overall reaction order?

1d1 mark

Determine the units of the rate constant, k, for the following rate equation:

rate = k[NO]2[O2]

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2a
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1 mark

The rate of hydrolysis of sucrose under acidic conditions can be determined experimentally. The following data was obtained:

Experiment Initial [HCl] / mol dm-3 Initial [sucrose] / mol dm-3 Rate of reaction / mol dm-3 s-1
1 0.10 0.10 0.024
2 0.10 0.15 0.036
3 0.20 0.10 0.048

Determine the order of reaction with respect to HCl.

2b
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1 mark

Determine the order of reaction with respect to sucrose.

2c
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3 marks

Determine the overall order of reaction, write the rate expression and state the units of the rate constant, k.

2d
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2 marks

Determine the following:

i)
The value of k, using Experiment 1

[1]

ii)
The rate of reaction if the concentration of HCl and sucrose are both 0.20 mol dm-3

[1]

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3a2 marks

Sketch graphs of a first order and second order reaction of concentration against time.

3b2 marks

Draw sketch graphs for a first and second order reaction of rate against concentration.

3c1 mark

Deduce the units of the rate constant, k, for a first order reaction.

3d4 marks

State, with a reason, how the value of the rate constant, k, varies with increased temperature for a reaction.

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4a2 marks

State what is meant by the terms rate determining step and molecularity in a chemical reaction.

4b2 marks

The following reaction mechanism has been proposed for the formation of nitrosyl bromide, NOBr, from nitrogen monoxide and bromine:

Step 1: NO + NO → N2O2

Step 2:  N2O2 + Br2 → 2NOBr

Deduce the overall reaction equation and comment on the molecularity of Step 1 and 2.

4c2 marks

A student proposes an alternative one step mechanism for the formation of nitrosyl bromide.

NO + NO + Br2  → NOBr2

Explain why this mechanism is not likely to take place.

4d1 mark

State the role of N2O2 in the mechanism in part b).

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5a3 marks

Draw a labelled diagram, on the follow grid, showing a potential energy profile in a two step reaction. The second step is the slow step of the reaction.

ib-hl-16-1-e-rate-expression--reaction-mechanism-q5a
5b1 mark

State which step of the mechanism in a) is affected by the addition of a catalyst.

5c2 marks

A reaction mechanism is shown below. 

Step 1: NO2 + NO2  →  NO + NO (slow)

Step 2: NO3 + CO →  NO2 + CO2 ( fast)

Deduce the overall reaction equation and the rate equation for the reaction.

5d2 marks

State the overall reaction order in part c) and state the units of the rate constant.

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1a2 marks

The conversion of hydrogen and iodine into hydrogen iodide proceeds via a three step reaction mechanism:

  1. I2 (g) rightwards harpoon over leftwards harpoon  2I (g)                   fast
  2. H2 (g) + I (g) rightwards harpoon over leftwards harpoon H2I (g)      fast
  3. H2I (g) + I (g) → 2HI (g)    slow

Write the rate equation for this reaction and show how the mechanism is consistent with the stoichiometric equation.

1b3 marks

An investigation into the rate of reaction between hydrogen and iodine was carried out at 298 K and the data obtained is shown below.

Experiment [H2] / mol dm-3 [I2] / mol dm-3 Initial rate/ mol dm-3 s-1
1 0.0258 0.0137 6.43 x 10-22
2 0.0258 0.0274 1.29 x 10-21
3 0.0516 0.0137 1.29 x 10-21

Determine the rate equation for the reaction and justify your answer.

1c
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1 mark

Calculate the rate constant using Expt 2 data, including its units.

1d1 mark

Using section 11 of the Data booklet, determine whether the forward reaction is favoured by an increase in temperature.

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2a
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2 marks

The reaction between iodide ions and persulfate ions is a 'clock' reaction and often used to study reaction kinetics.

2I- (aq) + S2O82- (aq) → I2 (aq) + 2SO42- (aq)

Deduce the redox changes taking place in the reaction.

2b
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2 marks

A persulfate-iodide clock reaction was studied and the following rate data obtained.

Experiment [S2O82-] / mol dm-3 [I-] / mol dm-3 Initial rate/ mol dm-3 s-1
1 0.25 0.10 8.0 x 10-3
2 0.10 0.10 3.2 x 10-3
3 0.20 0.30 1.92 x 10-2

Deduce the order with respect to persulfate ions and iodide ions.

2c
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2 marks

Determine the rate equation for the reaction and calculate rate constant, including the units.

2d3 marks

Four mechanisms are proposed for the persulfate-iodide reaction. Deduce which mechanism(s) is/are consistent with the rate equation in part c) and justify your answer.

Mechanism 1:

  1. I- (aq) + I- (aq) → I22- (aq)                                         slow
  2.  I22- (aq) + S2O82- (aq) → I2 (aq) + 2SO42- (aq)    fast

Mechanism 2:

  1. I- (aq) + S2O82- (aq) → S2O8I3- (aq)                     slow
  2.  S2O8I3- (aq) + I- (aq) → I2 (aq) + 2SO42- (aq)    fast

Mechanism 3:

  1. I- (aq) + S2O82- (aq)  → S2O8I3- (aq)                    fast
  2.  S2O8I3- (aq) + I- (aq) → I2 (aq) + 2SO42- (aq)    slow

Mechanism 4:

  1.  2I- (aq) + S2O82- (aq) → I2 (aq) + 2SO42- (aq)    slow

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3a1 mark

The reaction between nitrogen monoxide and hydrogen produces nitrogen and water:

2NO (g) + 2H2 (g) →  N2 (g) + 2H2O (g)

Rate data for this reaction is shown below.

Experiment  [NO] / mol dm-3  [H2] / mol dm-3  Initial rate/ mol dm-3 s-1

1

0.001 0.004 0.002
2 0.002 0.004 0.008
3 0.004 0.001 0.016

What is the molecularity of the reaction?

3b2 marks

Draw a sketch graphs of:

i)
Rate against concentration of NO.
[1]
ii)
Rate against concentration of H2
[1]
3c2 marks

Suggest a possible mechanism for the reaction.

3d2 marks

Suggest a Lewis structure for N2O2 and draw the shape of the molecule.

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4a
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3 marks

The rate of reaction between manganate(VII) ions and oxalate ions, C2O42-, can be investigated by measuring how the concentration of manganate(VII) varies with time.

2MnO4- (aq) + 16H+ (aq) + 5C2O42-  →  2Mn2+(aq) + 8H2O(l) + 10CO2(g) 

The rate is first order with respect to oxalate ions and the general rate equation for the reaction is:

rate = k [MnO4-]p[C2O42-]q[H+]r

i)
Suggest how the change in manganate(VII) concentration can be measured.

[1]

ii)
A student investigated how the concentration of manganate(VII) affected the rate of reaction and produced the following results. The oxalate ions and acid were in excess.

ib-16-1-q4a
Determine the rate of reaction.

[2]

4b2 marks

The student used an acid concentration of 1.0 mol dm-3. She then varied it, keeping the other concentrations constant. She measured the rate of reaction and found the following results:

[H+]/ mol dm-3 Relative rate of reaction
0.5 0.0025
0.25 0.0013
0.01 0.0005

Identify the relationship between the relative rate of reaction and H+, and hence determine the order of reaction with respect to H+ ions.

4c3 marks

The student varied the concentration of [MnO4-] and plotted the rate against time at three different concentrations:

ib-16-1-q4c-ans
i)
Deduce, with a reason, the order of reaction with respect to MnO4-.

[2]

ii)
Write the rate expression for the reaction.
[1]
4d
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2 marks

The student then measured the reaction time for different concentrations of C2O42- and obtained a curve as follows:

ib-16-1-q4d

Comment on the shape of the graph.

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5a4 marks

A reaction proceeds by a three step mechanism. The energy profile for the reaction is shown below:

ib-hl-16-1-q5a

Explain the difference between points A, C, E and B, D on the profile.

5b2 marks

Deduce which step is the rate determining step of the reaction, giving a reason.

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1a2 marks

For the reaction below, consider the following experimental data.
 

X (aq) + Y (aq) → Z (aq)

Experiment

Initial [X] / mol dm-3

Initial [Y] / mol dm-3

Initial rate / mol dm-3 s-1

1

0.030

0.040

4.0 x 10-4

2

0.045

0.040

6.0 x 10-4

3

0.060

0.120

2.4 × 10-3

 

Deduce the order of reaction with respect to X.

1b2 marks

Deduce the order of the reaction with respect to Y.

1c1 mark

Write the rate expression for the reaction between X and Y.

1d3 marks

Determine the rate constant, k, correct to three significant figures and state its units, using data from Experiment 2. 


Experiment

Initial [X] / mol dm-3

Initial [Y] / mol dm-3

Initial rate / mol dm-3 s-1

1

0.030

0.040

4.0 x 10-4

2

0.045

0.040

6.0 x 10-4

3

0.060

0.120

2.4 × 10-3

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2a2 marks

Explain why the reaction represented below is a redox reaction. 

2ClO2 (aq) + 2NaOH (aq) → NaClO3 (aq) + NaClO2 (aq) + H2O (l) 

2b3 marks

For the reaction below, consider the following experimental data. 

2ClO2 (aq) + 2OH- (aq) → ClO3- (aq) + ClO2- (aq) + H2O (l)

Experiment

Initial [ClO2]

/ mol dm-3

Initial [OH-]

/ mol dm-3

Initial rate

/ mol dm-3 s-1

1

0.85

1.70

9.30 x 10-5

2

1.70

1.70

3.72 x 10-4

3

1.70

0.85

1.86 x 10-4

Deduce the rate expression.

2c3 marks

Determine the rate constant, k, and state its units, using data from Experiment 3. 

2d2 marks

Calculate the rate when [ClO2 (aq)] = 3.10 x 10-2 mol dm-3 and [OH- (aq)] = 1.50 x 10-2 mol dm-3.

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3a1 mark

Sketch a graph to show how the rate constant, k, varies with temperature.

3b3 marks

The following mechanism is proposed for the reaction where ethanal dimerises in dilute alkaline solution to form 3-hydroxybutanal: 

Step 1: CH3CHO + :OH- → :CH2CHO + H2
Step 2: CH3CHO + :CH2CHO → CH3CH(O:-)CH2CHO
Step 3: CH3CH(O:-)CH2CHO + H2O → CH3CH(OH)CH2CHO + :OH- 

Classify OH-, CH2CHO and CH3CH(O:-)CH2CHO as reactant, product, catalyst or intermediate, based on the proposed mechanism.

3c1 mark

Using the following information about the proposed mechanism, deduce the rate expression. 

Step 1: CH3CHO + :OH- → :CH2CHO + H2O                                                slow step
Step 2: CH3CHO + :CH2CHO → CH3CH(O:-)CH2CHO                                fast step
Step 3: CH3CH(O:-)CH2CHO + H2O → CH3CH(OH)CH2CHO + :OH-         fast step

3d1 mark

Calculate the initial rate of reaction for experiment 2, if measured under the same conditions.


Experiment

Initial [CH3CHO]

/ mol dm-3

Initial [OH-]

/ mol dm-3

Initial rate

/ mol dm-3 s-1

1

0.25

0.20

4.2 x 10-2

2

0.25

0.30

 

3e1 mark

State the effect, if any, increasing the concentration of a reactant would have on the value of the rate constant, k.

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4a1 mark

Nitrogen dioxide and carbon monoxide react according to the following equation. 

            NO2 (g) + CO (g) → NO (g) + CO2 (g) 

Using the following graph, what is the order with respect to NO2?

q4a_16-1_ib_hl_medium_sq 

4b2 marks

The rate expression for the reaction of nitrogen dioxide and carbon monoxide at T < 227 OC is: 

            Rate = k [NO2]2 

Sketch a labelled graph of concentration against time for carbon monoxide.

4c2 marks

A student proposed the following single step mechanism for the reaction of nitrogen dioxide and carbon monoxide. 

NO2 + CO → NO + CO2         slow 

Rate = k [NO2]2 

Justify whether the student’s proposed mechanism is correct.

4d1 mark

Another student proposed the following mechanism for the reaction of nitrogen dioxide and carbon monoxide. 

Step 1:             NO2 + NO2 → NO + NO3
   Step 2:             NO3 + 2CO → NO + 2CO2 

Rate = k [NO2]2 

Explain which of the student’s proposed mechanism steps is the rate determining step.

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5a1 mark

Nitrogen(II) oxide is oxidised according to the following equation. 

2NO (g) + O2 (g) → 2NO2 (g) 

The following mechanism is proposed for the two-step oxidation of nitrogen(II) oxide. 

Step 1:             NO (g) + NO (g) → N2O2 (g)
   Step 2:             N2O2 (g) + O2 (g) → 2NO2 (g) 

The potential energy profile for this two-step reaction is shown.q5a_16-1_ib_hl_medium_sq

Explain which step is the rate determining step.

5b3 marks

Energy profile diagrams give evidence for or against a proposed mechanism or proposed rate expression.

i)

Explain why the rate expression for the oxidation of nitrogen(II) oxide is not rate = k [N2O2] [O2].

ii)
Deduce the rate expression for the oxidation of nitrogen(II) oxide.

5c2 marks

Explain why the following reaction between iodide ions and peroxodisulfate ions has a high activation energy. 

S2O82- (aq) + 2I- (aq) → 2SO42- (aq) + I2 (aq)

5d2 marks

Sketch the potential energy diagram for the reaction of iodide ions with peroxodisulfate ions catalysed by iron(II) ions according to the following mechanism. 

2Fe2+ (aq) + S2O82- (aq) → 2Fe3+ (aq) + 2SO42- (aq)             slow
2Fe3+ (aq) + 2I- (aq) → 2Fe2+ (aq) + I2 (aq)                             fastq5d_16-1_ib_hl_medium_sq

5e1 mark

Deduce the rate expression for the reaction of iodide ions with peroxodisulfate ions catalysed by iron(II) ions according to the following mechanism. 

2Fe2+ (aq) + S2O82- (aq) → 2Fe3+ (aq) + 2SO42- (aq)             slow
2Fe3+ (aq) + 2I- (aq) → 2Fe2+ (aq) + I2 (aq)                               fast

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