A2 20 Qu - Frankly Chemistry...(ii) Rate constant ... → 2NOBr(g) the rate equation was found by experiment to be rate = k[NO]2[Br 2]. Explain why the proposed mechanisms below are

REACTION KINETICS IIA2 TOPIC 20

Questionsheet 1

Questionsheet 2

Questionsheet 3

Questionsheet 4

Questionsheet 5

Questionsheet 6

Questionsheet 7

Questionsheet 8

Questionsheet 9

Questionsheet 10

QUESTIONSHEETS A2 LevelCHEMISTRY

16 marks

14 marks

15 marks

15 marks

14 marks

14 marks

17 marks

10 marks

18 marks

18 marks

MEASUREMENT OF REACTION RATES I

MEASUREMENT OF REACTION RATES II

ORDER OF REACTION I

ORDER OF REACTION II

FIRST ORDER REACTIONS AND HALF-LIFE TIMES

DEDUCTION OF REACTION MECHANISMS

TEST QUESTION I

TEST QUESTION II

TEST QUESTION III

TEST QUESTION IV

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AuthorsTrevor Birt John BrockingtonDonald E Caddy Kevin FrobisherAndrew Jones Andy ShepherdAdrian Bond Stuart Barker

EditorsJohn BrockingtonStuart Barker

Do notwrite inmargin

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MEASUREMENT OF REACTION RATES 1TOPIC 20 Questionsheet 1

a) Assuming that you had access to normal laboratory glassware and reagents, but no chemical instruments, howcould you monitor the rate of the following reaction at a given temperature?

CH3COOC

2H

5(l) + NaOH(aq) → CH

3COO- Na+(aq) + C

2H

5OH(l)

...........................................................................................................................................................................

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [5]

b) Why are results from the method in a) never very accurate?

....................................................................................................................................................................... [1]

c) For each of the following reactions, suggest a method by which the reaction rate could be measured accurately,and give reasons for your choice of method.

(i) C6H

5N≡N Cl-(aq) + H

2O(l) → C

6H

5OH(aq) + N

2(g) + HCl(aq)

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

(ii) CH3CH

2CH

2CH

2Br(l) + H

2O(l) → CH

3CH

2CH

2CH

2OH(l) + HBr(aq)

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

(ii) 4[Co(NH3)

6]2+(aq) + O

2(g) + 2H

2O(l) → 4[Co(NH

3)

6]3+(aq) + 4OH-(aq)

Pale brown solution Dark brown solution

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

(Continued.....)

+


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MEASUREMENT OF REACTION RATES I

d) The following graph relates to the decomposition of ethanedioic acid according to the equation:

H2C

2O

4(aq) → CO(g) + CO

2(g) + H

2O(l)

TOPIC 20 Questionsheet 1 Continued

From the graph, work out the rate of reaction after 24 × 103 s from the start. Show your working clearly.

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [4]

Mol

ar

conc

entr

atio

n of

H2C

2O4

0 10 × 103 20 × 103 30 × 103 40 × 103

Time/s

13

15

17

19

21

23


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MEASUREMENT OF REACTION RATES IITOPIC 20 Questionsheet 2

a) What is meant by the term autocatalysis?

...........................................................................................................................................................................

....................................................................................................................................................................... [1]

b) The reaction between an acidified solution of potassium manganate(VII) and ethanedioic acid is representedby the ionic equation: 2MnO

4-(aq) + 16H+(aq) + 5C

2O

42-(aq) → 2Mn2+(aq) + 8H

2O(l) + 10CO

2(g)

(i) Describe an instrumental method to investigate the rate of change in concentration of the purple MnO4

-

ions, at room temperature, as they are replaced by the almost colourless Mn2+ ions.

...........................................................................................................................................................................

...........................................................................................................................................................................

...........................................................................................................................................................................

...........................................................................................................................................................................

...........................................................................................................................................................................

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [8]

(ii) What is the relationship between the rate of formation of the Mn2+ ions and the rate of disappearance ofthe MnO

4- ions?

....................................................................................................................................................................... [1]

(iii) The change in concentration of the Mn2+ ions with time is shown by the graph:

Explain how the shape of the curve demonstrates that Mn2+ ions act as an autocatalyst in this reaction.

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

(iv) What are the implications of this when carrying out a titration of an ethanedioate solution with KMnO4

(aq)?

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

[Mn2+]

Time


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ORDER OF REACTION ITOPIC 20 Questionsheet 3

a) Define the following terms.

(i) Overall order of reaction

...........................................................................................................................................................................

....................................................................................................................................................................... [3]

(ii) Rate constant

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

b) The rate of the reaction between hydrogen and iodine was studied over a series of experiments using a rangeof initial concentrations of the two reactants. Some of the results are:

[H2(g)] initial

/ mol dm-3

0.01

0.02

0.04

0.02

Experiment

1

2

3

4

Initial rate/ mol dm-3 s-1

4.70 × 10-6

9.40 × 10-6

1.88 ×10-5

1.88 × 10-5

[I2(g)] initial

/ mol dm-3

0.01

0.01

0.01

0.02

(i) State, with reasoning, the order of the reaction with respect to both hydrogen and iodine.

Order with respect to H2(g)

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]

Order with respect to I2(g)

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]

(ii) Write down the experimental rate equation.

....................................................................................................................................................................... [1]

(iii) Calculate a value, with correct units, for the rate constant.

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]


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ORDER OF REACTION IITOPIC 20 Questionsheet 4

For the reaction: H2O

2(aq) + 2I-(aq) + 2H+(aq)

→ 2H

2O(l) + I

2(aq)

the following kinetic data were obtained:

Experiment

1

2

3

4

Initial rate/ mol dm-3 s-1

1.75 × 10-6

3.50 × 10-6

7.00 × 10-6

1.75 × 10-6

[H+(aq)]initial/ mol dm-3

0.2

0.2

0.2

0.4

[I-(aq)]initial/ mol dm-3

0.01

0.01

0.02

0.01

[H2O

2(aq)]initial

/ mol dm-3

0.01

0.02

0.02

0.01

a) State, with reasoning, the order of the reaction with respect to the following species.

(i) H2O

2(aq)

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

(ii) I-(aq)

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

(iii) H+(aq)

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

b) Write the rate equation for the reaction.

....................................................................................................................................................................... [1]

c) What is the overall order of the reaction?

....................................................................................................................................................................... [1]

d) Calculate a value, and state the units, for the rate constant, k.

...........................................................................................................................................................................

....................................................................................................................................................................... [3]

e) What would be the initial rate for this reaction if the initial concentrations of H2O

2(aq), I-(aq) and H+(aq)

were 0.02, 0.5 and 0.10 mol dm-3 respectively?

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

f) State, with an explanation for your answer, the role of the H+(aq) ions in this reaction.

...........................................................................................................................................................................

....................................................................................................................................................................... [2]


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FIRST ORDER REACTIONS AND HALF-LIFE TIMES

TOPIC 20 Questionsheet 5

The nucleophilic substitution of a bromoalkane, RBr, by aqueous hydroxide ions gave the following data whichshow how the concentration of the organic compound changes with time.

0.0100 0.0064 0.0042 0.0027 0.0018 0.0012 0.0008 0.0005

0 5 10 15 20 25 30 35

[RBr(l)]/ mol dm-3

Time/ min

a) Plot a suitable graph of this data and show that the reaction is first order with respect to the bromoalkane.

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [9]

b) Given that the reaction is also first order with respect to the hydroxide ions, write down the experimental rateequation.

...................................................................................................................................................................... [1]

c) Write down the simplest possible mechanistic equations for this reaction. Show clearly the rate-determiningstage, and show that your proposed mechanism is consistent with the overall equation for the reaction.(Note. You may represent the reaction intermediate by X.)

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [4]


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DEDUCTION OF REACTION MECHANISMSTOPIC 20 Questionsheet 6

a) For each of the following reactions, deduce the rate equation from the information given.(i) Stoichiometric equation: 2NO(g) + Cl

2(g) → 2NOCl(g)

Mechanism: NO(g) + Cl2(g) → NOCl

2(g) slow

NO(g) + NOCl2(g) → 2NOCl(g) fast

Rate equation ................................................................................................................................................. [1]

(ii) Stoichiometric equation: H2O

2(aq) +2HI(aq) → 2H

2O(l) + I

2(aq)

Mechanism: H2O

2(aq) + HI(aq) → H

2O(l) +HIO(aq) slow

HIO(g) + HI(aq) → H2O(l) + I

2(aq) fast

Rate equation ................................................................................................................................................. [1]

(iii) Stoichiometric equation: 2N2O(g) → 2N

2(g) + O

2(g)

Mechanism: N2O(g) → N

2(g) + O (g) slow

N2O(g) + O (g) → N

2(g) + O

2(g) fast

Rate equation ................................................................................................................................................. [1]

(iv) Stoichiometric equation: A(g) + B(g) + C(g) → D(g) + E(g)Mechanism: A(g) + B(g) → X(g) + E(g) slow

X(g) + C(g) →D(g) fast

Rate equation ................................................................................................................................................. [1]

b) Complete the blank spaces in the mechanisms for the following reactions.(i) Stoichiometric equation: 2NO

2(g) + F

2(g) → 2NO

2F(g); rate = k[NO

2][F

2]

Mechanism: ……… + ……. → ………..+ F (g) slow

……….. + F (g) → 2NO2F(g) fast [3]

(ii) Stoichiometric equation: H2(g) +ICl(g) →2HCl(g) + I

2(g)

Mechanism: ……… + ………. → ……….. + HCl(g) slow

……… + ICl(g) → HCl(g) + …….. fast [3]

c) For the reaction: 2NO(g) + Br2(g) → 2NOBr(g) the rate equation was found by experiment to be rate

= k[NO]2[Br2]. Explain why the proposed mechanisms below are inconsistent with this experimental result.

(i) NO(g) + Br2(g) → NOBr

2(g) slow

NO(g) + NOBr2(g) → 2NOBr(g) fast

...........................................................................................................................................................................

....................................................................................................................................................................... [1](ii) 2NO(g) → N

2O

2(g) slow

N2O

2(g) + Br

2(g) → 2NOBr(g) fast

...........................................................................................................................................................................

....................................................................................................................................................................... [1]d) The reaction between HBr and O

2 is thought to go by the mechanism shown below:

O2+ HBr → HBrO

2slow

HBrO2 +

HBr → 2HBrO fast

2HBrO + 2HBr → 2H2O + 2Br

2fast

Hence deduce the rate equation and stoichiometric equation.

(i) Rate equation ......................................................................................................................................... [1]

(ii) Stoichiometric equation: ......................................................................................................................... [1]


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TEST QUESTION ITOPIC 20 Questionsheet 7

This Questionsheet concerns the following reaction, referred to as ‘Reaction I’.

Reaction I 6H+(aq) + 5Br-(aq) + BrO3- (aq) → 3Br

2(aq) + 3H

2O(l)

a) Explain, in terms of the oxidation number of bromine, why Reaction I is regarded as a redox reaction.

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

b) Suggest how the rate of Reaction I could be measured in the laboratory.

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

c) Reaction I was studied experimentally and the data shown in the table were found.

Experiment [BrO3-(aq)]/mol dm-3 [Br-(aq)]/mol dm-3 [H+(aq)]/mol dm-3 Relative initial rate

1 0.050 0.250 0.300 1.0

2 0.100 0.250 0.300 2.0

3 0.050 0.125 0.300 0.5

4 0.050 0.250 0.600 4.0

5 0.200 0.500 0.300

6 0.100 0.500 2.0

State, with reasons, the order of reaction with respect to each of the three reactants.

Order with respect to BrO3-(aq) .................................................................................................................. [1]

Reason .......................................................................................................................................................... [1]

Order with respect to H+(aq) ...................................................................................................................... [1]

Reason .......................................................................................................................................................... [1]

Order with respect to Br-(aq) ...................................................................................................................... [1]

Reason .......................................................................................................................................................... [1]

d) Write down the experimental rate equation and state the overall order of reaction.

Experimental rate equation ......................................................................................................................... [1]

Overall order of reaction ............................................................................................................................ [1]

e) Complete the above table. [2]

f) Reaction I can be catalysed by aqueous solutions of certain compounds of transition elements. Suggest an explanation.

...........................................................................................................................................................................

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]


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TEST QUESTION II

10


Two substances, A and B, react together according to the following equation:

A(g) + 3B(g) → AB3(g)

A series of experiments gave the following results at 298 K.

Experiment Initial [A] / mol dm-3 Initial [B] / mol dm-3 Initial rate / mol dm-3 s-1

1 0.050 0.100 0.003

2 0.100 0.100 0.006

3 0.050 0.200 0.012

a) Determine the order of the reaction with respect to each reactant.

(i) Substance A .......................................................................................................................................... [1]

(ii) Substance B .......................................................................................................................................... [1]

b) Write a rate equation for the reaction between A and B.

....................................................................................................................................................................... [1]

c) Calculate a value for the rate constant for the reaction, stating its units.

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

d) Suggest a possible two-step mechanism for the formation of AB3 from the reactants A and B. Indicate which

is the rate-determining step.

Step 1 ............................................................................................................................................................ [1]

Step 2 ............................................................................................................................................................ [1]

Rate-determining step .................................................................................................................................. [1]

e) Calculate the initial rate of reaction at 298 K when [A] = [B] = 0.400 mol dm-3

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [2]


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TEST QUESTION III


TEST QUESTION IIITEST QUESTION III

For the following gaseous reaction:

2A(g) + B(g) → C(g) + D(g)

it has been shown by experiment that

Rate = k [A] [B]

and that the reaction proceeds by the two-step mechanism

A(g) + B(g) → C(g) + X(g) Step 1

X(g) + A(g) → D(g) Step 2

a) Using the above information, write down:

(i) the overall order of the reaction. ........................................................................................................... [1]

(ii) Which step (1 or 2) is the rate determining step? Explain the reason for your choice.

...........................................................................................................................................................................

....................................................................................................................................................................... [2](iii) the molecularity of the reaction, i.e. the number of molecules undergoing change in the rate-determining step.

....................................................................................................................................................................... [1]

b) State and explain what would happen to the rate of reaction if:

(i) the concentration of A is doubled and the concentration of B remains constant.

Effect on rate ...................................................................................................................................... [1]

Explanation ......................................................................................................................................... [1]

(ii) the concentration of A is doubled and the concentration of B is halved.

Effect on rate ....................................................................................................................................... [1]

Explanation .......................................................................................................................................... [1]

(iii) In a series of experiments it was found that k = 3.75 × 10-3 mol-1 dm3 s-1 at 298 K. The initial concentrationsof A and B in the experiments were as follows.

Experiment 1 [A] = 0.025 mol dm-3 [B] = 0.025 mol dm-3

Experiment 2 [A] = 0.050 mol dm-3 [B] = 0.025 mol dm-3

Calculate the initial rate of reaction for Experiment 1

...........................................................................................................................................................................

....................................................................................................................................................................... [1]

Calculate the initial rate of reaction for Experiment 2

...........................................................................................................................................................................

....................................................................................................................................................................... [1]

(Continued....)


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TEST QUESTION III

c) Look at the proposed two-step mechanism for the reaction.

(i) What description is given to the species like X in these schemes?

....................................................................................................................................................................... [1]

(ii) Show how the reaction mechanism agrees with the stoichiometric equation.

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

d) In this reaction, A is a dark brown gas whilst all the other substances are colourless gases. State and brieflydescribe which method would be used to determine the concentration of A at any given time.

Method .......................................................................................................................................................... [1]

Description .......................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3] Quality of language [1]


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A2 Level TOPIC 20 Questionsheet 10

TEST QUESTION IV

Iodine reacts with propanone in the presence of acid to give iodopropanone. In order to measure the rateof reaction, the concentration of iodine was measured by titration every minute. This was accomplishedby withdrawing a sample of the reaction mixture using a pipette and transferring it to a conical flaskcontaining aqueous sodium hydrogen carbonate, which stops the reaction. The iodine in the flask istitrated in the usual way against aqueous sodium thiosulfate.

THE KINETICS OF THE IODINE PROPANONE REACTION

Results

(A) Propanone = 1.00 mol dm-3, sulfuric acid = 1.00 mol dm-3, iodine = 1.00 x 10-3 mol dm-3.

Time/minutes 0 1 2 3 4 5 6 7 8 9 10

Iodine concentration 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0/mol dm-3 x 10-4

(B) Propanone = 2.00 mol dm-3, sulfuric acid = 1.00 mol dm-3, iodine = 1.00 x 10-3 mol dm-3.

Time/minutes 0 1 2 3 4 5 6 7 8 9 10

Iodine concentration 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0/mol dm-3 x 10-4

(C) Propanone = 2.00 mol dm-3, sulfuric acid = 2.00 mol dm-3, iodine = 1.00 x 10-3 mol dm-3.

Time/minutes 0 1 2 3 4 5 6 7 8 9 10

Iodine concentration 10.0 8.0 6.0 4.0 2.0 0.0/mol dm-3 x 10-4

a) Plot out a graph of iodine concentration against time in seconds, labelling the lines. [3]

b) Measure the gradient of each of the three lines A, B and C, this gives the rate of reaction.

Line A ............................................................................................................................................................ [1]

Line B ............................................................................................................................................................ [1]

Line C ............................................................................................................................................................ [1]

c) By comparing the rates of reaction with the initial concentrations deduce the order of reaction with respect to both

(i) propanone .....................................and (ii) acid. .................................... [2]

d) (i) How does the rate of reaction change as iodine is used up in the reaction?

....................................................................................................................................................................... [1]

(ii) What does this tell you about the order with respect to iodine? ............................................................ [1]

(iii) What is the total order of the reaction? ................................................................................................. [1]

(Continued....)


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e) (i) Write out the rate equation for this reaction. ......................................................................................... [1]

(ii) Calculate the value of the rate constant including the units

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]

f) Why was it necessary to stop the reaction in the samples before titrating?

...........................................................................................................................................................................

....................................................................................................................................................................... [1]

g) Sketch on your graph the line that you would expect for the following samples:

(D) Propanone = 2.00 mol dm-3, sulfuric acid = 2.00 mol dm-3, iodine = 5.00 x 10-4 mol dm-3.

(E) Propanone = 0.500 mol dm-3, sulfuric acid = 1.00 mol dm-3, iodine = 1.00 x 10-3 mol dm-3. [2]


TEST QUESTION IV

THE KINETICS OF THE IODINE PROPANONE REACTION

18

A2 20 Qu - Frankly Chemistry...(ii) Rate constant ... → 2NOBr(g) the rate equation was found by experiment to be rate = k[NO]2[Br 2]. Explain why the proposed mechanisms below are

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