a)
The conversion of hydrogen and iodine into hydrogen iodide proceeds via a three step reaction mechanism:
I2 (g) (g) fast
H 2 (g) + I (g) 2 I (g) fastH2 I (g) + I (g) → 2HI (g) slow
Write the rate equation for this reaction and show how the mechanism is consistent with the stoichiometric equation.
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b)
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.
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c)
Calculate the rate constant using Expt 2 data, including its units.
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d)
Using section 11 of the Data booklet, determine whether the forward reaction is favoured by an increase in temperature.
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a)
The reaction between iodide ions and persulfate ions is a 'clock' reaction and often used to study reaction kinetics.
2I- (aq) + S2 O8 2- (aq) → I2 (aq) + 2SO4 2- (aq)
Deduce the redox changes taking place in the reaction.
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b)
A persulfate-iodide clock reaction was studied and the following rate data obtained.
Experiment
[S2 O8 2- ] / 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.
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c)
Determine the rate equation for the reaction and calculate rate constant, including the units.
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d)
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.
I- (aq) + I- (aq) → I2 2- (aq) slow
I2 2- (aq) + S2 O8 2- (aq) → I2 (aq) + 2SO4 2- (aq) fast
Mechanism 2:
I- (aq) + S2 O8 2- (aq) → S2 O8 I3- (aq) slow
S2 O8 I3- (aq) + I- (aq) → I2 (aq) + 2SO4 2- (aq) fast
Mechanism 3:
I- (aq) + S2 O8 2- (aq) → S2 O8 I3- (aq) fast
S2 O8 I3- (aq) + I- (aq) → I2 (aq) + 2SO4 2- (aq) slow
Mechanism 4:
2I- (aq) + S2 O8 2- (aq) → I2 (aq) + 2SO4 2- (aq) slow
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a)
The reaction between nitrogen monoxide and hydrogen produces nitrogen and water:
2NO (g) + 2H2 (g) → N2 (g) + 2H2 O (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?
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b)
i)
Rate against concentration of NO.
[1]
ii)
Rate against concentration of H2
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c)
Suggest a possible mechanism for the reaction.
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d)
Suggest a Lewis structure for N2 O2 and draw the shape of the molecule.
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a)
The rate of reaction between manganate(VII) ions and oxalate ions, C
2 O
4 2- , can be investigated by measuring how the concentration of manganate(VII) varies with time.
2MnO4 - (aq) + 16H+ (aq) + 5C2 O4 2- → 2Mn2+ (aq) + 8H2 O(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 [C2 O4 2- ]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.
Determine the rate of reaction.
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b)
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.
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c)
The student varied the concentration of [MnO4 - ] and plotted the rate against time at three different concentrations:
i)
Deduce, with a reason, the order of reaction with respect to MnO4 -.
[2]
ii)
Write the rate expression for the reaction.
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d)
The student then measured the reaction time for different concentrations of C2 O4 2- and obtained a curve as follows:
Comment on the shape of the graph.
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a)
A reaction proceeds by a three step mechanism. The energy profile for the reaction is shown below:
Explain the difference between points A, C, E and B, D on the profile.
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b)
Deduce which step is the rate determining step of the reaction, giving a reason.
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2I (g) fast
