08 Reaction Kinetics, Set B To print: Ctrl + P, paper A4, margins Default.
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08
Reaction Kinetics
Set BExam practice
Name
Time 1 hour 15 minutes
Total 37 marks
A Data Booklet may be used.
A
Multiple choice
5 marks
1
CommonA certain first order reaction took 42 minutes for the initial concentration of the reactant to decrease from 0.48 mol dm−3 to 0.06 mol dm−3. How long will it take for the concentration of the same reactant to decrease from 0.40 mol dm−3 to 0.10 mol dm−3?
The dissociation of a compound RS is shown below. RS(g) → R(g) + S(g)
The rate equation for the reaction is rate = k[RS] and the rate constant is 3.6 × 10−3 s−1. If the initial concentration of RS is 2.0 × 10−2 mol dm−3, what will be the concentration of RS after 385 seconds?
Which statements about order of reaction are correct?
1Only first order reactions have constant half-lives.2Measurements of initial rates of reaction can be used to determine the overall order of a reaction.3The units of a rate constant are independent of the order of the reaction.
A1, 2 and 3
B1 and 2 only
C2 and 3 only
D1 only
Answer [1]
Source: A Level 2006 P1 Q34, via EJC 2022 JC1 Reaction Kinetics Tutorial, A1
4
The following data were obtained from studies of the reaction between NO and O2 in a vessel at constant temperature.
experiment
I
II
III
IV
total initial pressure / atm
1.00
1.60
2.00
x
initial pressure of NO / atm
0.40
0.40
0.80
0.20
initial rate of reaction / atm s−1
1.08
2.16
8.64
1.08
Which of the following statements is correct regarding the above system?
The reaction of acidified aqueous potassium iodide with aqueous hydrogen peroxide, 2I−(aq) + H2O2(aq) + 2H+(aq) → I2(aq) + 2H2O(l), is thought to involve the following steps.
step 1
H2O2 + I− → H2O + OI−
slow
step 2
OI− + H+ → HOI
step 3
HOI + H+ + I− → I2 + H2O
Which of the following conclusions can be drawn from this information?
1The acid is acting as a catalyst.2OI− and HOI are intermediates.3Step 2 has a lower activation energy than step 1.4The reaction is overall second order.
CommonAt 800 K, nitrogen monoxide reacts with hydrogen according to the equation 2H2(g) + 2NO(g) → 2H2O(l) + N2(g). The table shows how the initial rate of this reaction depends on the initial partial pressures of the reactants.
experiment
pH2 / atm
pNO / atm
initial rate / atm s−1
1
0.64
1.60
1.50 × 10−7
2
0.64
0.80
3.75 × 10−8
3
0.32
1.60
7.50 × 10−8
(a)
Determine the order of reaction with respect to H2 and NO. Hence state, in terms of partial pressures, the rate equation for the reaction.
[3]
(b)
Using the rate equation, calculate a value for the rate constant. Include units in your answer.
[2]
Source: 2023 VJC J2 Prelim P2 Q4(a)
7
Isoprene, CH2=C(CH3)CH=CH2, reacts with bromine. To determine the rate equation for this reaction, a student obtained the following data.
expt
[isoprene] / mol dm−3
[Br2] / mol dm−3
initial rate / mol dm−3 s−1
1
0.04
0.064
0.88 × 10−3
2
0.08
0.064
1.76 × 10−3
3
0.08
0.128
3.52 × 10−3
(a)
Using data from the table, determine the order of reaction with respect to Br2 and isoprene, showing your working clearly.
[2]
(b)
Hence, construct the rate equation for the reaction.
[1]
Source: 2023 YIJC J2 Prelim P3 Q2(b)(i) and (ii)
8
The rate of reaction between propanone, CH3COCH3, and acidified iodine to give iodopropanone, CH3COCH2I, is found to be independent of [I2], but directly proportional to [H+] and directly proportional to [propanone].
(a)
Write the rate equation for this reaction and state the overall order.
[1]
(b)
Suggest, with reasons, which of the following two possible mechanisms, A or B, fits the observed kinetic data. X and Y are intermediates.
A
CH3COCH3 + H+ → X (slow)
X + I2 → products (fast)
B
CH3COCH3 + I2 → Y (slow)
Y + H+ → products (fast)
[3]
(c)
The reaction between propanone and bromine to give bromopropanone proceeds by a similar mechanism. Explain how the rate of the reaction between propanone and bromine would compare with that of the reaction between propanone and iodine.
[1]
Source: RJC H2 Chemistry Tutorial 6 (Reaction Kinetics), Standard Q5
9
The half-life of a second-order reaction with respect to a reactant A can be calculated using t½ = 1k[A]t, where k is the rate constant at a specific temperature and [A]t is the concentration of A at time t. The decomposition of nitrogen dioxide, 2NO2(g) → 2NO(g) + O2(g), follows second-order kinetics. The rate constant at 300 °C is 0.54 mol−1 dm3 s−1. At 300 °C, a sample of NO2 in a closed container took 46.3 s for its concentration to decrease to one-fourth of its original value. Use the formula to determine the original concentration, in mol dm−3, of NO2 in the container.
[2]
Source: 2024 DHS J2 Prelim P3 Q2(d)(ii)
10
The reaction between iron(III) ions and iodide ions is shown. The initial concentration of Fe3+(aq) is 0.150 mol dm−3.
Fe3+(aq) + I−(aq) → Fe2+(aq) + ½I2(aq)
The rate of this reaction was followed by the change in the concentration of I−(aq), shown in the graph.
(a)
CommonUse the graph to calculate the initial rate of reaction and the rate of reaction at 20 s. Show your working on the graph.
[2]
(b)
Hence, determine the order of reaction with respect to iodide ions. Show your working.
[1]
(c)
Explain why it is important that the concentration of Fe3+(aq) is in large excess compared with the concentration of I−(aq) in this experiment.
[1]
(d)
The reaction is first order with respect to Fe3+(aq). Write the rate equation for the reaction.
[1]
(e)
Use the rate equation and the initial rate from (a) to calculate the rate constant, stating its units.
[2]
(f)
In a new experiment of the same reaction at a different temperature, the rate constant was found to be 2.10, in the same units. Deduce whether the new experiment was carried out at a higher or lower temperature than the original experiment.
[1]
Source: 2024 VJC J2 Prelim P2 Q2(a)
11
In the presence of chlorine atoms, ozone decomposes exothermically to produce oxygen. The reaction mechanism has two steps.
Use this mechanism to state and explain the role of Cl•.
[1]
(b)
CommonComplete the labelled energy profile diagram for this reaction.
[2]
Source: 2025 NYJC J2 Prelim P2 Q5(d)(ii) and (iii)
12
ChallengeExplain each of the following observations as fully as you can.
(a)
When ammonia gas is passed over platinum gauze in an enclosed vessel, the rate of decomposition of ammonia into nitrogen and hydrogen is independent of the partial pressure of ammonia, except at very low pressures, when the rate is directly proportional to the partial pressure of ammonia.
[3]
(b)
A relatively small increase in temperature can cause a large increase in the rate of a chemical reaction.
[3]
Source: RJC H2 Chemistry Tutorial 6 (Reaction Kinetics), Standard Q6(b) and (d)