R2.2 How fast? The rate of chemical change
Practice exam-style IB Chemistry questions for How fast? The rate of chemical change, aligned with the syllabus and grouped by topic.
Question 1
For the reaction (\ce{N2O5
A.
1.33 × 10⁻³ mol dm⁻³ s⁻¹
B.
5.00 × 10⁻⁴ mol dm⁻³ s⁻¹
C.
2.00 × 10³ mol dm⁻³ s⁻¹
D.
8.33 × 10⁻⁴ mol dm⁻³ s⁻¹
Question 2
A graph of volume of gas produced against time is obtained for a reaction. What gives the instantaneous rate at 40 s?
A.
The total gas volume divided by 40 s
B.
The final gas volume divided by the total reaction time
C.
The gradient of a tangent to the curve at 40 s
D.
The change in time divided by the change in volume near 40 s
Question 3
What is required for a collision between reactant particles to be successful?
A.
High pressure and low temperature
B.
Sufficient energy and suitable orientation
C.
A high concentration and equal masses
D.
A catalyst and an exothermic pathway
Question 4
Calcium carbonate reacts with dilute hydrochloric acid. Which change increases the initial rate while keeping the total amount of calcium carbonate unchanged?
A.
Diluting the hydrochloric acid at constant volume of acid added
B.
Using larger marble chips of the same mass
C.
Lowering the temperature of the acid
D.
Using the same mass of powdered calcium carbonate
Question 5
A mechanism contains the steps: (\ce{A + B -> C}); (\ce{C + D -> E}). What is C?
A.
A product of the overall reaction
B.
An intermediate
C.
A transition state
D.
A catalyst
Question 6
The concentration of (\ce{H2O2(aq)}) decreases from 0.800 mol dm⁻³ to 0.520 mol dm⁻³ in 70.0 s.
Calculate the average rate of disappearance of (\ce{H2O2}).
[1]
Question 7
In a Maxwell–Boltzmann distribution at constant temperature, what does the area to the right of the activation energy represent?
A.
Particles with sufficient kinetic energy to react if correctly oriented
B.
The average kinetic energy of all particles
C.
The enthalpy change of the reaction
D.
Particles that have already formed products
Question 8
What is changed by adding a catalyst to a reaction mixture at constant temperature?
A.
The overall enthalpy change is made more negative
B.
The average kinetic energy of particles is increased
C.
The activation energy of the pathway is decreased
D.
The equilibrium composition is shifted toward products
Question 9
In a clock reaction, the same fixed colour change is used in each trial. Which quantity is commonly taken as proportional to the initial rate?
A.
Temperature divided by final time
B.
Time to reach the colour change
C.
1/time to reach the colour change
D.
Final volume divided by concentration
Question 10
What is the molecularity of the elementary step (\ce{NO2
A.
Bimolecular
B.
Unimolecular
C.
Second order overall
D.
Termolecular
Question 11
The initial rate doubles when [A] is doubled while [B] is constant. The initial rate is unchanged when [B] is doubled while [A] is constant. What is the rate equation?
A.
\(r = k[B]\)
B.
\(r = k[A]^2\)
C.
\(r = k[A]\)
D.
\(r = k[A][B]\)
Question 12
For a reaction that is first order in A, which plot is linear with a negative gradient?
A.
rate against 1/[A]
B.
ln[A] against time
C.
1/[A] against time
D.
[A] against time
Question 13
A proposed mechanism has a slow step (\ce{X + Y -> Z}), but the experimentally determined rate equation is (r = k[X]^2). What conclusion is most appropriate?
A.
The rate equation should be changed to match the mechanism
B.
The mechanism is proved because X appears in both expressions
C.
The mechanism is inconsistent with the kinetic evidence as written
D.
The slow step must be termolecular
Question 14
Methane and chlorine react slowly in diffuse light.
State two requirements for a collision between reactant species to lead to reaction.
[1]
Explain why temperature should be expressed in kelvin when relating it to average kinetic energy.
[1]
Question 15
A student investigates the reaction between magnesium ribbon and hydrochloric acid by collecting hydrogen in a gas syringe.
Suggest one change that would increase the initial rate.
[1]
State two variables that should be controlled when investigating the effect of acid concentration.
[1]
Question 16
For (\ce{3X(aq) -> 2Y(aq)}), ([Y]) increases at (1.20 imes10^{-3}) mol dm⁻³ s⁻¹.
Determine the rate of disappearance of X.
[1]
Question 17
A proposed mechanism is:
Step 1: (\ce{A + B -> C}) fast
Step 2: (\ce{C + A -> D}) slow
Identify the intermediate.
[1]
Write the overall equation.
[1]
State which step is rate-determining.
[1]
Question 18
Classify the molecularity of each elementary step.
(\ce{O3 -> O2 + O})
[1]
(\ce{O + O3 -> 2O2})
[1]
(\ce{A + B + C -> D})
[1]
Question 19
Hydrogen gas was collected when magnesium reacted with excess hydrochloric acid.
Use the graph to determine the approximate initial rate of hydrogen production.
[1]
Describe how the rate changes as the reaction proceeds.
[1]
Suggest why the rate changes.
[1]
Question 20
For (\ce{2A(aq) -> B(aq)}), the concentration of A decreases at 0.040 mol dm⁻³ s⁻¹. What is the rate of formation of B?
A.
0.020 mol dm⁻³ s⁻¹
B.
0.080 mol dm⁻³ s⁻¹
C.
0.040 mol dm⁻³ s⁻¹
D.
2.50 mol dm⁻³ s⁻¹
Question 21
A rate equation is (r = k[A]^2[B]). If rate is in mol dm⁻³ s⁻¹ and concentration is in mol dm⁻³, what are the units of (k)?
A.
s⁻¹
B.
mol dm⁻³ s⁻¹
C.
dm³ mol⁻¹ s⁻¹
D.
dm⁶ mol⁻² s⁻¹
Question 22
In an Arrhenius plot of ln (k) against (1/T), what is equal to the gradient?
A.
\(-E_a/R\)
B.
\(E_a/R\)
C.
ln \(A\)
D.
\(A e^{-E_a/RT}\)
Question 23
Two reactions have similar activation energies at the same temperature, but one has a much smaller Arrhenius factor, (A). What is the most likely interpretation?
A.
The reaction has a larger enthalpy change
B.
A smaller fraction of collisions has the correct orientation
C.
The products are less stable than the reactants
D.
The reactant concentration is necessarily lower
Question 24
The same reaction is carried out at 298 K and 318 K.
Describe how the Maxwell–Boltzmann distribution changes when the temperature is increased.
[1]
Explain why the rate increases.
[1]
Question 25
A catalyst is added to an exothermic reaction.
State the effect of the catalyst on the activation energy.
[1]
Explain why the enthalpy change of the reaction is unchanged.
[1]
Question 26
In a rate experiment, measurements are repeated three times at each concentration.
Distinguish between random error and systematic error in rate data.
[1]
Suggest one way to reduce random error.
[1]
Question 27
Initial-rate data for a reaction between A and B are shown.
| Experiment | [A] / mol dm⁻³ | [B] / mol dm⁻³ | Initial rate / mol dm⁻³ s⁻¹ |
|---|---|---|---|
| 1 | 0.100 | 0.100 | 2.0 × 10⁻⁵ |
| 2 | 0.200 | 0.100 | 8.0 × 10⁻⁵ |
| 3 | 0.200 | 0.300 | 8.0 × 10⁻⁵ |
Determine the order with respect to A.
[1]
Determine the order with respect to B.
[1]
Write the rate equation.
[1]
Calculate (k) using experiment 1.
[1]
Question 28
A reaction is zero order with respect to reactant A.
State the shape of a graph of rate against [A].
[1]
State the shape of a graph of [A] against time.
[1]
Explain what zero order in A means experimentally.
[1]
Question 29
For a reaction, (r = k[A][B]^2).
State the overall order.
[1]
Deduce the units of (k) when rate is in mol dm⁻³ s⁻¹.
[1]
State whether changing [A] changes (k) at constant temperature.
[1]
Question 30
A multistep energy profile has three peaks and two valleys between reactants and products.
State the number of elementary steps.
[1]
State the number of intermediates.
[1]
Explain how the rate-determining step is identified from the profile.
[1]
Question 31
A student measures the time for a sulfur precipitate to obscure a cross in the reaction between sodium thiosulfate and hydrochloric acid at different thiosulfate concentrations. The same total volume and temperature are used in each trial.
| [Na₂S₂O₃] / mol dm⁻³ | Time / s | 1/t / s⁻¹ |
|---|---|---|
| 0.010 | 120 | 0.0083 |
| 0.020 | 62 | 0.016 |
| 0.030 | 40 | 0.025 |
| 0.040 | 30 | 0.033 |
| 0.050 | 24 | 0.042 |
Identify the dependent variable in this clock method.
[1]
Explain why (1/t) can be used as a measure of relative rate.
[1]
Describe the relationship shown between thiosulfate concentration and relative rate.
[1]
Suggest two variables, other than thiosulfate concentration, that must be controlled.
[1]
Question 32
The graph shows Maxwell–Boltzmann distributions for the same gas sample at two temperatures.
Identify the curve at the higher temperature.
[1]
State what the area under each curve represents.
[1]
Explain why the higher-temperature sample reacts faster for the same activation energy.
[1]
Question 33
The energy profile shows uncatalysed and catalysed pathways for the same endothermic reaction.
Identify which pathway is catalysed.
[1]
State whether the overall enthalpy change is positive or negative.
[1]
Explain why the catalyst increases the rate.
[1]
Question 34
Initial-rate data are collected for the reaction (\ce{P + Q -> products}).
| Run | [P] / mol dm⁻³ | [Q] / mol dm⁻³ | Initial rate / mol dm⁻³ s⁻¹ |
|---|---|---|---|
| 1 | 0.100 | 0.100 | 5.00 × 10⁻⁴ |
| 2 | 0.200 | 0.100 | 1.00 × 10⁻³ |
| 3 | 0.100 | 0.200 | 2.00 × 10⁻³ |
Determine the order with respect to P.
[1]
Determine the order with respect to Q.
[1]
Write the rate equation.
[1]
Calculate the units of (k).
[1]
Explain why the orders cannot be deduced from the balanced equation alone.
[1]
Question 35
The decomposition of hydrogen peroxide is catalysed by manganese(IV) oxide.
Define catalyst.
[1]
Explain, using collision theory, why the catalysed reaction is faster at the same temperature.
[1]
Question 36
An Arrhenius plot of ln (k) against (1/T) has a gradient of (-6.20 imes10^3) K.
Calculate the activation energy in kJ mol⁻¹.
[1]
State the meaning of the y-intercept of the plot.
[1]
State why temperature must be in kelvin.
[1]
Question 37
For a first-order reaction, an Arrhenius plot gives a y-intercept of 18.4.
Determine the Arrhenius factor, (A).
[1]
State the units of (A).
[1]
Explain what the Arrhenius factor represents.
[1]
Question 38
A reaction producing carbon dioxide is followed by measuring mass loss. The graph includes data from repeated trials.
State what the gradient of the mass–time curve represents.
[1]
Identify one feature of the data that suggests random error.
[1]
Identify one feature that could suggest systematic error.
[1]
Suggest two improvements to the experimental method.
[1]
Question 39
Concentration–time data for reactant A are analysed using three possible linear plots.
| Time / s | [A] / mol dm^-3 | ln([A]/mol dm^-3) | 1/[A] / dm^3 mol^-1 |
|---|---|---|---|
| 0 | 0.800 | -0.223 | 1.25 |
| 20 | 0.536 | -0.623 | 1.87 |
| 40 | 0.359 | -1.023 | 2.79 |
| 60 | 0.241 | -1.423 | 4.15 |
| 80 | 0.161 | -1.823 | 6.21 |
| 100 | 0.108 | -2.223 | 9.26 |
Identify the order of the reaction with respect to A.
[1]
State how (k) is obtained from the correct plot.
[1]
State the units of (k) for this order.
[1]
Explain why a concentration–time curve alone may be insufficient to distinguish first and second order visually.
[1]
Question 40
The energy profile for a multistep reaction is shown.
State the number of elementary steps.
[1]
Identify the rate-determining step.
[1]
Identify one transition state and one intermediate from the profile.
[1]
Question 41
A student investigates how temperature affects the rate of the reaction between calcium carbonate chips and hydrochloric acid by measuring the volume of carbon dioxide produced.
Outline how the initial rate can be determined from a volume–time graph.
[1]
Explain why increasing temperature increases the initial rate, and state two variables that should be controlled to make the investigation fair.
[1]
Question 42
A reaction producing a gas is investigated using either a gas syringe or a balance recording mass loss.
Describe one situation in which a gas syringe is more suitable and one situation in which a balance is more suitable.
[1]
Evaluate how reliable rate data can be obtained from such experiments, including treatment of errors.
[1]
Question 43
A proposed two-step mechanism for (\ce{2X + Y -> Z}) is shown with experimental kinetic data.
Step 1: (\ce{X + Y -> I}) slow
Step 2: (\ce{I + X -> Z}) fast
The experimental rate equation is (r=k[X][Y]).
| Entry | Reaction or measurement | [X] / mol dm⁻³ | [Y] / mol dm⁻³ | Initial rate / mol dm⁻³ s⁻¹ |
|---|---|---|---|---|
| Step 1 | X + Y → I (slow) | |||
| Step 2 | I + X → Z (fast) | |||
| Overall | 2X + Y → Z | |||
| 1 | Initial-rate experiment | 0.100 | 0.100 | 2.50 × 10⁻⁵ |
| 2 | Initial-rate experiment | 0.200 | 0.100 | 5.00 × 10⁻⁵ |
| 3 | Initial-rate experiment | 0.100 | 0.200 | 5.00 × 10⁻⁵ |
| 4 | Initial-rate experiment | 0.200 | 0.200 | 1.00 × 10⁻⁴ |
Show that the mechanism is consistent with the overall equation.
[1]
Identify the intermediate.
[1]
Explain whether the mechanism is consistent with the rate equation.
[1]
State one reason why this mechanism is still considered a possible mechanism rather than proven.
[1]
Question 44
Rate constants for a reaction were measured at different temperatures and used to construct an Arrhenius plot.
Use the graph to determine the gradient of the best-fit line.
[1]
Calculate the activation energy in kJ mol⁻¹.
[1]
Determine the Arrhenius factor, (A), from the intercept.
[1]
State why using at least five temperatures improves the investigation.
[1]
Question 45
Consider an exothermic reaction carried out with and without a catalyst.
Sketch, in words, the key features that should appear on an energy profile for the uncatalysed reaction.
[1]
Compare and contrast the effects of increasing temperature and adding a catalyst on the rate of reaction using Maxwell–Boltzmann ideas.
[1]
Question 46
Collision theory is used to explain why reaction rates change under different conditions.
State the two conditions required for a successful collision and define activation energy.
[1]
Discuss how concentration, pressure, surface area and catalysts affect reaction rate in terms of collisions.
[1]
Question 47
The following mechanism is proposed for the reaction (\ce{2A + B -> E}):
Step 1: (\ce{A + B <=> C}) fast
Step 2: (\ce{C + A -> E}) slow
Identify the intermediate and state the molecularity of the slow step.
[1]
Evaluate the proposed mechanism if the experimental rate equation is (r=k[A]^2[B]). Include stoichiometric and kinetic evidence in your answer.
[1]
Question 48
Initial-rate data for (\ce{A + B + C -> products}) are obtained at constant temperature.
| Experiment | [A] / mol dm⁻³ | [B] / mol dm⁻³ | [C] / mol dm⁻³ | Initial rate / mol dm⁻³ s⁻¹ |
|---|---|---|---|---|
| 1 | 0.100 | 0.100 | 0.100 | 1.50 × 10⁻⁵ |
| 2 | 0.200 | 0.100 | 0.100 | 3.00 × 10⁻⁵ |
| 3 | 0.100 | 0.200 | 0.100 | 6.00 × 10⁻⁵ |
| 4 | 0.100 | 0.100 | 0.300 | 1.50 × 10⁻⁵ |
Determine the order with respect to each reactant.
[1]
Write the rate equation, calculate (k) using experiment 1, and state the units of (k).
[1]
Question 49
A reaction occurs by a three-step mechanism. Kinetic evidence shows that the second step is rate-determining and the overall reaction is exothermic.
Describe the features of an energy profile for this reaction.
[1]
Discuss how kinetic evidence can be used to construct and evaluate a multistep energy profile.
[1]
Question 50
The rate constant for a reaction is measured at several temperatures.
State the linear form of the Arrhenius equation and identify the gradient and intercept for a plot of ln (k) against (1/T).
[1]
Explain how the activation energy and Arrhenius factor are determined from the plot, and discuss two experimental precautions needed for reliable values.
[1]
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