IB Chemistry SL 2.2 Rate of Chemical Change Questions

Practise IB Chemistry SL 2.2 by following reaction progress, explaining collision-based rate changes, and interpreting Maxwell–Boltzmann distributions and catalyst energy profiles.

Exam points

Choose a measure—gas volume, mass, colour or concentration—to track reaction progress over time.
Calculate average or initial rates from concentration–time graphs; use gradients or tangents with units.
State that successful collisions need energy ≥ Ea and the correct particle orientation.
Explain pressure, concentration and surface-area effects through collision frequency on rate curves.
Explain temperature effects on Maxwell–Boltzmann curves by locating Ea and comparing particles above it.
Compare catalysed and uncatalysed energy profiles: the pathway changes, but ΔH does not.
Distinguish homogeneous intermediates from heterogeneous adsorption, weakened bonds and desorption.
Interpret product–time curves to separate faster reaction rates from changes in final amount or yield.

Covered syllabus points

2.2.1—Rate of reaction 2.2.2—Collision theory 2.2.3—Factors affecting rate 2.2.4—Activation energy (Ea)2.2.5—Catalysts

[Maximum number: 7]

Iron rusts in the presence of oxygen and water. Rusting is a redox process involving several steps that produces hydrated iron(III) oxide, $\mathrm{Fe}_{2} \mathrm{O}_{3} \bullet \mathrm{nH}_{2} \mathrm{O}$ , as the final product. The half-equations involved for the first step of rusting are given below.

Half-equation 1: $\quad \mathrm{Fe}(\mathrm{s}) \rightarrow \mathrm{Fe}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-}$

Half-equation 2: $\quad \mathrm{O}_{2}(\mathrm{aq})+4 \mathrm{e}^{-}+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow 4 \mathrm{OH}^{-}(\mathrm{aq})$

(a)

Hydrogen peroxide decomposes according to the equation below.

2 \mathrm{H}_{2} \mathrm{O}_{2}(\mathrm{aq}) \rightarrow 2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})+\mathrm{O}_{2}(\mathrm{~g})

The rate of the decomposition can be monitored by measuring the volume of oxygen gas released. The graph shows the results obtained when a solution of hydrogen peroxide decomposed in the presence of a CuO catalyst.

Figure for Question (a) — IB Chemistry SL

[ 7 ]

(i)

The experiment is repeated with the same amount of a more effective catalyst, $\mathrm{MnO}_{2}$ , under the same conditions and using the same concentration and volume of hydrogen peroxide. On the graph above, sketch the curve you would expect.

[ 1 ]

steeper curve with a similar shape that reaches same maximum volume of $\mathrm{O}_{2}$ ;

(ii)

Outline how the initial rate of reaction can be found from the graph.

[ 2 ]

(iii)

Outline a different experimental procedure that can be used to monitor the decomposition rate of hydrogen peroxide.

[ 1 ]

measure/monitor mass/pressure/ $\left[\mathrm{H}_{2} \mathrm{O}_{2}\right]$ ;

Marking guidance:

Accept measure/monitor temperature of system.

(iv)

A Maxwell-Boltzmann energy distribution curve is drawn below. Label both axes and explain, by annotating the graph, how catalysts increase the rate of reaction.

Figure for Question (iv) — IB Chemistry SL

[ 3 ]

Figure for Answer to Question (iv) — IB Chemistry SL

y-axis: probability / fraction of molecules/particles / probability density Allow "number of particles/molecules" on y-axis.
and
x-axis: (kinetic) energy;
Accept "speed/velocity" on x-axis.

correct relative position of $E_{\mathrm{a}}$ catalysed and $E_{\mathrm{a}}$ uncatalysed;
more/greater proportion of molecules/collisions have the lower/required/catalysed $E_{\mathrm{a}}$ (and can react upon collision);

M3 can be scored by shading and annotating the graph.
Accept a greater number/proportion of successful collisions as catalyst reduces $E_{a}$ .

[Maximum number: 9]

(a)

A purple solution of potassium manganate( VII $), \mathrm{KMnO}_{4}$ , reacts with ethanedioate ions according to the following equation.

2 \mathrm{MnO}_{4}^{-}(\mathrm{aq})+5 \mathrm{C}_{2} \mathrm{O}_{4}^{2-}(\mathrm{aq})+16 \mathrm{H}^{+}(\mathrm{aq}) \rightarrow 10 \mathrm{CO}_{2}(\mathrm{~g})+2 \mathrm{Mn}^{2+}(\mathrm{aq})+8 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})

[ 9 ]

(i)

Outline an experimental procedure which may be used to measure the rate of this reaction.

[ 3 ]

(ii)

Sketch a graph to show the results of the experimental procedure outlined in (a) (i).

[ 2 ]

Figure for Answer to Question (ii) — IB Chemistry SL

axes labelled correctly;

Units not required for axes.
correct shape of curve;

Curve must have a slope of a gradually decreasing magnitude (except the pH curve) but does not have to show the end of the reaction/plateau.
Accept curve to start or end at zero or non-zero.
Accept slight initial horizontal line for mass, volume and pressure curves due to slight solubility of $\mathrm{CO}_{2}$ released.
Accept zero-order graphs.
M2 can only be scored if M1 correct.
Examples of graph:

(iii)

Outline how the rate of reaction at a particular time could be determined from the graph.

[ 1 ]

(iv)

Discuss, in terms of collision theory, the effect of increasing temperature on the rate of this reaction.

[ 3 ]

(rate increases due to)
increase in (average) kinetic energy/speed of the particles;
increase in frequency of collisions/collisions per unit time;
greater proportion/number of particles have energy $\geq E_{a}$ ;

[Maximum number: 3]

(a)

0.100 g of magnesium ribbon is added to $50.0 \mathrm{~cm}^{3}$ of $1.00 \mathrm{moldm}^{-3}$ sulfuric acid to produce hydrogen gas and magnesium sulfate.

\mathrm{Mg}(\mathrm{~s})+\mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq}) \rightarrow \mathrm{H}_{2}(\mathrm{~g})+\mathrm{MgSO}_{4}(\mathrm{aq})

[ 3 ]

(i)

The graph shows the volume of hydrogen produced against time under these experimental conditions.

Figure for Question (i) — IB Chemistry SL

Sketch two curves, labelled I and II, to show how the volume of hydrogen produced (under the same temperature and pressure) changes with time when:

I. using the same mass of magnesium powder instead of a piece of magnesium ribbon;
II. 0.100 g of magnesium ribbon is added to $50 \mathrm{~cm}^{3}$ of $0.500 \mathrm{moldm}^{-3}$ sulfuric acid.

[ 2 ]

(ii)

Outline why it is better to measure the volume of hydrogen produced against time rather than the loss of mass of reactants against time.

[ 1 ]

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Covered syllabus points

Question 1

Question (a)

Question (i)

Question (ii)

Question (iii)

Question (iv)

Question 2

Question (a)

Question (i)

Question (ii)

Question (iii)

Question (iv)

Question 3

Question (a)

Question (i)

Question (ii)