IB Chemistry SL Reactivity 2.2 Rate of Chemical Change

Practise calculating rates from data or graph gradients and explaining concentration, pressure, temperature, surface-area and catalyst effects with collision theory.

Exam points

calculate average or instantaneous reaction rate from data or the gradient of a tangent
explain pressure, concentration, surface-area or temperature effects through successful collisions
interpret Maxwell-Boltzmann curves and catalyst energy profiles using activation energy

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 $\ce{CO2}$ 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: 2]

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})

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

Figure for Question 2.2 Rate of chemical change question 3 — 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.

All question bank results loaded

Covered syllabus points

2.2 Rate of chemical change question 1

Question (a)

Question (i)

Question (ii)

Question (iii)

Question (iv)

2.2 Rate of chemical change question 2

Question (a)

Question (i)

Question (ii)

Question (iii)

Question (iv)

2.2 Rate of chemical change question 3