IB Chemistry SL Reactivity 2: Change & Equilibrium

[Maximum number: 1]

Iron may be extracted from iron (II) sulfide, FeS.

The first step in the extraction of iron from iron (II) sulfide is to roast it in air to form iron (III) oxide and sulfur dioxide.

Write the equation for this reaction.

$4 \mathrm{FeS}(\mathrm{s})+7 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{Fe}_{2} \mathrm{O}_{3}(\mathrm{~s})+4 \mathrm{SO}_{2}(\mathrm{~g})$

Marking guidance:

Accept any correct ratio.

[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: 5]

(a)

Describe two characteristics of a reaction at equilibrium.

[ 2 ]

(b)

Describe how a catalyst increases the rate of a reaction.

[ 2 ]

Provides an alternative pathway of lower energy.

Lowers activation energy of the reaction / more particles have $E \geq E_{\mathrm{a}}$ .

(c)

Ethanoic acid reacts with ethanol to form the ester ethyl ethanoate.

\mathrm{CH}_{3} \mathrm{COOH}(\mathrm{l})+\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}(\mathrm{l}) \stackrel{\mathrm{H}^{+}}{\rightleftharpoons} \mathrm{CH}_{3} \mathrm{COOCH}_{2} \mathrm{CH}_{3}(\mathrm{l})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})

The esterification reaction is exothermic. State the effect of increasing temperature on the value of the equilibrium constant ( $K_{\mathrm{c}}$ ) for this reaction.

[ 1 ]

[Maximum number: 6]

The reaction between ethanoic acid and ethanol is homogeneous and reversible.

\mathrm{C}_{2} \mathrm{H}_{6} \mathrm{O}(\mathrm{l})+\mathrm{C}_{2} \mathrm{H}_{4} \mathrm{O}_{2}(\mathrm{l}) \rightleftharpoons \mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}(\mathrm{l})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \quad \Delta H^{\theta}{ }_{\mathrm{r}}=-4 \mathrm{~kJ} \mathrm{~mol}^{-1}

(a)

Deduce the expression for the equilibrium constant, K, for this reaction.

[ 1 ]

$\langle\mathrm{K}=\rangle \frac{\left[\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}\right]\left[\mathrm{H}_{2} \mathrm{O}\right]}{\left[\mathrm{C}_{2} \mathrm{H}_{6} \mathrm{O}\right]\left[\mathrm{C}_{2} \mathrm{H}_{4} \mathrm{O}_{2}\right]}$

Marking guidance:

Do not accept expression without
$\mathrm{H}_{2} \mathrm{O}$ .

(b)

$\quad 0.6$ moles each of ethanol and ethanoic acid at $60^{\circ} \mathrm{C}$ reacted in the presence of an acid catalyst. The volume remained constant. At equilibrium 0.2 moles of ethanoic acid remained in the reaction mixture.

Calculate the amounts, in mol, of ethanol, ethyl ethanoate and water at equilibrium.

[ 2 ]

(c)

Explain the effect of reducing the temperature on the value of the equilibrium constant.

[ 1 ]

(d)

Sketch the Maxwell-Boltzmann energy distribution curve for this reaction. Label the activation energy with and without a catalyst on the diagram.

Figure for Question (d) — IB Chemistry SL

[ 2 ]

Fraction of particles
$\xrightarrow[\text { Kinetic energy }]{E_{a} \text { with catalyst }}$
correct shape curve starting at the origin, without touching the x axis at high energy.
( $\mathrm{E}_{\mathrm{a}}$ ) catalysed $<\left(\mathrm{E}_{\mathrm{a}}\right)$ uncatalysed on x axis.

[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 $\geq E_{a}$ ;

IB Chemistry SL Reactivity 2: How Much, How Fast and How Far?

Covered syllabus points

Covered syllabus points

Reactivity 2. How much, how fast and how far? question 1

Reactivity 2. How much, how fast and how far? question 2

Question (a)

Question (i)

Question (ii)

Question (iii)

Question (iv)

Reactivity 2. How much, how fast and how far? question 3

Question (a)

Question (b)

Question (c)

Reactivity 2. How much, how fast and how far? question 4

Question (a)

Question (b)

Question (c)

Question (d)

Reactivity 2. How much, how fast and how far? question 5

Question (a)

Question (i)

Question (ii)

Question (iii)

Question (iv)