IB Chemistry SL 1.5 Chemical Quantities

[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: 7]

(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)

State and explain the effect of a catalyst on the position of equilibrium.

[ 2 ]

(d)

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.

IB Chemistry SL 1.5 Chemical Quantities Question Bank

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)

Question (d)

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

Question (a)

Question (b)

Question (c)

Question (d)