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Date	November 2020	Marks available	3	Reference code	20N.2.hl.TZ0.4
Level	HL	Paper	2	Time zone	TZ0
Command term	Discuss	Question number	4	Adapted from	N/A

Question

Nickel catalyses the conversion of propanone to propan-2-ol.

Outline how a catalyst increases the rate of reaction.

[1]

Explain why an increase in temperature increases the rate of reaction.

[2]

Discuss, referring to intermolecular forces present, the relative volatility of propanone and propan-2-ol.

[3]

The diagram shows an unlabelled voltaic cell for the reaction

${Pb}^{2 +} (aq) + Ni (s) \to {Ni}^{2 +} (aq) + Pb (s)$

Label the diagram with the species in the equation.

[1]

d(i).

Calculate the standard cell potential, in $V$ , for the cell at $298 K$ . Use section 24 of the data booklet

[1]

d(ii).

Calculate the standard free energy change, $∆ G^{⦵}$ , in $kJ$ , for the cell using sections 1 and 2 of the data booklet.

[1]

d(iii).

Suggest a metal that could replace nickel in a new half-cell and reverse the electron flow. Use section 25 of the data booklet.

[1]

d(iv).

Describe the bonding in metals.

[2]

d(v).

Nickel alloys are used in aircraft gas turbines. Suggest a physical property altered by the addition of another metal to nickel.

[1]

d(vi).

Markscheme

provides an alternative pathway/mechanism AND lower E_a ✔

Accept description of how catalyst lowers E_a (e.g. “reactants adsorb on surface «of catalyst»”, “reactant bonds weaken «when adsorbed»”).

more/greater proportion of molecules with E E_a✔

greater frequency/probability/chance of collisions «between the molecules»
OR
more collision per unit of time/second ✔

hydrogen bonding/bonds «and dipole–dipole and London/dispersion forces are present in» propan-2-ol ✔

dipole–dipole «and London/dispersion are present in» propanone ✔

propan-2-ol less volatile AND hydrogen bonding/bonds stronger «than dipole–dipole »
OR
propan-2-ol less volatile AND «sum of all» intermolecular forces stronger ✔

d(i).

$« - 0.13 V - (- 0.26 V) = + » 0.13 « V »$ ✔

d(ii).

$« {ΔG}^{Θ} = - {nFE}^{Θ} = - 2 \times 96 500 \times \frac{0.13}{1000} = » - 25 « k J »$ ✔

d(iii).

$Bi / Cu / Ag / Pd / Hg / Pt / Au$ ✔

Accept $S b$ OR $A s$ .

d(iv).

electrostatic attraction ✔

between «a lattice of» metal/positive ions/cations AND «a sea of» delocalized electrons ✔

Accept “mobile/free electrons”.

d(v).

Any of:

malleability/hardness
OR
«tensile» strength/ductility
OR
density
OR
thermal/electrical conductivity
OR
melting point
OR
thermal expansion ✔

Do not accept corrosion/reactivity or any chemical property.

Accept other specific physical properties.

d(vi).

Examiners report

Although fairly well done some candidates did not mention that providing an alternate pathway to the reaction was how the activation energy was lowered and hence did not gain the mark.

Almost all candidates earned at least 1 mark for the effect of temperature on rate. Some missed increase in collision frequency, others the idea that more particles reached the required activation energy.

The average mark was 1.9/3. Almost all candidates could recognize hydrogen bonding in alcohol but many missed the dipole-dipole attraction in propanone. There was also some confusion on the term volatility, with some thinking stronger IMF meant higher volatility.

A surprising number of No Response for a question where candidates simply had to label a diagram with the species in the equation. Some candidates had the idea but did not use the species for electrolytic cell, e.g., Pb(SO₄) instead of Pb²⁺(aq).

d(i).

80% of candidates could correctly calculate a cell potential by using a reduction table and a balanced redox reaction.

d(ii).

This was similar to 2f(ii) where many could apply the formula for Gibbs free energy change, ΔG^ө, correctly however some did not get the units correct.

d(iii).

80% could correctly pick a metal to reverse the electron flow, however some candidates thought a more reactive, rather than a less reactive metal than nickel would reverse the electron flow.

d(iv).

Most candidates were aware that metallic bonding involved a "sea of electrons", but were unsure about surrounding what and could not identify that it was electrostatic attraction holding the metal together.

d(v).

Almost all candidates could correctly identify a physical property of a metal which might be altered when alloying.

d(vi).