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Date	November 2019	Marks available	2	Reference code	19N.2.hl.TZ0.6
Level	HL	Paper	2	Time zone	TZ0
Command term	State	Question number	6	Adapted from	N/A

Question

Copper forms two chlorides, copper(I) chloride and copper(II) chloride.

Two electrolysis cells were assembled using graphite electrodes and connected in series as shown.

Copper(I) chloride undergoes a disproportionation reaction, producing copper(II) chloride and copper.

2Cu⁺ (aq) → Cu (s) + Cu²⁺ (aq)

Dilute copper(II) chloride solution is light blue, while copper(I) chloride solution is colourless.

State the electron configuration of the Cu⁺ ion.

[1]

a(i).

Copper(II) chloride is used as a catalyst in the production of chlorine from hydrogen chloride.

4HCl (g) + O₂ (g) → 2Cl₂ (g) + 2H₂O (g)

Calculate the standard enthalpy change, ΔH^θ, in kJ, for this reaction, using section 12 of the data booklet.

[2]

a(ii).

The diagram shows the Maxwell–Boltzmann distribution and potential energy profile for the reaction without a catalyst.

Annotate both charts to show the activation energy for the catalysed reaction, using the label E_{a (cat)}.

[2]

a(iii).

Explain how the catalyst increases the rate of the reaction.

[2]

a(iv).

Solid copper(II) chloride absorbs moisture from the atmosphere to form a hydrate of formula CuCl₂• $x$ H₂O.

A student heated a sample of hydrated copper(II) chloride, in order to determine the value of $x$ . The following results were obtained:

Mass of crucible = 16.221 g
Initial mass of crucible and hydrated copper(II) chloride = 18.360 g
Final mass of crucible and anhydrous copper(II) chloride = 17.917 g

Determine the value of $x$ .

[3]

State how current is conducted through the wires and through the electrolyte.

Wires:

Electrolyte:

[2]

c(i).

Write the half-equation for the formation of gas bubbles at electrode 1.

[1]

c(ii).

Bubbles of gas were also observed at another electrode. Identify the electrode and the gas.

Electrode number (on diagram):

Name of gas:

[1]

c(iii).

Deduce the half-equation for the formation of the gas identified in (c)(iii).

[1]

c(iv).

Determine the enthalpy of solution of copper(II) chloride, using data from sections 18 and 20 of the data booklet.

The enthalpy of hydration of the copper(II) ion is −2161 kJ mol⁻¹.

[2]

Calculate the cell potential at 298 K for the disproportionation reaction, in V, using section 24 of the data booklet.

[1]

e(i).

Comment on the spontaneity of the disproportionation reaction at 298 K.

[1]

e(ii).

Calculate the standard Gibbs free energy change, ΔG^θ, to two significant figures, for the disproportionation at 298 K. Use your answer from (e)(i) and sections 1 and 2 of the data booklet.

[1]

e(iii).

Suggest, giving a reason, whether the entropy of the system increases or decreases during the disproportionation.

[1]

e(iv).

Deduce, giving a reason, the sign of the standard enthalpy change, ΔH^θ, for the disproportionation reaction at 298 K.

[1]

e(v).

Predict, giving a reason, the effect of increasing temperature on the stability of copper(I) chloride solution.

[1]

e(vi).

Describe how the blue colour is produced in the Cu(II) solution. Refer to section 17 of the data booklet.

[3]

f(i).

Deduce why the Cu(I) solution is colourless.

[1]

f(ii).

When excess ammonia is added to copper(II) chloride solution, the dark blue complex ion, [Cu(NH₃)₄(H₂O)₂]²⁺, forms.

State the molecular geometry of this complex ion, and the bond angles within it.

Molecular geometry:

Bond angles:

[1]

f(iii).

Examine the relationship between the Brønsted–Lowry and Lewis definitions of a base, referring to the ligands in the complex ion [CuCl₄]²⁻.

[2]

f(iv).

Markscheme

[Ar] 3d¹⁰
OR
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ ✔

a(i).

ΔH^θ = ΣΔH^θ_f (products) − ΣΔH^θ_f (reactants) ✔
ΔH^θ = 2(−241.8 «kJ mol⁻¹») − 4(−92.3 «kJ mol⁻¹») = −114.4 «kJ» ✔

NOTE: Award [2] for correct final answer.

a(ii).

E_{a (cat)} to the left of E_a ✔

peak lower AND E_{a (cat)} smaller ✔

a(iii).

«catalyst provides an» alternative pathway ✔

«with» lower E_a
OR
higher proportion of/more particles with «kinetic» E ≥ E_a(cat) «than E_a» ✔

a(iv).

mass of H₂O = «18.360 g – 17.917 g =» 0.443 «g» AND mass of CuCl₂ = «17.917 g – 16.221 g =» 1.696 «g» ✔

moles of H₂O = « $\frac{{0.443{\text{g}}}}{{18.02{\text{g mo}}{{\text{l}}^{ - 1}}}}$ =» 0.0246 «mol»
OR
moles of CuCl₂ =« $\frac{{1.696{\text{g}}}}{{134.45{\text{g mo}}{{\text{l}}^{ - 1}}}}$ = » 0.0126 «mol» ✔

«water : copper(II) chloride = 1.95 : 1»

« $x$ =» 2 ✔

NOTE: Accept « $x$ =» 1.95.

NOTE: Award [3] for correct final answer.

Wires:
«delocalized» electrons «flow» ✔

Electrolyte:
«mobile» ions «flow» ✔

c(i).

2Cl⁻ → Cl₂ (g) + 2e⁻
OR
Cl⁻ → $\frac{1}{2}$ Cl₂ (g) + e⁻ ✔

NOTE: Accept e for e⁻.

c(ii).

«electrode» 3 AND oxygen/O₂ ✔

NOTE: Accept chlorine/Cl₂.

c(iii).

2H₂O (l) → 4H⁺ (aq) + O₂ (g) + 4e^– ✔

NOTE: Accept 2Cl^– (aq) → Cl₂ (g) + 2e^–.
Accept 4OH⁻ → 2H₂O + O₂ + 4e⁻

c(iv).

enthalpy of solution = lattice enthalpy + enthalpies of hydration «of Cu²⁺ and Cl⁻» ✔

«+2824 kJ mol^–1 − 2161 kJ mol^–1 − 2(359 kJ mol^–1) =» −55 «kJ mol^–1» ✔

NOTE: Accept enthalpy cycle.
Award [2] for correct final answer.

E^θ = «+0.52 – 0.15 = +» 0.37 «V» ✔

e(i).

spontaneous AND E^θ positive ✔

e(ii).

ΔG^θ = «−nFE = −1 mol × 96 500 C Mol^–1 × 0.37 V=» −36 000 J/−36 kJ ✔

NOTE: Accept “−18 kJ mol^–1 «per mole of Cu⁺»”.

Do not accept values of n other than 1.

Apply SF in this question.

Accept J/kJ or J mol⁻¹/kJ mol⁻¹ for units.

e(iii).

2 mol (aq) → 1 mol (aq) AND decreases ✔

NOTE: Accept “solid formed from aqueous solution AND decreases”.
Do not accept 2 mol → 1 mol without (aq).

e(iv).

ΔG^θ < 0 AND ΔS^θ < 0 AND ΔH^θ < 0
OR
ΔG^θ + TΔS^θ < 0 AND ΔH^θ < 0 ✔

e(v).

TΔS more negative «reducing spontaneity» AND stability increases ✔

NOTE: Accept calculation showing non-spontaneity at 433 K.

e(vi).

«ligands cause» d-orbitals «to» split ✔

light absorbed as electrons transit to higher energy level «in d–d transitions»
OR
light absorbed as electrons promoted ✔

energy gap corresponds to «orange» light in visible region of spectrum ✔

colour observed is complementary ✔

f(i).

full «3»d sub-level/orbitals
OR
no d–d transition possible «and therefore no colour» ✔

f(ii).

octahedral AND 90° «180° for axial» ✔

NOTE: Accept square-based bi-pyramid.

f(iii).

Any two of:
ligand/chloride ion Lewis base AND donates e-pair ✔
not Brønsted–Lowry base AND does not accept proton/H⁺ ✔
Lewis definition extends/broader than Brønsted–Lowry definition ✔

f(iv).

Examiners report

[N/A]

a(i).

[N/A]

a(ii).

[N/A]

a(iii).

[N/A]

a(iv).

[N/A]

c(i).

[N/A]

c(ii).

[N/A]

c(iii).

[N/A]

c(iv).

[N/A]

e(i).

[N/A]

e(ii).

[N/A]

e(iii).

[N/A]

e(iv).

[N/A]

e(v).

[N/A]

e(vi).

[N/A]

f(i).

[N/A]

f(ii).

[N/A]

f(iii).

[N/A]

f(iv).

Syllabus sections

Core » Topic 9: Redox processes » 9.2 Electrochemical cells

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