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Date November 2019 Marks available 1 Reference code 19N.2.hl.TZ0.6
Level HL Paper 2 Time zone TZ0
Command term Formulate and Apply 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, ΔHθ, 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 Ea (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 CuCl2xH2O.

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]
b.

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]
d.

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, ΔGθ, 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, ΔHθ, 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).

ΔHθ = ΣΔHθf (products) − ΣΔHθf (reactants) ✔
ΔHθ = 2(−241.8 «kJ mol−1») − 4(−92.3 «kJ mol−1») = −114.4 «kJ» ✔

 

NOTE: Award [2] for correct final answer.

a(ii).

Ea (cat) to the left of Ea ✔                        

 

peak lower AND Ea (cat) smaller ✔

a(iii).

«catalyst provides an» alternative pathway ✔

«with» lower Ea
OR
higher proportion of/more particles with «kinetic» EEa(cat) «than Ea» ✔

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 = «0.443g18.02g mol1=» 0.0246 «mol»
OR
moles of CuCl2 =«1.696g134.45g mol1= » 0.0126 «mol» ✔

 

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

«x =» 2 ✔

NOTE: Accept «x =» 1.95.

NOTE: Award [3] for correct final answer.

b.

Wires:
«delocalized» electrons «flow» ✔

Electrolyte:
«mobile» ions «flow» ✔

c(i).

2Cl → Cl2 (g) + 2e
OR
Cl 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.

d.

Eθ = «+0.52 – 0.15 = +» 0.37 «V» ✔

e(i).

spontaneous AND Eθ positive ✔

e(ii).

ΔGθ = «−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).

ΔGθ < 0 AND ΔSθ < 0 AND ΔHθ < 0
OR
ΔGθ + TΔSθ < 0 AND ΔHθ < 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]
b.
[N/A]
c(i).
[N/A]
c(ii).
[N/A]
c(iii).
[N/A]
c(iv).
[N/A]
d.
[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 1: Stoichiometric relationships » 1.1 Introduction to the particulate nature of matter and chemical change
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