OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2018: Question 21
18 marks · Hard difficulty · Structured Questions
Complete electron configurations, determine standard cell potentials and explain redox reactions, equilibria, and copper chemistry involving transition elements.
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Question text
21 This question is about some reactions of d block elements and their ions.
Table 21.1 shows standard electrode potentials which will be needed within this question.
Zn2+(aq) + 2e– Zn(s) Eө = –0.76 V
Cr3+(aq) + e– Cr2+(aq) Eө = –0.42 V
Ni2+(aq) + 2e– Ni(s) Eө = –0.25 V
I (aq) + 2e– 2I–(aq) Eө = +0.54 V
Fe3+(aq) + e– Fe2+(aq) Eө = +0.77 V
Cr O 2–(aq) + 14H+(aq) + 6e– 2Cr3+(aq) + 7H O(l) Eө = +1.33 V
27 2
H O (aq) + 2H+(aq) + 2e– 2H O(l) Eө = +1.78 V
22 2
Table 21.1
(a) Complete the electron configuration of
a Ni atom: 1s2 …
a Ni2+ ion: 1s2 … [2]
(b) A standard cell is set up in the laboratory with the cell reaction shown below.
Ni(s) + I (aq) Ni2+(aq) + 2I–(aq)
(i) Draw a labelled diagram to show how this cell could be set up to measure its standard
cell potential.
Include details of apparatus, solutions and the standard conditions required.
Standard conditions …
… [4]
(ii) Predict the standard cell potential of this cell.
standard cell potential = … V [1]
(c) Use the information in Table 21.1 to help you answer both parts of this question.
(i) Write the overall equation for the oxidation of Fe2+ by acidified H O .
… [1]
(ii) Zinc reacts with acidified Cr O 2– ions to form Cr2+ ions in two stages.
Explain why this happens in terms of electrode potentials and equilibria.
Include overall equations for the reactions which occur.
… [4]
(d)* Three different reactions of copper compounds are described below.
Reaction 1: Aqueous copper(II) sulfate reacts with excess aqueous ammonia in a ligand
substitution reaction. A deep-blue solution is formed, containing an octahedral
complex ion, C, which is a trans isomer.
Reaction 2: Copper(I) oxide reacts with hot dilute sulfuric acid in a disproportionation
reaction. A blue solution, D, and a brown solid, E are formed.
Reaction 3: Copper(II) oxide reacts with warm dilute nitric acid in a neutralisation reaction,
to form a blue solution. Unreacted copper(II) oxide is filtered off, and the
solution is left overnight in an evaporating basin.
A hydrated salt, F, crystallises, with the percentage composition by mass:
Cu, 26.29%; H, 2.48%; N, 11.59%; O, 59.63%.
Identify C–F by formulae or structures, as appropriate.
Include equations, any changes in oxidation number, and working. [6]
Additional answer space if required.
Mark scheme
Show the mark scheme
Question Answer Marks Guidance
21 (a) Ni: 1s22s22p63s23p63d84s2 2 ALLOW 4s before 3d, ie 1s22s22p63s23p64s23d8
ALLOW 1s2 written after answer prompt (ie 1s2 twice)
Ni2+: 1s22s22p63s23p63d8 ALLOW upper case D, etc and subscripts, e.g … 4S 3D
ALLOW for Ni2+ … 4s0
DO NOT ALLOW [Ar] as shorthand for 1s22s22p63s23p6
Look carefully at 1s22s22p63s23p6 – there may be a mistake
(b) (i) 4
Circuit: complete circuit AND voltmeter AND salt Voltmeter must be shown AND salt bridge must be
bridge linking two half-cells labelled
ALLOW small gaps in circuit
Half cells: Pt AND I– AND I ALLOW half cells drawn either way around
IGNORE 2 before I–(aq)
Ni AND Ni2+ DO NOT ALLOW I (g) OR I (s) OR I (l)
22 2
Standard conditions: ALL conditions required
1 mol dm–3 solutions BUT ALLOW 1 mol dm–3/1M if omitted here but shown for
AND 298 K / 25ºC just one solution in diagram
Look on diagram in addition to answer lines
IGNORE pressure
Not relevant for this cell
DO NOT ALLOW 1 mol for concentration
(b) (ii) E = 0.79 (V) 1 IGNORE sign
(c) (i) 1 ALLOW multiples
H O (aq) + 2H+(aq) + 2Fe2+(aq) 2Fe3+-(aq) + 2H O(l) IGNORE state symbols, even if wrong
22 2
(c) (ii) Equations 4 ALLOW multiples
2– + IGNORE state symbols, even if wrong
3Zn(s) + Cr2O7 (aq) + 14H (aq)
2+ 3+
3Zn (aq) + 2Cr (aq) + 7H2O(l)
Zn(s) + 2Cr3+(aq) Zn2+(aq) + 2Cr2+(aq)
Comparison of E values (seen once)
ALLOW E is (+) 2.09V for Zn/Cr O 2– cell
cell 2 7
E of Zn is more negative/less positive than E of OR
Cr O 2– 3+
27 ALLOW Ecell is (+) 0.34V for Zn/Cr cell
OR IGNORE ‘lower/higher’
E of Zn is more negative/less positive than E of Cr3+
Equilibrium shift related to E values
For ‘shifts left’:
More negative/less positive OR Zn system shifts left ALLOW ‘(Zn) is oxidised’ OR ‘electrons are lost (from Zn)’
OR For ‘shifts right’,
2– ALLOW ‘(Cr) is reduced’ OR ‘electrons are gained’
Less negative/more positive Cr2O7 system shifts
right OR Less negative/more positive Cr3+ system
shifts right
(d) Please refer to the marking instructions on page 5 of this 6 Indicative scientific points may include:
mark scheme for guidance on how to mark this question.
REACTION 1 (CuSO4/NH3)
Level 3 (5–6 marks) Product
All three reactions are covered in detail with C, D, E and F C : 2+
[Cu(NH3)4(H2O)2]
identified with clear explanations. Equation
2+ 2+
[Cu(H2O)6] + 4NH3 [Cu(NH3)4(H2O)2] + 4H2O
There is a well-developed line of reasoning which is clear Structure of trans stereoisomer
and logically structured with clear chemical
communication and few omissions. The information
presented is relevant and substantiated.
Level 2 (3–4 marks)
All three reactions are covered but explanations may be
Correct connectivity
incomplete
OR REACTION 2 (Cu O/H SO )
22 4
Two reactions are explained in detail.
Products
D : CuSO OR [Cu(H O) ]2+
42 6
There is an attempt at a logical structure with a line of E: Cu
reasoning. The information is relevant e.g. formulae may
Equation
contain missing brackets or numbers and supported by
Cu2O + H2SO4 CuSO4 + Cu + H2O
some evidence.
Oxidation numbers
Level 1 (1–2 marks) Cu(+1) Cu(+2) + Cu(0)
Make two simple explanations from any one reaction.
OR REACTION 3 (CuO/HNO3)
Makes one simple explanation from each of two reactions Equation
CuO + 2HNO3 Cu(NO3)2 + H2O
There is an attempt at a logical structure with a line of Molar ratios
reasoning The information is in the most part relevant. Cu : H : N : O
26.29 2.49 11.59 59.63
= 63.5 : 1.0 : 14.0 : 16.0
0 marks No response worthy of credit.
Formula of F
CuH6N2O9
F: Cu(NO3)2•3H2O (OR Cu(NO3)2(H2O)3)
H032/01 Mark Scheme26 June 2017
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Further guidance on use of wedges
Must contain 2 ‘out wedges’, 2 ‘in wedges’ and 2 lines in
plane of paper OR 4 lines, 1 ‘out wedge’ and 1 ‘in
wedge’:
For bond into paper, ALLOW:
ALLOW following geometry:
Total 18
How to answer it
Reactions of d-Block Elements and Their Ions
This comprehensive synoptic question tests your mastery of transition element chemistry and electrochemistry. Key competencies assessed include: writing electron configurations for atoms and transition metal ions (remembering the filling/emptying order of 4s and 3d orbitals), designing standard electrochemical cells and calculating standard cell potentials, using electrode potentials to predict the feasibility of redox reactions and equilibrium shifts, writing complex multi-step redox equations, and performing complex empirical formula calculations coupled with deep knowledge of copper chemistry (ligand substitution, disproportionation, and neutralisation).
Electron Configurations of d-Block Elements
✅ Correct Answers
- Ni atom: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁸ 4s²
- Ni²⁺ ion: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁸ (or 1s² 2s² 2p⁶ 3s² 3p⁶ 4s⁰ 3d⁸ )
💡 Key Knowledge
- Filling order: 4s fills before 3d because it is lower in energy.
- Ionisation order: When transition metals form ions, electrons are always removed from the 4s orbital before the 3d orbital due to electrostatic repulsion and shielding changes.
❌ Common Errors
- Failing to remove electrons from 4s first when writing the Ni²⁺ configuration (incorrectly removing from 3d to leave 3d⁶ 4s² ).
- Writing subshells out of principal quantum number order without checking conventions (though 4s before 3d is allowed on entry, subshell energy ordering rules must be carefully followed).
Setting Up a Standard Electrochemical Cell
✅ Correct Answers
A fully labelled diagram containing:
- Complete external circuit: Connecting wires with a high-resistance voltmeter.
- Salt bridge: Linking the two half-cells (usually filter paper soaked in saturated KNO₃ or KCl).
- Half-cells:
• Ni rod dipped in Ni²⁺(aq) solution.
• Pt electrode immersed in a mixture of I⁻(aq) and I₂(aq) . - Standard conditions: Solution concentrations of 1.0 mol dm⁻³ and temperature of 298 K .
🧠 Exam Technique & Diagram Rules
- Platinum is mandatory as an inert conductor for the I₂ / I⁻ half-cell because neither reactant is a solid metal.
- Make sure the salt bridge dips into both solutions. Do not use metal wire for the salt bridge!
- State conditions explicitly near your diagram: 1 mol dm⁻³ for solutions and 298 K .
❌ Common Errors
- Omitting the inert platinum electrode and trying to dip a wire directly into the solution.
- Forgetting to state concentration units ( mol dm⁻³ ) or temperature units ( K ).
Calculating Standard Cell Potential
✅ Correct Answer
+0.79 V (ignoring sign conventions unless specified, but standard calculation is E°(reduction) - E°(oxidation) or +0.54 - (-0.25) = +0.79 V ).
📐 Calculation Steps
- Identify half-equations from Table 21.1:
• I₂(aq) + 2e⁻ ⇌ 2I⁻(aq) ( E° = +0.54 V )
• Ni²⁺(aq) + 2e⁻ ⇌ Ni(s) ( E° = -0.25 V ) - Apply formula: E°(cell) = E°(positive electrode) - E°(negative electrode)
- E°(cell) = +0.54 - (-0.25) = +0.79 V
Overall Equation for Oxidation of Fe²⁺ by Acidified H₂O₂
✅ Correct Answer
H₂O₂(aq) + 2H⁺(aq) + 2Fe²⁺(aq) → 2Fe³⁺(aq) + 2H₂O(l)
Multiples are fully accepted by the mark scheme.
💡 Key Knowledge
Combine the two half-equations from Table 21.1: multiply the iron(II) oxidation equation by 2 so that electrons cancel out with the reduction of hydrogen peroxide.
Zinc Reduction of Acidified Dichromate (Two-Stage Reaction)
✅ Correct Answers & Equations
- Stage 1: 3Zn(s) + Cr₂O₇²⁻(aq) + 14H⁺(aq) → 3Zn²⁺(aq) + 2Cr³⁺(aq) + 7H₂O(l)
- Stage 2: Zn(s) + 2Cr³⁺(aq) → Zn²⁺(aq) + 2Cr²⁺(aq)
🧠 Explanation Using Electrode Potentials
- Compare electrode potentials: E°(Zn) is more negative than both E°(Cr₂O₇²⁻/Cr³⁺) and E°(Cr³⁺/Cr²⁺) .
- Because zinc has a more negative E° , the Zn system shifts left (oxidation of Zn), driving the other systems right (reduction of Cr₂O₇²⁻ to Cr³⁺ , and subsequently Cr³⁺ to Cr²⁺ ).
Copper Chemistry: Complexes, Disproportionation, and Stoichiometry
✅ Identifications (C to F)
- C: [Cu(NH₃)₄(H₂O)₂]²⁺ (Deep blue solution, octahedral trans isomer)
- D: CuSO₄ or [Cu(H₂O)₆]²⁺ (Blue solution)
- E: Cu (Brown/red solid copper metal)
- F: Cu(NO₃)₂·3H₂O (Hydrated copper(II) nitrate trihydrate)
💡 Reaction Summaries
- Reaction 1 (Ligand Substitution): [Cu(H₂O)₆]²⁺ + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O . Forms a deep-blue octahedral complex. The trans isomer features opposite pairs of identical ligands (e.g., H₂O molecules axially opposed, NH₃ molecules in equatorial planes with proper wedge/dash 3D geometry).
- Reaction 2 (Disproportionation): Cu₂O + H₂SO₄ → CuSO₄ + Cu + H₂O . Oxidation number of Cu changes from +1 in Cu₂O to both +2 in CuSO₄ and 0 in elemental Cu .
- Reaction 3 (Neutralisation & Crystallisation): CuO + 2HNO₃ → Cu(NO₃)₂ + H₂O .
📐 Step-by-Step Calculation for Salt F
- Find moles/percentages of each element:
• Cu: 26.29 / 63.5 = 0.4140
• H: 2.49 / 1.0 = 2.4900
• N: 11.59 / 14.0 = 0.8279
• O: 59.63 / 16.0 = 3.7269 - Divide by the smallest value (0.4140):
• Cu: 0.4140 / 0.4140 = 1
• H: 2.4900 / 0.4140 = 6
• N: 0.8279 / 0.4140 = 2
• O: 3.7269 / 0.4140 = 9 - Deduce empirical formula: CuH₆N₂O₉ , which rearranges to the hydrated salt formula: Cu(NO₃)₂·3H₂O .
🧠 Top-Level Exam Strategy
To achieve Level 3 (5–6 marks), top responses explicitly detail all three reactions with clear equations, correct oxidation state changes for the disproportionation reaction, proper 3D stereochemical drawings (using solid lines, wedges, and dotted/hatched bonds for the octahedral complex), and a flawless empirical formula calculation showing clear ratios.
Topics
Module 2: Foundations in chemistry · Module 5: Physical chemistry and transition elements · Practical Activity Groups · 2.2 Electrons, bonding and structure · 5.2 Energy · 5.3 Transition elements · PAG 8: Electrochemical cells
Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.