Edexcel A-Level Chemistry Paper 1, June 2023: Question 7

13 marks · Hard difficulty · Open Response

Assess chromium and chromium compounds by defining isotopes, writing electron configurations, explaining ionisation energies and radii, discussing the acidity of complex ions, and evaluating cell potential feasibility.

Practise this question

Question

Exam question about chromium and chromium compounds. Part (a) asks to define isotopes given four chromium isotopes. Part (b) asks to complete the electronic configuration of Cr2+ and explain using a table of atomic numbers, ionisation energies, and metallic radii whether chromium or calcium more easily forms a +2 ion. Part (c) asks for the colour of the [Cr(H2O)6]3+ complex ion and to explain why its aqueous solution is acidic. Part (d) provides standard electrode potentials for dichromate and chlorine half-cells and asks to comment on a statement regarding the feasibility of oxidising chloride ions.
Question text

7 This question is about chromium and chromium compounds.

(a) Naturally occurring chromium has four isotopes, 50Cr, 52Cr, 53Cr and 54Cr.

State what is meant by the term isotopes.

(2)

(b) Both chromium and calcium can form ions with a +2 charge.

(i) Complete the electronic configuration of a Cr2+ ion.

(1)

1s2

(ii) Explain which of chromium or calcium most easily forms a +2 ion using all of

the data in the table.

Atomic 1st ionisation energy 2nd ionisation energy Metallic radius

Element –1 –1

number / kJ mol / kJ mol / nm

Chromium 24 653 1592 0.129

Calcium 20 590 1145 0.197

(3)

(c) Chromium(III) sulfate, Cr2(SO4)3, dissolves in water to form the

complex ion [Cr(H O)6]3+(aq).

(i) State the colour of this complex ion.*P71912A01528*

(1)

(ii) Explain why the aqueous solution of this complex ion has an acidic pH by

considering the interaction between the metal ion and the ligands.

(2)

(d) A student researching the role of dichromate(VI) ions, Cr O2– , as an oxidising

agent made the statement shown.

‘Standard electrode potential data shows that it is never feasible

for a 1.00 mol dm–3 solution of potassium dichromate(VI) to oxidise

the chloride ions in hydrochloric acid.’

Comment on this statement using the data and equilibria shown.

Equilibrium 1

Cr O2–(aq) + 14H+(aq) + 6e– 2Cr 3+(aq) + 7H O(l) E d = +1.33 V

27 2

Equilibrium 2

Cl (aq) + 2e– 2Cl–(aq) E d = +1.36 V

(4)

… *P71912A01628*

(Total for Question 7 = 13 marks)

Mark scheme

Show the mark scheme Mark scheme providing detailed points for each part of the question. Part (a) awards marks for atoms with the same number of protons and different numbers of neutrons. Part (b)(i) gives the electron configuration for Cr2+. Part (b)(ii) awards marks for comparing sum of first two ionisation energies, nuclear charge, and atomic radius. Part (c)(i) gives green. Part (c)(ii) awards marks for polarisation of O-H bonds by Cr3+ and release of hydrogen ions. Part (d) awards marks for calculating E_cell = -0.03V, noting that changing conditions can make it positive, considering non-standard conditions, and shifting equilibrium positions.

7(a) An answer that makes reference to the following points (2)

• (atoms with) the same atomic number / number of protons (1) Ignore atoms of the same element

Ignore comments about same number of

electrons

Do not award compounds

• but with a different mass number / number of neutrons (1) Do not award different number of

electrons

Note

Allow both M1 and M2 in terms of Cr

i.e.

M1 Cr has 24 protons

M2 but may have 26, 28, 29 or 30 /

different numbers of neutrons

Question

Answer Additional Guidance Mark

Number

7(b)(i) An answer that makes reference to the following point Allow numbers of electrons as subscripts (1)

but not as large numbers

• (1s2)2s22p63s23p63d4 Allow the 2p6 written as 2p 22p 22p 2 and

x y z

similar for 3p6

Ignore 1s2 repeated

Ignore 4s0

Question

Answer Additional Guidance Mark

Number

7(b)(ii) An explanation that makes reference to the following points: Allow reverse arguments (3)

Ignore references to group / period trends

• calcium forms a +2 ion more readily as (the sum of) its first and second Allow it takes 1735 (kJ mol−1) to form

ionisation energies is lower (1) Ca2+ but 2245 (kJ mol−1) to form Cr2+ so

calcium forms a +2 ion most easily

Allow just ‘the ionisation energies of

calcium are lower so it forms a +2 ion

more easily’

• as (calcium’s atomic number is lower so) its nuclear charge is less Allow calcium atoms have fewer protons

or Ignore shielding

(calcium’s metallic / atomic radius is greater so) its outer / valence

electrons are further from the nucleus (1)

• so the outer / valence electrons experience weaker attraction from the

nucleus (1)

Question

Answer Additional Guidance Mark

Number

7(c)(i) (1)

• green Allow violet

Ignore shades of colours e.g. pale / light

Do not award mixed colours e.g. blue-green

Question

Answer Additional Guidance Mark

Number

7(c)(ii) An explanation that makes reference to the following points Mark independently (2)

• the (highly charged, small) Cr3+ ion weakens / polarises / Allow Cr3+ forms dative bonds with the O (of H O) so

distorts the O-H bonds (in the water ligands) (1) the H is less strongly attached

• allowing some water ligands to release hydrogen ions / to Accept M2 for correct equation

form the oxonium ion [Cr(H O) ]3+ + H O ⇌ [Cr(H O) OH]2+ + H O+

26 2 2 5 3

or Allow [Cr(H O) ]3+ ⇌ [Cr(H O) OH]2+ + H+

26 2 5

water molecule (from the solution) deprotonates (one of) the +

Allow equations forming [Cr(H2O)4(OH)2]

water ligands (1)

Allow just ‘deprotonation of water ligand occurs’

Allow H+ ions released / donated from water ligands

Ignore formation of Cr(H2O)3(OH)3 and any

chromium anions

Ignore an acid-base reaction occurs

Ignore chromium(III) hydroxide is amphoteric

Ignore reference to sulfate ions forming sulfuric acid

Do not award reference to ligand exchange

Question

Answer Additional Guidance Mark

Number

7(d) An answer that makes reference to the following points: (4)

• Eo = −0.03V and so reaction / oxidation of chloride o

cell Allow E cell = +0.03V for the reduction of chlorine (so

ions is not feasible (under standard conditions) (1) oxidation of chloride not feasible)

Allow Eqm 1 Eo is more negative / less positive

Allow proof using anti-clockwise rule

• (this is small so) changing the conditions may make Allow Eo is close to 0V / only just negative (so some Cl−

cell

Eo positive so the oxidation reaction becomes feasible

cell ions are oxidised)

(1)

• (it is possible under non-standard conditions if you) Allow increase the concentration of H+ / Cl−

increase the concentration of HCl / H+ / Cl‒ (1) Ignore increasing the concentration of Cr O 2− ions

Allow changing / increasing / decreasing the temperature

• so equilibrium 1 moves to RHS Allow so eqm 1 Eo becomes more positive / better at

or accepting electrons

equilibrium 2 moves to LHS Allow eqm 2 Eo becomes more negative / better at donating

(so oxidation of Cl− ions is more likely / feasible) (1) electrons

Allow Cl2 that forms escapes driving eqm 2 to LHS

Allow increasing the concentration of Cr O 2−ions moves

equilibrium 1 to RHS (so oxidation of Cl− ions is more likely

/ feasible)

(Total for Question 7 = 13 marks)

How to answer it

Chromium and Chromium Compounds Study Guide

Edexcel A-Level Chemistry — Exam Question Breakdown

What this question tests

This comprehensive question assesses core inorganic and physical chemistry concepts across transition metal chemistry:

  • Fundamental atomic structure definitions (isotopes).
  • Transition metal and s-block electronic configurations (specifically testing anomalous transition metal configurations like Cr).
  • Ionisation energy trends and nuclear charge arguments to explain reactivity.
  • Transition metal complex ion colours and complex ion acidity via hydrolysis/polarisation of water ligands.
  • Electrochemical cell potentials (E-theta values), feasibility under standard conditions, and applying Le Chatelier's principle to non-standard conditions.

Part (a): Isotopes Definition

State what is meant by the term isotopes. (2 marks)

✅ Correct Answer

Atoms of the same element (or atoms with the same atomic number / number of protons) with a different mass number / number of neutrons.

❌ Common Errors

Students frequently lose marks by referring to "atoms of the same mass" or confusing isotopes with ions. Do not mention electrons or compounds when defining isotopes.

🎯 Mark breakdown: 1 mark for mentioning same protons / atomic number; 1 mark for different neutrons / mass number.

Part (b): Electronic Configurations and Ionisation Energies

(i) Complete the electronic configuration of a Cr²⁺ ion. (1 mark)

1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁴ (or 3d² 4s² configuration modified correctly).

(ii) Explain which of chromium or calcium most easily forms a +2 ion using all of the data in the table. (3 marks)

💡 Key Knowledge

To form a 2+ ion, an atom must lose two electrons. This requires the input of the 1st + 2nd ionisation energies. Calcium requires 590 + 1145 = 1735 kJ mol⁻¹, whereas Chromium requires 653 + 1592 = 2245 kJ mol⁻¹.

🧠 Exam Technique

Structure your answer logically:
1. Compare the total ionisation energies (sum of 1st and 2nd IE).
2. Link calcium's lower IE to its smaller atomic number / lower nuclear charge / greater atomic radius.
3. Conclude that outer electrons experience weaker attraction in calcium, making it easier to form the +2 ion.

🎯 Mark breakdown: 1 mark for stating calcium's sum of 1st and 2nd IE is lower; 1 mark for referencing lower nuclear charge / larger atomic radius; 1 mark for linking this to weaker attraction on outer electrons.

Part (c): Complex Ions and Acidity

(i) State the colour of this complex ion [Cr(H₂O)₆]³⁺. (1 mark)

✅ Correct Answer

Green (Allow violet; ignore qualifiers like 'pale' or 'dark').

(ii) Explain why the aqueous solution of this complex ion has an acidic pH by considering the interaction between the metal ion and the ligands. (2 marks)

✅ Correct Answer

The highly charged, small Cr³⁺ ion strongly polarises/distorts the O-H bonds in the coordinated water ligands. This weakens the O-H bonds, allowing water molecules to release hydrogen ions (protons) into solution (or undergo deprotonation).

❌ Common Errors

Do not talk about simple ligand exchange. The key mechanism here is metal-ion polarisation of coordinated water leading to proton release (hydrolysis).

🎯 Mark breakdown: 1 mark for stating Cr³⁺ polarises/weakens O-H bonds in water ligands; 1 mark for explaining that this allows H⁺ ions to be released/donated.

Part (d): Electrode Potentials and Feasibility

Comment on the statement that it is never feasible for a 1.00 mol dm⁻³ solution of potassium dichromate(VI) to oxidise chloride ions in hydrochloric acid using the data and equilibria shown. (4 marks)

💡 Key Knowledge & Calculations

Under standard conditions:
Equilibrium 1: Cr₂O₇²⁻(aq) + 14H⁺(aq) + 6e⁻ ⇌ 2Cr³⁺(aq) + 7H₂O(l) (E-theta = +1.33 V)
Equilibrium 2: Cl₂(aq) + 2e⁻ ⇌ 2Cl⁻(aq) (E-theta = +1.36 V)
E-theta(cell) = +1.33 - (+1.36) = -0.03 V.

🧠 Exam Technique

A top-level response must address both standard and non-standard conditions:
1. Calculate/state that E-theta cell is negative (-0.03 V), so it is not feasible under standard conditions.
2. Note that the value is very close to zero.
3. Apply Le Chatelier's principle: increasing reactant concentrations (H⁺ or Cl⁻) shifts equilibrium 1 right and equilibrium 2 left.
4. Conclude that changing conditions makes E-cell positive, rendering the oxidation feasible.

🎯 Mark breakdown: 1 mark for standard E-cell calculation/conclusion (-0.03V, not feasible under standard conditions); 1 mark for noting the value is small; 1 mark for stating changing conditions can make E-cell positive; 1 mark for explaining the shift in equilibrium (moving eqm 1 to RHS / increasing H⁺ or Cl⁻ concentration).

Topics

Physical Chemistry · Inorganic Chemistry · Topic 1: Atomic Structure and the Periodic Table · Topic 14: Redox II · Topic 15: Transition Metals

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.