Edexcel A-Level Chemistry Paper 1, June 2022: Question 7

14 marks · Hard difficulty · Calculations

Analyze chromium chemistry including ionisation energies, dot-and-cross diagrams, electrode potentials, cell calculations, d-orbital color explanations, and a titration calculation involving an iron nail.

Practise this question

Question

Question text

7 This question is about chromium and some of its compounds.

(a) The common oxidation numbers of chromium are +2, +3 and +6.

Give a reason, in terms of ionisation energies, why chromium can show variable

oxidation numbers.

(1)

(b) The bonding in chromate(VI) ions, CrO2–, is similar to that in sulfate(VI) ions, SO2–.

Draw a possible dot-and-cross diagram for a chromate(VI) ion.

(2)

potassium dichromate(VI).

Some standard electrode potentials are given in the table.

Right-hand electrode system d

E / V

2+ – –0.76

Zn (aq) + 2e Zn(s)

3+ – 2+ –0.41

Cr (aq) + e Cr (aq)

2– + – 3+ +1.33

Cr2O7 (aq) + 14H (aq) + 6e 2Cr (aq) + 7H2O(l)

(i) Write the overall equation for the reduction of dichromate(VI) ions to

chromium(III) ions by zinc in acid conditions.

State symbols are not required.

(2)

(ii) Calculate Ecell for the reaction in (c)(i).

(1)

*P67093RA02132*

(iii) Predict whether or not a further reduction of chromium(III) ions to

chromium(II) ions will occur. Justify your answer.

(1)

(iv)Aqueous solutions containing chromium(III) ions and chromium(II) ions have*P67093RA02232*

different colours.

Explain why these solutions differ in colour.

An explanation of the origin of the colours is not required.

(2)

(d) An iron nail was analysed using the following outline procedure.

• An iron nail was placed in a beaker and excess dilute sulfuric acid

was added.

• After all the iron had reacted to form iron(II) ions, the solution was made

up to 1.00dm in a volumetric flask.

• 25.0 cm3 portions of the solution were acidified and titrated with

–3

potassium dichromate(VI) solution of concentration 0.0167moldm .

Results

mass of iron nail = 3.54g

mean titre = cm15.503

The table shows the percentage by mass of iron in four different brands of nail.

Brand of nail Percentage by mass of iron

A 92

B 94

C 96

D 98

Potassium dichromate(VI) in acid solution oxidises iron(II) ions as shown in

the equation

2– + 2+ 3+ 3+

Cr2O7 + 14H + 6Fe → 2Cr + 6Fe + 7H2O

Determine, using the experimental data, the brand of nail that was analysed.*P67093RA02332*

(5)

(Total for Question 7 = 14 marks)

Mark scheme

Show the mark scheme

Question

Answer Additional Guidance Mark

Number

7(a) An answer that makes reference to the following point: (1)

Allow they / the (successive) ionisation

• there is only a gradual / steady increase in (successive energies are close in value / similar

ionisation energies)

Allow the extra ionisation energy to

increase oxidation state is similar to the

increase in hydration enthalpy / lattice

energy

Ignore chromium is a transition element

Ignore 3d (and 4s) orbitals have similar

energy

Ignore Cr is [Ar]3d54s1 so can lose 6

electrons

Ignore reference to electrons being

removed from the d-orbital

Question

Answer Additional Guidance Mark

Number

7(b) Examples of diagrams (2)

• 2 double bonds and 2 single bonds shown as dots

and crosses (1)

• Another 4 electrons around each oxygen involved in

the double bond and another 6 electrons around

each oxygen involved in the single bond with one

different symbol on each of two oxygens to indicate Penalise extra electrons on chromium

the extra electrons in the ion (1) In both examples, M2 is conditional on M1

Or Allow overlapping circles with electrons in

correct places

• 2 single bonds shown as dots and crosses

2 dative bonds with the electrons being donated from Ignore missing brackets and charge / shape

the chromium (1) Ignore lines representing covalent bonds

• another 6 electrons around each oxygen with one

different symbol on two of the oxygens to indicate

the extra electrons (1)

Question

Answer Additional Guidance Mark

Number

7(c)(i) Example of equation (2)

• correct species (1) Cr O 2− + 14H+ + 3Zn → 2Cr3+ + 7H O + 3Zn2+

27 2

Allow multiples

Allow ⇌ provided equation written in direction

shown

• balancing (1)

Ignore state symbols even if incorrect

Do not award uncancelled electrons

Question

Answer Additional Guidance Mark

Number

7(c)(ii) Example of calculation (1)

• calculation of Eo (Eo = 1.33 – (−0.76) )

cell cell

= (+) 2.09 (V)

Allow –2.09 (V) if equation written in reverse in (c) (i)

Correct answer with no working scores (1)

Question

Answer Additional Guidance Mark

Number

7(c)(iii) (1)

yes/zinc and acid will reduce chromium(III) ions

to chromium(II) ions

and because

Eo for the reaction between Zn and Cr3+ is (+) Allow positive or >0 if not calculated

cell

0.35 (V)

Zn2+ / Zn electrode potential / SEP / Eo value is Allow explanations in terms of the anti-

more negative / less positive / lower than the clockwise rule

Cr3+ / Cr2+ value

Zn/ Zn2+ electrode potential / SEP / Eo value is

less negative / more positive / higher than the

Cr3+ / Cr2+

Question

Answer Additional Guidance Mark

Number

7(c)(iv) An explanation that makes reference to the following (2)

points:

• the energy difference between the two sets of d Allow the d orbital energies are different

orbitals is different in the two ions / Cr3+ and Cr2+ Allow different charges / oxidation numbers

or alter the d orbital energies differently

there is different splitting of the d orbitals / d Do not award reference to a single d orbital

subshell (1) splitting/ d orbital splitting

Ignore references to charges/charge

density/oxidation numbers/electron

configurations of the ions

• electrons undergo different d-d transitions/ are Do not award references to electrons

promoted to a higher d-orbital absorbing/requiring a being excited and falling back to the

different amount of energy ground state (or words to that effect)

a different amount of energy is absorbed the Allow the frequency / wavelength of

frequency / wavelength/colour of (visible) light (visible) light transmitted / reflected is

absorbed is different different

(1) Do not award emitted instead of

absorbed

Ignore reference to different ligands

Question

Answer Additional Guidance Mark

Number

7(d) Example of calculation (5)

• calculation of mol of Cr O 2− (1) mol Cr O 2− used = (15.50 x 0.0167) ÷1000

27 2 7

= 2.5885 x 10−4 / 0.00025885 (mol)

• calculation of mol Fe2+ in 25.0 cm3 mol Fe2+ in 25.0 cm3 = 6 x 2.5885 x 10−4

(1) = 1.5531 x 10−3 = 0.0015531 (mol)

• calculation of mol Fe2+ in 1.00 dm3 mol Fe2+ in 1.00 dm3 = (1.5531 x 10−3 x 1000) /25

(1) = 6.2124 x 10−2 / 0.062124 (mol)

• calculation of mass of Fe in 1 nail mass of Fe = 6.2124 x 10−2 x 55.8 = 3.4665 (g)

(1) Allow 3.4789 (g) from Ar of 56

• calculation of percentage of iron Percentage of iron = (3.4665/3.54) x 100 (=97.924)= 98 / 97.9 (%)

and Allow 98 / 98.3 from Ar of 56

brand of nail (1) and

Brand D

Do not award for a percentage of 84% or below

Ignore SF except 1 SF

Correct answer with some relevant working scores 5

Correct percentage (98%) and brand (D) with no working scores

(1)

Any other percentage and brand with no working scores (0)

(Total for Question 7 = 14 marks)

How to answer it

Edexcel A-Level Chemistry: Chromium Chemistry Study Guide

What this question tests

This comprehensive question assesses transition metal chemistry, specifically: explaining variable oxidation numbers via successive ionisation energies, drawing dot-and-cross structures for complex ions, combining half-equations to calculate overall redox equations and standard cell potentials ( E↻cell ), predicting reaction feasibility using electrode potentials, explaining colour differences in transition metal ions based on d-orbital splitting, and performing a multi-step redox titration calculation involving iron and dichromate.

Part (a)

Variable Oxidation States & Ionisation Energies

✅ Correct Answer

There is only a gradual / steady increase in successive ionisation energies.

💡 Key Knowledge

Transition metals have inner 3d and 4s orbitals with very similar energy levels. Removing multiple electrons does not encounter a massive jump in energy (unlike breaking into a noble gas core), which allows transition elements to exhibit multiple stable oxidation states.

❌ Common Errors

Students often lose marks by stating vague answers like "chromium is a transition element" or discussing 3d/4s orbital energy similarities without linking them to ionisation energy values. The mark scheme explicitly requires reference to gradual successive ionisation energies.

Mark: 1 mark

Part (b)

Dot-and-Cross Diagram for Chromate(VI) Ion

✅ Correct Answer

Show 2 double bonds and 2 single bonds around the central chromium atom, with 4 extra electrons distributed correctly on the oxygen atoms to account for the 2- charge (or alternatively, 2 single bonds and 2 dative covalent bonds).

🧠 Exam Technique

When drawing oxoanions like CrO₄²⁻ , ensure you clearly distinguish between electrons originating from different atoms using dots and crosses. Clearly show the extra 2 electrons on the oxygen atoms, and penalise if extra electrons are incorrectly placed on the chromium center.

Marks: 2 marks

Part (c)(i)

Redox Equation for Dichromate Reduction

✅ Correct Answer

Cr₂O₇²⁻ + 14H⁺ + 3Zn → 2Cr³⁺ + 7H₂O + 3Zn²⁺

(State symbols are not required by the prompt).

🧠 Exam Technique

Derive this by combining the reduction half-equation for dichromate ( Cr₂O₇²⁻ + 14H⁺ + 6e⁻ ⇌ 2Cr³⁺ + 7H₂O ) with the oxidation half-equation for zinc ( Zn ⇌ Zn²⁺ + 2e⁻ ) multiplied by 3 to balance electrons. Ensure electrons cancel out completely.

Marks: 2 marks

Part (c)(ii)

Calculating Standard Cell Potential

📐 Calculation Step

Formula: E↻cell = E↻right - E↻left

E↻cell = 1.33 - (-0.76) = +2.09 V

❌ Common Errors

Forgetting to account for the double negative when subtracting a negative electrode potential, leading to an incorrect subtraction error ( 1.33 - 0.76 ).

Mark: 1 mark

Part (c)(iii)

Feasibility of Further Reduction

✅ Correct Answer

Yes. Zinc and acid will further reduce chromium(III) ions ( Cr³⁺ ) to chromium(II) ions ( Cr²⁺ ).

💡 Justification

The calculated E↻cell for the reaction between Zn and Cr³⁺ is positive ( +0.35 V ), meaning the reaction is thermodynamically feasible. Alternatively, explain that the Zn / Zn²⁺ electrode potential is more negative than the Cr³⁺ / Cr²⁺ potential.

Mark: 1 mark

Part (c)(iv)

Origin of Colour Differences in Chromium Ions

💡 Key Knowledge

1. The energy difference ( ΔE ) between the split sets of d-orbitals is different in Cr³⁺ compared to Cr²⁺ due to different oxidation numbers / charge densities.
2. Therefore, electrons undergo different d-d transitions, absorbing a different amount of energy (frequency/wavelength) of visible light.

❌ Common Errors

Students lose marks by talking about electrons being "excited and falling back to the ground state" and emitting light—remember, transition metal colours arise from absorption of visible light during d-d transitions, not emission!

Marks: 2 marks

Part (d)

Redox Titration & Iron Nail Analysis

📐 Step-by-Step Calculation

Moles of dichromate used:
(15.50 × 0.0167) / 1000 = 2.5885 × 10⁻⁴ mol
Moles of Fe²⁺ in 25.0 cm³ portion:
Using 1:6 stoichiometry ( Cr₂O₇²⁻ : 6Fe²⁺ ):
2.5885 × 10⁻⁴ × 6 = 1.5531 × 10⁻³ mol
Moles of Fe²⁺ in total 1.00 dm³ flask:
1.5531 × 10⁻³ × (1000 / 25.0) = 6.2124 × 10⁻² mol
Mass of Iron (Fe) in the nail:
6.2124 × 10⁻² × 55.8 = 3.4665 g
Percentage by mass & Brand Identification:
(3.4665 / 3.54) × 100 = 97.9%
Closest match in table: 98%, corresponding to Brand D.

🧠 Exam Technique & Traps

Reacting Ratios: Never forget to multiply by 6 when converting from dichromate to iron(II) moles based on the balanced stoichiometric equation!
Volumetric Scaling: Remember to scale up from the 25.0 cm³ titre aliquot to the full 1.00 dm³ volumetric flask volume ( × 1000 / 25 ).

Marks: 5 marks

Topics

Physical Chemistry · Inorganic Chemistry · Core Practicals · Core Practical 3: Find the concentration of a solution of hydrochloric acid · Core Practical 11: Find the amount of iron in an iron tablet using redox titration · Topic 1: Atomic Structure and the Periodic Table · Topic 2: Bonding and Structure · Topic 5: Formulae, Equations and Amounts of Substance · Topic 14: Redox II · Topic 15: Transition Metals

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.