Edexcel A-Level Chemistry Paper 3, June 2022: Question 2

12 marks · Medium difficulty · Short Open Response

Explain the colour, octahedral shape, and crystallisation steps of ammonium cobalt(II) sulfate complex ions, and identify reasons for a percentage yield of less than 100%.

Practise this question

Question

Exam question about the preparation of ammonium cobalt(II) sulfate crystals via a four-step procedure. Part (a)(i) asks to explain why the [Co(H2O)6]2+ solution is coloured (4 marks). Part (a)(ii) asks to explain the shape of [Co(H2O)6]2+ using electron-pair repulsion theory (3 marks). Part (a)(iii) asks to give reasons for carrying out Steps 3 and 4, referring to the words in bold (3 marks). Part (b) asks to give two possible reasons other than incomplete reaction why the yield is 70.0% (2 marks).
Question text

2 Ammonium cobalt(II) sulfate is made by mixing aqueous solutions of

ammonium sulfate and excess cobalt(II) sulfate.

(a) Dry crystals of ammonium cobalt(II) sulfate, (NH4)2SO4·CoSO4·6H2O, are obtained

by the procedure shown.

Step 1 The reaction mixture is transferred to an evaporating basin, heated

gently and then left to crystallise.

Step 2 The crystals are separated by gravity filtration.

Step 3 The crystals are then rinsed with a small amount of ice‑cold water.

Step 4 The rinsed crystals are placed in a warm oven for 30 minutes.

(i) The colour of the cobalt(II) sulfate solution used is pink due to the complex

cobalt(II) ion, [Co(H O) ]2+.

Explain why the solution is coloured.

(4)

(ii) Explain the shape of the cobalt(II) ion, [Co(H O) ]2+ , using electron-pair repulsion theory.

(3)

(iii)Give the reasons for carrying out Steps*P67095A0436*3and4of the procedure, referring

particularly to the words in bold.

(3)

(b) The percentage yield of this reaction is 70.0%.

Give two possible reasons, other than an incomplete reaction, why the yield is

less than 100%.

(2)

(Total for Question 2 = 12 marks)

Mark scheme

Show the mark scheme Mark scheme providing detailed marking points for 2(a)(i) regarding d-orbital splitting and electron promotion; 2(a)(ii) regarding 6 dative bonds and octahedral shape; 2(a)(iii) for explaining the purpose of rinsing with ice-cold water and drying in a warm oven; and 2(b) for acceptable causes of lower percentage yield such as transfer losses or crystals remaining in solution.

Question

Answer Additional Guidance Mark

Number

2(a)(i) An explanation that makes reference to the following points: Penalise omission of ‘d’ once only (4)

Splitting

• (ligand / water molecule causes) d orbitals to split (into 2 Allow d subshell / shell for d orbitals

energy levels) (1) Do not award ‘a d orbital is split’

Do not award ‘electrons are split’

Absorption

• electrons absorb energy (in the visible region) / photons (of Allow energy / photons / light absorbed

visible light) (1)

Promotion

• to promote electrons (to higher d orbitals) Allow d-d transitions occur

or Allow electrons are excited / jump for promote

electrons move from lower to higher energy (d) orbitals / Ignore reference to electron(s) relaxing / dropping to

levels (1) ground state

Do not award d-s transitions

Colour

• the remaining light / unabsorbed light / complementary Allow reflected / emerged / seen

colour / pink light is transmitted (1) Do not award ‘emitted’

Question

Answer Additional Guidance Mark

Number

2(a)(ii) An explanation that makes reference to the following points: Pairs only needs to be mentioned once in M1 or (3)

M2

Allow areas of electron density for pairs of

electrons

Electron pairs

• 6 (dative) pairs of (bonding) electrons (around cobalt ion) (1) Allow 6 bond(ing) pairs

May be shown on diagram but dative bonds must

be between O and Co2+

Do not award mention of having any lone pairs

Minimise repulsion

• (electron / bond pairs) arranged in order to minimise Allow to maximise separation between electron /

repulsion (1) bond pairs or the electron / bond pairs are as far

apart as possible

Ignore equal repulsion between bond pairs

Ignore comments based on repulsion / separation

between bonds / atoms

Ignore comments on repulsion between bond

pairs and lone pairs

Shape

• so shape is octahedral (1) Allow 3-D diagram to show octahedral shape

Allow square based bipyramidal

Do not award octagonal

No TE on incorrect number of electron pairs

Ignore bond angles

Question

Answer Additional Guidance Mark

Number

2(a)(iii) An answer that makes reference to the following points: (3)

Rinsed

• rinsed to remove cobalt(II) sulfate (solution) (1) Allow to remove remaining solution

Allow to remove impurities that didn’t

crystallise

Allow just ‘to remove impurities’

Ignore to remove ammonium sulfate

Ignore to remove solvent

Do not award to remove insoluble impurities

Ice-cold water

• ice-cold water minimises / prevents ammonium cobalt(II) Allow the crystals are insoluble / less soluble

sulfate / crystals (re)dissolving (1) in cold water

Ignore to stop the reaction

Do not award to stop the crystals melting

Warm oven

• warm oven (rather than hot) to ensure water of crystallisation is Allow to dry crystals / remove water

not removed (during drying) Do not award to remove water of

or crystallisation / heat to constant mass

to stop the crystals melting (1)

Question

Answer Additional Guidance Mark

Number

2(b) An answer that makes reference to two of the following points: Allow e.g. crystals / salt / solid / product for (2)

ammonium cobalt(II) sulfate

• some ammonium cobalt(II) sulfate solution lost if it ‘spits’ out Do not award crystals evaporated for M1 only

of basin when heated (in Step 1) (1)

• some ammonium cobalt(II) sulfate remains in solution Allow the crystals weren’t left to crystallise for

(in Step 1) (1) long enough

• some ammonium cobalt(II) sulfate is soaked into the filter Allow just ‘solid is lost during filtration’

paper/ some ammonium cobalt(II) sulfate crystals remain on

filter paper (in Step 2) (1)

• transfer losses from reaction flask / beaker to evaporating basin Allow any type of specific transfer loss e.g.

/ from evaporating basin to filter funnel (in Steps 1 and 2) (1) some product left behind in the beaker / flask /

evaporating basin

• some water of crystallisation is lost during the drying process Allow crystals decompose during drying

(in Step 4) (1)

Allow some ammonium cobalt(II) sulfate

dissolves in ice-cold water (in Step 3)

Ignore formation of alternative product

Ignore reaction is reversible

(Total for Question 2 = 12 marks)

How to answer it

Preparation and Properties of Ammonium Cobalt(II) Sulfate

What this question tests

This question assesses core transition metal chemistry and practical preparation techniques. You will be tested on explaining d-block colour via d-orbital splitting, applying VSEPR theory to coordination numbers/shapes, justifying experimental steps in salt purification, and evaluating reasons for sub-100% percentage yields in laboratory preparations.

Question 2(a)(i) — Colour of Transition Metal Ions (4 Marks)

Explain why the solution is coloured.

✅ Correct Answer Structure

  • Splitting: The surrounding water ligands cause the partially filled d orbitals to split into two different energy levels.
  • Absorption: Electrons absorb energy (photons) in the visible region of the light spectrum.
  • d-d Transition / Promotion: Electrons are promoted from lower to higher energy d orbitals.
  • Transmission: The remaining unabsorbed (complementary) wavelengths of light are transmitted/seen (pink).

💡 Key Knowledge

Colour in transition metal complexes arises due to incomplete d-subshells. When ligands bond dative-covalently to the central metal ion, electrostatic repulsion splits the degenerate d orbitals into non-degenerate energy levels.

❌ Common Errors

  • Saying "d-orbitals are split" without mentioning that ligands cause it.
  • Stating that light is "emitted" or "reflected" instead of absorbed and transmitted.
  • Using imprecise terminology like "d-electrons are split" or "d-shells".
Mark scheme requirement: 4 distinct marking points corresponding to splitting, absorption, promotion, and transmitted colour.
Question 2(a)(ii) — VSEPR & Complex Shapes (3 Marks)

Explain the shape of the cobalt(II) ion, [Co(H₂O)₆]²⁺, using electron-pair repulsion theory.

✅ Correct Answer

  • There are 6 dative covalent (bonding) pairs of electrons around the central cobalt(II) ion.
  • These electron pairs arrange themselves as far apart as possible to minimise repulsion.
  • The resulting 3D geometry is octahedral.

🧠 Exam Technique

Always link the coordination number directly to the number of electron pairs surrounding the central metal atom. Use standard phrasing from electron-pair repulsion theory ("minimize repulsion").

Mark scheme requirement: 1 mark for stating 6 pairs, 1 mark for minimising repulsion, 1 mark for naming "octahedral".
Question 2(a)(iii) — Practical Chemistry & Purification (3 Marks)

Give reasons for carrying out Steps 3 and 4 of the procedure, referring particularly to the words in bold.

✅ Correct Answer Breakdown

  • Rinsed: To remove any residual soluble impurities or remaining cobalt(II) sulfate solution stuck to the crystal surfaces.
  • Ice-cold water: Ensures the ammonium cobalt(II) sulfate crystals do not dissolve (solubility is minimized at low temperatures).
  • Warm oven: Dries the crystals by evaporating surface water without being hot enough to decompose the product or melt the crystals / remove water of crystallization.

❌ Common Errors

  • Writing that ice-cold water "stops the reaction" (no reaction is occurring at this filtering stage).
  • Forgetting that washing with room-temperature water would dissolve away part of your yield.
Mark scheme requirement: One mark awarded for each accurately justified term (Rinsed, Ice-cold water, Warm oven).
Question 2(b) — Percentage Yield (2 Marks)

Give two possible reasons, other than an incomplete reaction, why the yield is less than 100%.

✅ Valid Answers (Any two)

  • Product solution lost due to "spitting" during heating/evaporation in Step 1.
  • Some product remains dissolved in solution because it wasn't left to crystallise long enough.
  • Crystals lost on the filter paper or transfer vessels during separation/pouring (transfer losses).
  • Product lost during the washing/rinsing step because it slightly dissolved in the ice-cold water.
  • Loss of water of crystallisation due to overheating in the warm oven.

🧠 Exam Technique

When asked for reasons for yield losses in crystallisation preparations, standard practical errors like transfer losses, spitting, and solubility in wash water are reliable answers.

Mark scheme requirement: 1 mark per valid practical loss mechanism (max 2 marks). Do not accept "reversible reaction" unless specified context allows, and strictly avoid "incomplete reaction" as it's excluded by the stem.

Topics

Inorganic Chemistry · Physical Chemistry · Topic 15: Transition Metals · Topic 2: Bonding and Structure

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.