Edexcel A-Level Chemistry Paper 1, June 2018: Question 6

10 marks · Medium difficulty · Open Response

Answer questions on the solubility, equilibrium constant, enthalpy of solution, and Le Chatelier's principle for metal hydroxides including magnesium hydroxide.

Practise this question

Question

Question 6 consists of five parts: (a) Multiple-choice question asking which metal hydroxide is most soluble in water: barium hydroxide, calcium hydroxide, magnesium hydroxide, or potassium hydroxide. (b) Given the dissolution equilibrium Mg(OH)2(s) <=> Mg2+(aq) + 2OH-(aq) and Kc = [Mg2+(aq)][OH-(aq)]^2: (i) State why Mg(OH)2 is omitted from Kc. (ii) Give the units for Kc. (iii) Table of lattice energy (-2842 kJ/mol) and hydration enthalpies (-1920 kJ/mol for Mg2+ and -460 kJ/mol for OH-), requiring calculation of the enthalpy change of solution. (iv) Multiple-choice question showing four concentration-time graphs for [Mg2+(aq)] to choose the correct curve reaching equilibrium. (v) Predict and justify in terms of equilibrium the effect of adding magnesium sulfate solution and dilute hydrochloric acid.
Question text

6 This question is about the solubility of metal hydroxides.

(a) Which of these metal hydroxides is the most soluble in water?

(1)

A barium hydroxide

B calcium hydroxide

C magnesium hydroxide

D potassium hydroxide

(b) When excess magnesium hydroxide is added to water and shaken, a saturated

solution is formed and the mixture reaches equilibrium.

Mg(OH) (s) Mg2+(aq) + 2OH−(aq)

The equilibrium constant, Kc, for this reaction is

K = [Mg2+(aq)][OH−(aq)]2

c

(i) Give a reason why the magnesium hydroxide is not included in the expression

for Kc.

(1)

(ii) Give the units for Kc.

(1)

(iii) Calculate the enthalpy change of solution of magnesium hydroxide, using the

following data.

Energy or enthalpy change Value / kJ mol−1

Lattice energy of Mg(OH)2(s) −2842

∆ H (Mg2+(aq)) −1920

hyd

∆ H (OH−(aq)) −460

hyd

(2)

*P52302RA01028*

(iv) Which graph shows the change in the concentration of the Mg2+(aq) ions when some

solid magnesium hydroxide is shaken with water and left to reach equilibrium?

(1)

[Mg2+(aq)] [Mg2+(aq)]

A /mol dm−3 B /mol dm−3

Time Time

[Mg2+(aq)] [Mg2+(aq)]

C /mol dm−3 D /mol dm−3

Time Time

(v) Predict the effect, if any, of adding each of the following to a saturated

solution of magnesium hydroxide in contact with solid magnesium hydroxide.

Justify your answers in terms of the effect on the equilibrium. 11

*P52302RA01128*Mg(OH)2(s) Mg2+(aq)+2OH−(aq)

(4)

Magnesium sulfate solution

Dilute hydrochloric acid

(Total for Question 6 = 10 marks)

Mark scheme

Show the mark scheme Mark scheme for Question 6: (a) D is correct (potassium hydroxide). (b)(i) Concentration of solid is constant/unchanged. (b)(ii) mol^3 dm^-9. (b)(iii) Enthalpy of solution = -1920 + 2(-460) - (-2842) = +2 kJ mol^-1 (2 marks). (b)(iv) D is correct (curve starting at origin, rate decreasing, plateauing). (b)(v) Magnesium sulfate: shifts left / more solid forms, because [Mg2+] increases. Hydrochloric acid: shifts right / more solid dissolves, because H+ reacts with/neutralises OH- ions (4 marks). Total 10 marks.

Question Answer

Mark

Number

6(a) The only correct answer is D (1)

A is not correct because it is the 2nd most soluble

B is not correct because it is the 3rd most soluble

C is not correct because it is the least soluble

Question

Acceptable Answer Additional Guidance Mark

Number

6(b)(i) An answer that makes reference to the following Allow magnesium hydroxide is in a different (1)

point: phase / state (from the aqueous ions)

the concentration of a solid / Mg(OH)2 is constant / Ignore solids do not appear in Kc

unchanged / changes very little expressions / just ‘it is solid’

Ignore solid does not affect the

concentration of the solution

Ignore it is a heterogeneous equilibrium

Ignore it is difficult to measure the

concentration of a solid

Do not award the solid does not have a

concentration

Question

Acceptable Answer Additional Guidance Mark

Number

6(b)(ii) Allow (1)

mol3 dm−9

dm−9 mol3

mol3/dm9

Ignore any working before the answer

Question

Acceptable Answer Additional Guidance Mark

Number

6(b)(iii) Example of calculation (2)

use of ∆ H = ∆ H[Mg2+(aq)] + 2∆ H[OH−(aq)] − ∆ H = −1920 + 2(−460) − (−2842)

sol hyd hyd sol

∆lattH[Mg(OH)2(s)] (1) Allow this shown on a Hess cycle

calculation of ∆ H (1) ∆ H = (+)2 (kJ mol−1)

sol sol

Allow 2000 J mol−1

Correct answer with no working scores 2

Mark

Number

6(b)(iv) The only correct answer is D (1)

A is not correct because it should not be linear and should level off

B is not correct because it should not increase in that way and should level off

C is not correct because it should not be horizontal

Question

Acceptable Answer Additional Guidance Mark

Number

6(b)(v) An answer that makes reference to the following points: Mark independently (4)

Addition of magnesium sulfate solution:

equilibrium position shifts to the left / in the backwards Allow more magnesium hydroxide

direction (1) precipitates / forms

because increased concentration / amount of Allow more Mg2+ ions present

magnesium ions / Mg2+((aq)) (1)

Addition of dilute hydrochloric acid:

equilibrium shifts to the right / in the forwards direction Allow more magnesium hydroxide

(1) dissolves / dissociates

because the hydrogen ions / H+((aq)) react with / Allow H+((aq)) + OH−((aq)) → H O((l))

neutralise / removes the hydroxide ions / OH−((aq)) (1)

Allow magnesium hydroxide reacts with /

is neutralised by acid / equation to show

this

Allow acid / HCl reacts with / neutralises /

removes hydroxide ions

Penalise reference to Kc changing once

only

(Total for Question 6 = 10 marks)

How to answer it

Solubility of Metal Hydroxides & Dynamic Equilibria

📋 What this question tests

This question assesses core concepts across inorganic chemistry, energetic cycles, and dynamic equilibrium principles:

  • Group 1 vs Group 2 trends: Comparing hydroxide solubilities across groups and down Group 2.
  • Heterogeneous equilibria: Constructing equilibrium constant expressions ( Kc ) involving pure solids and deriving correct units.
  • Enthalpy of solution ( ΔsolH ): Using Hess's Law cycles with lattice energy ( ΔlattH ) and enthalpies of hydration ( ΔhydH ).
  • Graphical analysis of equilibrium: Interpreting concentration changes over time as dissolution reaches saturation.
  • Le Chatelier's principle & the Common Ion Effect: Predicting shifts in position when ions are introduced or removed.
Question 6(a)

Group 1 vs Group 2 Hydroxide Solubility

Identifying the most soluble hydroxide [1 Mark]

✅ Correct Answer

D — potassium hydroxide

Award [1 mark] for selecting option D.

💡 Key Knowledge

  • Group 1 hydroxides (e.g. KOH, NaOH) are soluble, strongly alkaline ionic compounds that dissolve completely in water.
  • Group 2 hydroxides become more soluble down the group (Mg(OH)₂ is sparingly soluble, Ca(OH)₂ is slightly soluble, Ba(OH)₂ is moderately soluble).
  • Even the most soluble Group 2 hydroxide listed (barium hydroxide) is significantly less soluble than any Group 1 hydroxide.

❌ Common Errors

  • Choosing Ba(OH)₂ (Option A): Students remember that "solubility increases down Group 2" and mistakenly pick barium hydroxide without noticing that potassium belongs to Group 1.
Question 6(b)(i)

Heterogeneous Equilibrium Expressions

Reason for omitting Mg(OH)₂(s) from Kc [1 Mark]

✅ Correct Answer

The concentration of a solid / Mg(OH)₂ is constant (or unchanged / changes very little).

Award [1 mark] for referencing the constant concentration/density of a pure solid.

🧠 Exam Technique

The expression for the equilibrium constant Kc is incorporated with the constant concentration of the solid to give a solubility product:

Kc × [Mg(OH)₂(s)] = [Mg²⁺(aq)][OH⁻(aq)]²

Also allowed by the mark scheme: "Magnesium hydroxide is in a different phase / state from the aqueous ions."

❌ Common Errors & Examiner Guidance

  • Do NOT say: "The solid has no concentration." (Mark scheme explicitly directs: Do not award). Solids have a constant concentration determined by their density and molar mass.
  • Vague answers: Stating merely that "it is a solid" or "solids are not in Kc expressions" earns 0 marks because it simply restates the question.
Question 6(b)(ii)

Units of the Equilibrium Constant

Deducing units for Kc = [Mg²⁺][OH⁻]² [1 Mark]

✅ Correct Answer

mol³ dm⁻⁹ (or dm⁻⁹ mol³ )

Award [1 mark] for correct units with correct indices.

📐 Step-by-Step Derivation

  1. Substitute units of concentration into the expression:
    units = (mol dm⁻³) × (mol dm⁻³)²
  2. Expand powers:
    units = (mol dm⁻³) × (mol² dm⁻⁶)
  3. Combine terms by adding indices:
    mol¹⁺² dm⁻³⁺⁽⁻⁶⁾ = mol³ dm⁻⁹

❌ Common Errors

  • Forgetting to square the hydroxide term: resulting in mol² dm⁻⁶ instead of mol³ dm⁻⁹ .
  • Sign errors on indices (e.g. writing mol³ dm⁹ ).
Question 6(b)(iii)

Enthalpy Change of Solution Calculation

Determining ΔsolH for magnesium hydroxide [2 Marks]

📐 Step-by-Step Calculation

Step 1: Write the thermodynamic relationship

ΔsolH = ΣΔhydH(ions) − ΔlattH

Alternative Hess's Cycle view: Break lattice into gaseous ions ( −ΔlattH ), then hydrate them ( +ΔhydH ):

Mg(OH)₂(s) → Mg²⁺(g) + 2OH⁻(g) → Mg²⁺(aq) + 2OH⁻(aq)

Step 2: Substitute values (remember the 1:2 stoichiometry!)

ΔsolH = [ΔhydH(Mg²⁺) + 2 × ΔhydH(OH⁻)] − ΔlattH

ΔsolH = [−1920 + 2(−460)] − (−2842)

ΔsolH = [−1920 − 920] + 2842

ΔsolH = −2840 + 2842 = +2 kJ mol⁻¹

✅ Mark Breakdown

  • Mark 1: Correct expression / Hess cycle setup with factor of 2 included for OH⁻ hydration:
    −1920 + 2(−460) − (−2842)
  • Mark 2: Final answer of +2 (kJ mol⁻¹) (allow +2000 J mol⁻¹ ).
Correct answer alone without working scores full [2 marks].

❌ Common Errors & Pitfalls

  • Stoichiometry error: Forgetting to multiply ΔhydH(OH⁻) by 2. (Gives +462 kJ mol⁻¹ , losing both marks).
  • Sign convention trap: Subtracting negative lattice energy means adding 2842. Getting confused with signs often gives −5682 kJ mol⁻¹ .
  • Missing sign: Enthalpy values must strictly include a sign ( +2 ). While positive values without sign are occasionally tolerated, standard convention requires + .
Question 6(b)(iv)

Graphical Rate & Dynamic Equilibrium

Selecting the graph for [Mg²⁺] approaching equilibrium [1 Mark]

✅ Correct Answer

D — Curved increase leveling off horizontally

Award [1 mark] for selecting option D.

🧠 Graph Analysis

  • At t = 0 , pure water contains no dissolved magnesium ions, so the curve must start at [Mg²⁺] = 0 .
  • Dissolution rate is initially fastest (steepest gradient) and decreases as reverse precipitation rate increases.
  • When dynamic equilibrium is established (saturated solution), rate of dissolution = rate of precipitation, so [Mg²⁺] becomes constant (plateaus to a horizontal line).

❌ Why Other Options Are Incorrect

  • A: Shows a straight line that never reaches equilibrium.
  • B: Shows rate increasing with time (exponential/accelerating), which is physically impossible here.
  • C: Shows concentration already at a constant value from t = 0 , ignoring the dissolution process.
Question 6(b)(v)

Le Chatelier's Principle & Perturbing Equilibrium

Predicting and justifying shifts in equilibrium [4 Marks]

Mg(OH)₂(s) ⇌ Mg²⁺(aq) + 2OH⁻(aq)

✅ Addition of Magnesium Sulfate Solution (2 Marks)

  • Equilibrium Shift: Equilibrium position shifts to the left / backwards direction (or more Mg(OH)₂ precipitates / forms). [1 Mark]
  • Justification: Because of an increase in concentration / amount of Mg²⁺ ions in solution. [1 Mark]

✅ Addition of Dilute Hydrochloric Acid (2 Marks)

  • Equilibrium Shift: Equilibrium position shifts to the right / forwards direction (or more Mg(OH)₂ dissolves / dissociates). [1 Mark]
  • Justification: The H⁺ ions react with / neutralise / remove OH⁻ ions ( H⁺ + OH⁻ → H₂O ). [1 Mark]

🧠 Exam Technique: Two-Part Answers

For each reagent, you must provide:

  1. The direction of shift (left vs right / forward vs backward / dissolve vs precipitate).
  2. The exact ion responsible and what happens to its concentration.

These points are marked independently, so you can still gain the shift mark even if your explanation is incomplete.

❌ Critical Examiner Warning

  • Do NOT claim Kc changes! The mark scheme states: "Penalise reference to Kc changing once only." Kc is a constant at constant temperature; adding reagents shifts the position of equilibrium, not the equilibrium constant.
  • Sulfate misconception: Mentioning that sulfate forms a precipitate with magnesium loses credit; magnesium sulfate is soluble. The added ion is Mg²⁺.

Topics

Inorganic Chemistry · Physical Chemistry · Topic 4: Inorganic Chemistry and the Periodic Table · Topic 10: Equilibrium I · Topic 11: Equilibrium II · Topic 13: Energetics II

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.