WJEC A-Level Chemistry Unit 3, June 2025: Question 2

1 mark · Easy difficulty · Short Answer

State why silicon tetrachloride reacts vigorously with water whereas carbon tetrachloride does not.

Practise this question

Question

Examination question 2: 'Give a reason why SiCl4 reacts vigorously with water whilst CCl4 does not. [1]', followed by two blank dotted answer lines.
Question text

2. Give a reason why SiCl4 reacts vigorously with water whilst CCl4 does not. [1]

Mark scheme

Show the mark scheme Mark scheme for question 2 awarding 1 mark for: 'silicon has available d-orbitals / d-orbitals in outer shell but carbon does not (so water can bond to silicon but not to carbon)'.

2 silicon has available d-orbitals / d-orbitals in outer shell but carbon

does not (so water can bond to silicon but not to carbon) 1 1

How to answer it

WJEC Chemistry A-Level • Unit 3 / Periodic Trends

Hydrolysis of Group 4 Tetrachlorides: SiCl₄ vs CCl₄

What this question tests

  • Understanding of the comparative reactivity and hydrolysis of Group 4 tetrachlorides.
  • Ability to relate electronic configurations and available orbitals ( 3d subshell) to reaction mechanisms.
  • Formulating concise comparative explanations (identifying what silicon has that carbon lacks).

Question 2

Hydrolysis mechanism comparison • [1 Mark]

✅ Correct Answer

Any statement clearly expressing both parts of the comparison:

  • Silicon has available (empty/accessible) d-orbitals in its outer valence shell, whereas carbon does not.
  • Optional supplementary context: This allows a lone pair from a water molecule ( H₂O ) to form a dative covalent bond to the silicon atom, initiating hydrolysis.

💡 Key Knowledge

  • Carbon (Period 2): Valence configuration is 2s² 2p² . The second quantum shell ( n=2 ) has no d-orbitals. Carbon cannot expand its octet to accommodate an incoming nucleophile.
  • Silicon (Period 3): Valence configuration is 3s² 3p² 3d⁰ . It has low-lying, unoccupied 3d orbitals available for coordinate bonding.
  • Reaction equation:
    SiCl₄(l) + 2H₂O(l) → SiO₂(s) + 4HCl(g)
    (Produces white fumes of HCl and a white precipitate of hydrated SiO₂ ).

🧠 Exam Technique

  • Always state the comparison: The question asks why SiCl₄ reacts whilst CCl₄ does not. Mentioning silicon alone can risk losing the mark on strict schemes—always write "Silicon has... but carbon does not".
  • Specify the orbital type: You must explicitly write "d-orbitals". Vague phrases like "empty orbitals" or "higher energy levels" are not awarded.
  • Use standard keywords: "Available / accessible / low-lying d-orbitals" or "d-orbitals in the valence/outer shell".

❌ Common Errors

  • Focusing on bond polarity or bond energy: Stating that the "Si–Cl bond is weaker" or "C–Cl bond is non-polar" misses the kinetic/mechanistic reason entirely. Both molecules are non-polar overall due to tetrahedral symmetry, but bond polarity does not explain the dramatic difference in reactivity.
  • Ignoring carbon: Giving an incomplete answer such as "Silicon has d-orbitals" without explicitly contrasting it with carbon.
  • Claiming C has full d-orbitals: Stating that carbon's d-orbitals are full (carbon has no d-orbitals at all in the n=2 shell).
Mark Scheme Breakdown:
[1 Mark] — Silicon has available d-orbitals / d-orbitals in outer shell but carbon does not (so water can bond / coordinate to silicon but not to carbon).

Topics

Inorganic Chemistry · 3.3 Chemistry of the p-block

Question and mark scheme from the WJEC A-Level Chemistry examination, Unit 3, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.