OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2022: Question 14

1 mark · Medium difficulty · Multiple Choice

Identify which given chemical reactions form a product with non-polar molecules from a multiple-choice list.

Practise this question

Question

Multiple choice question 14 asking which reaction(s) form a product with non-polar molecules. The reactions are: 1, 2CO + O2 -> ; 2, Si + 2Cl2 -> ; 3, S + 3F2 -> . Options A, B, C, and D provide different combinations of these reaction numbers, and a box is provided for the final answer.
Question text

14 The three reactions below each form one product only.

Which reaction(s) form(s) a product with non-polar molecules?

1 2CO + O2

2 Si + 2Cl2

3 S + 3F2

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is A for question number 14, carrying 1 mark.

14 A 1 1.2

How to answer it

Identifying Non-Polar Molecules in Synthesis Reactions

What this question tests

This question assesses your understanding of covalent bonding, electronegativity differences, molecular shapes (VSEPR theory), and molecular symmetry to determine overall molecular polarity across multiple inorganic reactions.

Question 14 Analysis

Exam Breakdown & Correct Answer

✅ Correct Answer: A (1, 2 and 3)

The correct option is A, meaning all three reactions produce molecules that are entirely non-polar.

💡 Key Knowledge: Polarity vs Symmetry

  • A bond is polar if there is a significant difference in electronegativity between the bonded atoms.
  • A molecule is non-polar if individual bond dipoles cancel out completely due to high molecular symmetry.

🧠 Exam Technique

Do not just look for polar bonds and stop. Always predict the 3D shape and consider whether bond dipoles oppose and cancel each other symmetrically.

Detailed Breakdown of the Three Reactions

Reaction 1: 2CO + O₂ → 2CO₂

Product formed: Carbon dioxide ( CO₂ )

Analysis: Although C=O bonds are polar due to the electronegativity difference between carbon and oxygen, CO₂ is a linear molecule. The two equal and opposite bond dipoles cancel each other out perfectly, making the overall molecule non-polar.

Reaction 2: Si + 2Cl₂ → SiCl₄

Product formed: Silicon tetrachloride ( SiCl₄ )

Analysis: Silicon and chlorine have different electronegativities, so the Si-Cl bonds are polar. However, SiCl₄ adopts a symmetrical tetrahedral shape. The four bond dipoles pull equally in opposite directions and cancel out, resulting in a non-polar molecule.

Reaction 3: S + 3F₂ → SF₆

Product formed: Sulfur hexafluoride ( SF₆ )

Analysis: Sulfur-fluorine bonds are highly polar. However, SF₆ has an octahedral geometry. Because it is completely symmetrical with opposite dipoles directly opposing one another, the individual bond moments cancel out, rendering the molecule non-polar.

Common Errors & Examiner Insights

❌ Common Misconceptions

  • The "Polar Bonds = Polar Molecule" Trap: Many students see polar bonds (like C=O or Si-Cl) and automatically assume the molecule must be polar. You must factor in molecular geometry and vector cancellation.
  • Misinterpreting Equation 1: Some students forget that carbon monoxide reacts with oxygen to form CO₂ (one product only), rather than remaining as CO.
Examiner Note: This question successfully discriminates between students who memorise surface-level rules and those who can apply VSEPR theory and vector addition of dipoles to 3D shapes. Full marks require recognising that symmetrical shapes ( linear , tetrahedral , octahedral ) cancel out polar bonds.

Topics

Module 2: Foundations in chemistry · 2.2 Electrons, bonding and structure

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.