OCR A-Level Chemistry Synthesis and analytical techniques (02), November 2021: Question 4

1 mark · Easy difficulty · Multiple Choice

Identify the correct balanced equation for the incomplete combustion of butan-1-ol producing carbon monoxide and water.

Practise this question

Question

Multiple choice question 4 asking for the correct equation for the incomplete combustion of butan-1-ol. Four options A, B, C, and D show different balanced or unbalanced equations involving butan-1-ol, oxygen, carbon monoxide, water, or hydrogen gas.
Question text

4 What is the correct equation for the incomplete combustion of butan-1-ol?

A CH3CH2CH2CH2OH + 1½O2 4CO + 5H2

B CH3CH2CH2CH2OH + 2O2 4CO + 5H2

C CH3CH2CH2CH2OH + 4O2 4CO + 5H2O

D CH3CH2CH2CH2OH + 4½O2 4CO + 5H2O

Your answer

[1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is C, worth 1 mark.

4 C 1 AO2.6

How to answer it

Incomplete Combustion of Butan-1-ol

OCR A-Level Chemistry • Multiple Choice Question

What this question tests

This question assesses your ability to recall, construct, and balance chemical equations for the incomplete combustion of alcohols, specifically accounting for carbon monoxide (CO) formation alongside water (H₂O), while carefully tracking the balancing of carbon, hydrogen, and oxygen atoms.

Question 4 Analysis

Balancing an Incomplete Combustion Reaction

✅ Correct Answer: C

CH₃CH₂CH₂CH₂OH + 4O₂ ➔ 4CO + 5H₂O

Option C correctly provides both the products of incomplete combustion (carbon monoxide and water) and balances every single atom on both sides of the equation.

💡 Key Knowledge

  • Complete combustion produces carbon dioxide (CO₂) and water (H₂O).
  • Incomplete combustion produces carbon monoxide (CO) or soot (C) alongside water (H₂O).
  • Always balance the carbon atoms first using the carbon chain length, then hydrogen atoms via water, and finally oxygen last.

🧠 Exam Technique

Elimination is key in multiple-choice questions:

  • Check products first: Incomplete combustion yielding CO must have H₂O as the hydrogen product, not hydrogen gas ( H₂ ). This immediately eliminates options A and B.
  • Check oxygen balancing: Butan-1-ol has 1 oxygen atom + 8 from 4O₂ = 9 oxygen atoms on the left. On the right, 4CO (4 oxygen) + 5H₂O (5 oxygen) = 9 oxygen atoms. Perfect balance!

❌ Common Errors

  • Mistake 1: Choosing options A or B because they list elemental hydrogen ( H₂ ) instead of water ( H₂O ). Combustion reactions always produce oxides of the elements involved.
  • Mistake 2: Failing to account for the oxygen atom already present within the alcohol functional group ( -OH ), leading to incorrect balancing of O₂ .
Mark Scheme Allocation: 1 mark awarded for selecting C (AO2.6 - Applying chemical knowledge to balancing equations).

Topics

Module 4: Core organic chemistry · 4.2 Alcohols, haloalkanes and analysis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.