OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2023: Question 15

1 mark · Medium difficulty · Multiple Choice

Identify which of the given ions contain bond angles of approximately 120 degrees.

Practise this question

Question

Multiple choice question 15 asking which ion(s) contain(s) bond angles of approximately 120 degrees, with three options: 1 for CH3COO-, 2 for C6H5O-, and 3 for (CH3)3C+. The four answer choices are A (1, 2 and 3), B (Only 1 and 2), C (Only 2 and 3), and D (Only 1). A box for the student's answer is shown at the bottom.
Question text

15 Which ion(s) contain(s) bond angles of approximately 120º?

1 CH COO–

2 C H O–

3 (CH ) C+

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 for question 15 showing the correct answer is A.

15 A 1 AO1.2

Total 15

SECTION B

How to answer it

Analysing Bond Angles in Organic Ions

What this question tests

This question assesses your ability to apply Electron Pair Repulsion (EPR) theory to complex organic ions (carboxylate, phenoxide, and carbocation species). You must determine the shape and associated bond angles around specific carbon and oxygen atoms by counting bonding pairs and lone pairs of electrons.

Question 15 Multiple Choice

Full Breakdown & Solutions

✅ Correct Answer: A (1, 2 and 3)

All three listed ions contain carbon or oxygen atoms surrounded by regions of electron density that result in bond angles of approximately 120°.

Mark Awarded: 1 / 1 (AO1.2)

💡 Key Knowledge: Trigonal Planar Geometry

  • Trigonal planar geometry yields a bond angle of 120°.
  • This occurs when a central atom has 3 regions of electron density (bonding pairs) and 0 lone pairs.
  • Double bonds count as a single region of electron density for shape determination under VSEPR theory.

🧠 Exam Technique

Break down each ion individually rather than guessing. Draw out or mentally visualise the skeletal/displayed structure of each chemical species to count the bonding pairs around the central atoms in question.

❌ Common Errors

Students often miscount lone pairs on oxygen or assume that because a carbon atom is bonded to three methyl groups (like in a carbocation), it must be tetrahedral (109.5°). Remember to check hybridization and charge!

Step-by-Step Analysis of Each Ion

1. Ethanoate ion ( CH₃COO⁻ )

The carboxylate carbon is bonded to a methyl group (C-C), a single-bonded oxygen (C-O⁻), and a double-bonded oxygen (C=O). This gives 3 regions of electron density around the carbon atom with no lone pairs, creating a trigonal planar arrangement with a bond angle of 120°.

2. Phenoxide ion ( C₆H₅O⁻ )

The carbon atoms in the delocalised benzene ring each have 3 regions of electron density and exhibit trigonal planar geometry at 120°. Furthermore, the oxygen atom bonded to the ring is involved in delocalisation, and the C-O-H (or in this case C-O⁻) framework lies within a planar arrangement associated with the aromatic system.

3. Tert-butyl carbocation ( (CH₃)₃C⁺ )

The positively charged carbon atom is bonded to three methyl groups with zero lone pairs. This results in 3 bonding regions around the central C⁺ atom, forcing it into a trigonal planar shape with bond angles of exactly 120° (contrasting with tetrahedral sp³ carbons).

Topics

Module 2: Foundations in chemistry · Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · 2.2 Electrons, bonding and structure · 4.1 Basic concepts and hydrocarbons · 6.1 Aromatic compounds, carbonyls and acids

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.