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A-Level Chemistry May/June 2025 Q5(b)(i): Deduce the number of sigma (ơ) bonds and pi (π) bonds present in Z. number of o bonds n…
A-Level Chemistry · Paper 9701/23 · May/June 2025 · Question 5(b)(i) · [2 marks]
Deduce the number of sigma (ơ) bonds and pi (π) bonds present in Z. number of o bonds number of π bonds
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
The structure of Z is assumed to be ethyl acrylate, CH₂=CHCOOCH₂CH₃, which has the required number of bonds.
To find the number of sigma (σ) and pi (π) bonds, we count them from the full displayed formula.
1. Sigma (σ) bonds: Every single bond is a σ bond, and every double bond contains one σ bond.
- Count all single bonds (C-H, C-C, C-O):
- C-H bonds: 2 + 1 + 2 + 3 = 8
- C-C single bonds: 2
- C-O single bonds: 2
- Count the σ component of each double bond:
- One C=C bond contributes 1 σ bond.
- One C=O bond contributes 1 σ bond.
Total σ bonds = (C-H) + (C-C) + (C-O) + (σ from C=C) + (σ from C=O) Total σ bonds =
2. Pi (π) bonds: Every double bond contains one π bond.
- The molecule contains one C=C double bond, which is 1 π bond.
- The molecule contains one C=O double bond, which is 1 π bond.
Total π bonds =
Final Answer:
number of σ bonds 14
number of π bonds 2
How the marks are awarded
- B1 — Awarded for correctly deducing there are 14 sigma (σ) bonds by systematically counting all single bonds and one bond from each double bond in the molecule.
- B1 — Awarded for correctly deducing there are 2 pi (π) bonds by identifying the two double bonds (C=C and C=O) present in the molecule.
Common mistakes
- Miscounting the number of C-H bonds, especially on the ethyl group, leading to an incorrect sigma bond count.
- Forgetting to include the sigma bonds within the C=C and C=O double bonds, leading to an answer of 12 σ bonds.
- Incorrectly counting a double bond as two sigma bonds instead of one sigma and one pi bond.
- Only counting the pi bond from the C=C double bond and ignoring the one in the C=O group, leading to an answer of 1 π bond.
Examiner tip: To find the number of sigma and pi bonds, always draw the full displayed formula of the molecule and remember that every single bond is a sigma bond, every double bond is one sigma and one pi bond, and every triple bond is one sigma and two pi bonds.
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- Count all single bonds (C-H, C-C, C-O):
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