IB DP Chemistry - Structure 3.2 Functional groups- IB Style Questions For HL Paper 1A -FA 2025
▶️ Answer/Explanation
Deca-3,5,7-triene-2,9-dione structure analysis:
- Three C=C double bonds (at positions 3, 5, 7) – each can be E/Z
- Two ketone groups (at positions 2 and 9)
- However, the conjugated system may restrict some stereoisomers due to symmetry or geometric constraints
- With 3 double bonds, maximum theoretical stereoisomers = 2³ = 8
- But due to molecular symmetry or other restrictions, the actual number is lower
✅ Answer: (B) – 6 stereoisomers
▶️ Answer/Explanation
An ether is an organic compound with the general functional group \( \text{R–O–R’} \), where the oxygen atom is single-bonded to two alkyl or aryl groups.
Option (A) \( C_6H_5OH \): This is phenol (an aromatic alcohol), not an ether.
Option (B) \( CH_3CHO \): This is ethanal (an aldehyde), not an ether.
Option (C) \( CH_3COCH_3 \): This is propanone/acetone (a ketone), not an ether.
Option (D) \( CH_3OCH_3 \): This is dimethyl ether, which has the structure \( \text{CH}_3 – O – \text{CH}_3 \), matching the ether functional group.
✅ Answer: (D)
▶️ Answer/Explanation
✅ Correct answer: (B)
The first ionization energy is the energy needed to remove one mole of electrons from one mole of gaseous atoms. Across a period, ionization energy generally increases because nuclear charge increases and electrons are held more strongly.
However, there is an exception between phosphorus and sulfur due to their electron configurations in the outer \(3p\) sub-level:
Phosphorus: \( [\mathrm{Ne}]\,3s^2\,3p^3 \) → three unpaired electrons in \(3p\).
Sulfur: \( [\mathrm{Ne}]\,3s^2\,3p^4 \) → one of the \(3p\) orbitals now contains a pair of electrons.
In sulfur, the pairing of two electrons in the same \(3p\) orbital causes extra electron–electron repulsion, making it easier to remove one of these electrons. This lowers the first ionization energy of sulfur compared with phosphorus.
Statement B correctly highlights that phosphorus does not have paired electrons in the outer \(p\) sub-level, while sulfur does. This difference explains why sulfur has a lower first ionization energy than phosphorus.