IB DP Chemistry Mock Exam SL Paper 1A Set 2 - 2025 Syllabus

IB DP Chemistry Mock Exam SL Paper 1A Set 2

Prepare for the IB DP Chemistry Exam with our comprehensive IB DP Chemistry Exam Mock Exam SL Paper 1A Set 2. Test your knowledge and understanding of key concepts with challenging questions covering all essential topics. Identify areas for improvement and boost your confidence for the real exam

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Question

Which row puts these molecules in order of decreasing bond angle?

(A) \(NH_{3}, H_{2}O, C_{2}H_{4}, CH_{4}\)
(B) \(H_{2}O, C_{2}H_{4}, CH_{4}, NH_{3}\)
(C) \(C_{2}H_{4}, CH_{4}, NH_{3}, H_{2}O\)
(D) \(CH_{4}, NH_{3}, H_{2}O, C_{2}H_{4}\)

▶️ Answer/Explanation

Detailed solution

1. Determine Molecular Geometries and Bond Angles:

\(C_{2}H_{4}\) (Ethene): Trigonal planar around carbons. Bond angle \(\approx 120^{\circ}\).
\(CH_{4}\) (Methane): Tetrahedral. No lone pairs. Bond angle \(= 109.5^{\circ}\).
\(NH_{3}\) (Ammonia): Trigonal pyramidal. 1 lone pair compresses the angle. Bond angle \(\approx 107^{\circ}\).
\(H_{2}O\) (Water): Bent. 2 lone pairs compress the angle more. Bond angle \(\approx 104.5^{\circ}\).

2. Order Decreasing (Largest to Smallest):
\(120^{\circ} > 109.5^{\circ} > 107^{\circ} > 104.5^{\circ}\)
\(C_{2}H_{4}, CH_{4}, NH_{3}, H_{2}O\)
This matches row C.
✅ Answer: (C)

Question

Aluminium trichloride, \(AlCl_{3}(g)\), and beryllium chloride, \(BeCl_{2}(g)\), have been known for many years. Which adaptation of the bonding model was made to account for the bonding in \(AlCl_{3}(g)\) and \(BeCl_{2}(g)\)?

(A) The existence of coordination bonding was proposed.
(B) Ideas about oxidation states were formed.
(C) Exceptions to the octet rule were introduced.
(D) Lewis formulas of molecules were developed.

▶️ Answer/Explanation

Detailed solution

1. Analyze Bonding in \(BeCl_2\):
Beryllium is in Group 2 and has 2 valence electrons. It forms two single bonds with chlorine. The central Be atom has only \(2 \times 2 = 4\) electrons in its valence shell. It is electron-deficient.

2. Analyze Bonding in \(AlCl_3\):
Aluminium is in Group 13 and has 3 valence electrons. It forms three single bonds with chlorine. The central Al atom has \(3 \times 2 = 6\) electrons in its valence shell. It is also electron-deficient.

3. Compare to Octet Rule:
The octet rule states atoms tend to gain, lose, or share electrons to achieve 8 valence electrons. Both Be and Al in these gaseous chlorides have fewer than 8 electrons. Therefore, they are recognized as exceptions to the octet rule (specifically, incomplete octets).
✅ Answer: (C)

Question

Which statement best explains the trend in first ionisation energy from sodium (Na) to chlorine (Cl)?

(A) Nuclear charge increases.
(B) Electronegativity decreases.
(C) Atomic radius increases.
(D) Shielding decreases.

▶️ Answer/Explanation

Detailed solution

1. Analyze the Trend Across a Period:
Moving from Na to Cl (left to right across Period 3), protons are added to the nucleus one by one. This increases the nuclear charge.

2. Analyze Shielding:
Electrons are added to the same principal energy shell (n=3). The inner shell electrons (n=1, n=2) remain constant, so the shielding effect is approximately constant, not decreasing.

3. Relate to Ionization Energy:
Since nuclear charge increases while shielding stays roughly the same, the effective nuclear charge increases. This pulls the valence electrons closer (decreasing atomic radius) and holds them more tightly. Therefore, more energy is required to remove an electron (higher first ionization energy). The primary driver is the increasing nuclear charge.
✅ Answer: (A)

IB DP Chemistry Mock Exam SL Paper 1A Set 2 - 2025 Syllabus