OCR A-Level Chemistry AS Depth in chemistry (02), November 2021: Question 1

✅ Correct Answer

Electrostatic attraction between oppositely charged ions.

❌ Common Errors

• Mentioning "transfer of electrons" instead of focusing on the electrostatic attraction between existing ions. (The transfer creates the ions, but the bond itself is the attraction).
• Using the word "force" instead of "attraction" (though examiners often ignore "force", sticking to "attraction" is safer).

✅ Correct Answer

Two separate [K]⁺ ions (or 2[K]⁺ ) and one [S]²⁻ ion.

• Potassium ions: show empty outer shell (or previous full shell) with a + charge outside brackets.
• Sulfur ion: show 8 electrons in the outer shell (mix of dots and crosses) with a 2- charge outside brackets.

🧠 Exam Technique

When drawing ionic lattices or individual ionic species, always include square brackets around the ions and clearly display the correct stoichiometric charges ( + and 2- ) as superscripts outside the brackets.

✅ Correct Answer

A central sulfur atom covalently bonded to two fluorine atoms.

• 1 single covalent bond (shared pair of electrons) between S and each F.
• Sulfur must have 2 bonding pairs and 2 lone pairs in its outer shell.
• Each fluorine atom must have 1 bonding pair and 3 lone pairs in its outer shell.

❌ Common Errors

• Forgetting to draw the unbonded lone pairs on sulfur or fluorine atoms.
• Giving sulfur an incorrect octet expansion or drawing ionic structures instead of covalent overlapping/shared pairs.

💡 Key Knowledge

Physical state depends entirely on the strength of the forces holding the structural units together.

✅ Correct Answer (3-Mark Breakdown)

1. K₂S structure/bonding: Giant ionic lattice with strong electrostatic forces of attraction between oppositely charged ions (K⁺ and S²⁻).
2. SF₂ structure/bonding: Simple molecular structure with weak intermolecular forces (induced dipole-dipole / London forces or permanent dipole interactions) between molecules.
3. Energy comparison: A lot of energy is required to break the strong ionic bonds in K₂S (high melting point, solid), whereas very little energy is needed to overcome the weak intermolecular forces between SF₂ molecules (low boiling point, gas).

✅ Correct Answer

• Shape: Octahedral
• Bond angle: 90° (and 180°)

🧠 Exam Technique

Do not confuse "tetrahedral" with "octahedral". SF₆ has 6 bonding pairs of electrons around the central sulfur atom and zero lone pairs, giving 6 electron pairs arranged symmetrically to minimize repulsion.

✅ Correct Answer

High activation energy / strong S-F bonds / sterically hindered by the six fluorine atoms preventing attack by nucleophiles or reagents.

❌ Common Errors

Vague statements like "it has a full outer shell" or "it's stable" without linking to bond strength, high activation energy, or steric shielding will not be credited by examiners.