Edexcel A-Level Chemistry Paper 1, June 2017: Question 4

✅ Correct Answer

B

[Ar] with 3d orbitals containing (↑↓, ↑, ↑, ↑, ↑) and 4s empty.

💡 Key Knowledge

Neutral iron (Fe) has the configuration [Ar] 3d⁶ 4s² . When forming positive ions, electrons are always removed from the 4s orbital first before the 3d orbitals. For Fe²⁺, two electrons are lost from 4s, leaving [Ar] 3d⁶ . Hund's rule states electrons fill degenerate orbitals singly before pairing up.

❌ Common Errors

Students often incorrectly remove electrons from the 3d sub-shell instead of the 4s sub-shell, or fail to apply Hund's rule correctly when distributing electrons across the five degenerate 3d boxes.

✅ Correct Answer

1. Zinc ions (Zn²⁺) have a full 3d sub-shell ( 3d¹⁰ / all 3d orbitals are full).
2. Therefore, d-d transitions cannot take place because electrons cannot be promoted or move between 3d orbitals.

🧠 Exam Technique

To secure full marks in explanations of transition metal colour, you must explicitly mention: (1) the specific electronic state of the d-subshell (full/empty), and (2) the consequence on d-d electron transitions. Vague references to "no electrons to move" will lose marks.

❌ Common Errors

Referring to zinc atoms rather than zinc ions (Zn²⁺), or stating that the d-subshell "does not split" (the orbitals do split in a ligand field, but transitions cannot occur because the sub-shell is completely full).

✅ Correct Answer

A drawn octahedral complex ion [Fe(C₂O₄)₂(H₂O)₂]²⁻ showing:

• Two water ligands ( H₂O ) coordinated to the central Fe through their oxygen atoms (can be cis or trans).
• Two bidentate ethanedioate ligands ( C₂O₄²⁻ ) correctly bonded through two single-bonded oxygen atoms each, forming 5-membered chelate rings.
• Overall enclosed in square brackets with a 2- charge.

💡 Key Knowledge

Ethanedioate ( C₂O₄²⁻ ) is a bidentate ligand. Since each ethanedioate donates two lone pairs and water donates one, the coordination number is 6 (octahedral geometry). Bidentate ligands must show bonds connecting from the donor oxygen atoms to the central Fe atom.

❌ Common Errors

Connecting ethanedioate to the iron via the double-bonded oxygens instead of the single-bonded (carboxylate oxygen) atoms, or failing to show the bidentate bite correctly.

✅ Correct Answer

1. There is an increase in the number of particles/moles from left to right (from 3 particles on the left to 5 particles on the right).
2. Therefore, ΔS_system increases (becomes more positive), making the reaction feasible.

🧠 Exam Technique

Always structure your entropy answers by first quoting the mole/particle change on both sides of the equation, linking this directly to the sign of ΔS_system, and explaining how that contributes to reaction feasibility (ΔS_total > 0).

❌ Common Errors

Stating simply that "disorder increases" without linking it explicitly to the number of moles or particles specified in the balanced equation.

✅ Correct Answer

Two ionic equations showing the redox catalytic cycle of Fe²⁺:

1) 2Fe²⁺ + S₂O₈²⁻ → 2Fe³⁺ + 2SO₄²⁻

2) 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂

Note: Equations can be in either order, and multiples are accepted. State symbols are not required.

💡 Key Knowledge

In homogeneous catalysis, the catalyst reacts with one reactant to form an intermediate (oxidising Fe²⁺ to Fe³⁺), which then reacts with the second reactant in a subsequent step, regenerating the original catalyst (reducing Fe³⁺ back to Fe²⁺).

❌ Common Errors

Writing incorrect chemical formulas for peroxodisulfate ( S₂O₈²⁻ ) or iodide ( I⁻ ), or failing to balance the charges and stoichiometry properly across both equations.