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2025 H2 Chem P2 Q2: Covalent Bonding & Structures Solution

Covalent Bonding & Structures 2025 A-Level H2 Chemistry · Paper 2 · Q2 · 9 marks

Question

(a)(i)[1]

When a sample of the non-metal sulfur and the metal iron are heated together, iron sulfide is made.

Covalent bonds are present between the atoms in an \(S_8\) molecule.

Describe a covalent bond.

(a)(ii)[2]

Draw a labelled diagram to show the bonding present in solid iron.

(b)[2]

Greigite is a mineral of iron sulfide. It contains both \(Fe^{2+}\) and \(Fe^{3+}\).

A 25.00 g sample of greigite is made of 18.86% by mass of \(Fe^{2+}\) and 37.73% by mass of \(Fe^{3+}\).

Deduce the oxidation number of sulfur in the sample.

(c)(i)[1]

\(Fe^{2+}(aq)\) is a catalyst for the reaction between \(S_2O_8^{2-}(aq)\) and \(I^-(aq)\). Without the catalyst, they react together slowly because the activation energy for the reaction is very high.

reaction 1 \(S_2O_8^{2-} + 2I^- \rightarrow 2SO_4^{2-} + I_2\)

Explain why the activation energy for reaction 1 is very high.

(c)(ii)[1]

Identify the type of catalytic behaviour shown by \(Fe^{2+}(aq)\) in reaction 1.

(c)(iii)[2]

Write two equations to show how \(Fe^{2+}(aq)\) acts as a catalyst for reaction 1.

Video Solution

Overview

Question 2 is a mixed bonding and reactivity question built around sulfur and iron. It begins by asking you to describe a covalent bond, using the S₈ molecule as the context, then to draw and label the metallic bonding in solid iron. The middle part is an oxidation-number calculation: greigite is an iron sulfide containing both Fe²⁺ and Fe³⁺, and you use the percentage masses to deduce the oxidation number of sulfur. The final section turns to the reaction between peroxodisulfate and iodide ions. You explain why the activation energy is so high when two negative ions must collide, identify the type of catalysis shown by Fe²⁺(aq), and write the two equations that show how it works.

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