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IB DP Chemistry -Structure 2.2 The covalent model - IB Style Questions For SL Paper 2 -FA 2025

IB DP Chemistry SL Paper 2 - All Topics

Question

Iron(II) sulfide can be made by heating powdered iron with sulfur.
(a) Explain the difference between an element and a compound.
(b) Explain why solid iron(II) sulfide can be classified as a polar covalent substance. Use sections 9 and 17 of the data booklet (Electronegativity values: \(\chi(\mathrm{Fe}) = 1.8\), \(\chi(\mathrm{S}) = 2.6\); bonding triangle uses \(\Delta \chi\) and average electronegativity).
(c) A student suggested that iron(II) sulfide is ionic and therefore could be decomposed back into iron and sulfur by electrolysis of the molten compound using inert electrodes.
(i) Label the electrolytic cell with the terms anode and cathode, and indicate the direction of ion movement.
(ii) Write half-equations for the reaction occurring at each electrode.
Negative electrode:
Positive electrode:
(iii) State one physical property that differs between iron and iron(II) sulfide.
Most-appropriate topic codes (IB Chemistry 2025):
• TOPIC S1.1: Particulate nature of matter — part (a)
• TOPIC S2.2: Covalent bonding — part (b)
• TOPIC R2.2: Electrolytic cells — part (c)
▶️ Answer/Explanation
Detailed solution
(a)
An element contains only one type of atom (atoms with the same number of protons) and cannot be broken down by chemical means.
A compound contains two or more different elements chemically bonded together in a fixed ratio.
(b)
From the data booklet (section 9), the electronegativity values are \(\chi(\mathrm{S}) = 2.6\) and \(\chi(\mathrm{Fe}) = 1.8\).
So, \(\Delta \chi = 2.6 – 1.8 = 0.8\), and the average electronegativity is \(\dfrac{2.6 + 1.8}{2} = 2.2\).
Using the triangular bonding diagram (section 17), a moderate \(\Delta \chi\) with a relatively high average electronegativity places the bonding in the polar covalent region rather than fully ionic.
(c)
(i)

• Left Electrode (Positive): Anode. \(\text{S}^{2-}\) ions move towards it.
• Right Electrode (Negative): Cathode. \(\text{Fe}^{2+}\) ions move towards it.
(ii)
• Negative electrode (Cathode): \(\text{Fe}^{2+} + 2\text{e}^- \rightarrow \text{Fe}\)
• Positive electrode (Anode): \(\text{S}^{2-} \rightarrow \text{S} + 2\text{e}^-\)
(iii)
Iron is a good conductor of electricity/heat (or is malleable/ductile/magnetic), whereas iron(II) sulfide is a non-conductor (or brittle/non-magnetic).
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