IB Chemistry HL S2.1 The ionic model questions

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Question 1

Multiple choiceEasy1 mark

A calcium fluoride crystal has the empirical formula CaF₂. Which description of its ionic lattice is consistent with this formula?

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Answer: B. An ionic empirical formula gives the simplest whole-number ratio of ions in the lattice. In CaF₂, one Ca²⁺ balances the charge of two F⁻ ions, giving a ratio of 1:2.

A: reverses the subscripts, giving a ratio of 2:1.
C: assigns calcium a +1 charge, so the stated ratio is not charge-balanced.
D: uses equal numbers of ions, leaving the charges unbalanced.

Remember: ionic formulas give ratios of ions in a lattice, not discrete molecules.

An investigation compares the ionic solids magnesium oxide, MgO, and calcium oxide, CaO. Both have the same arrangement of ions in their lattices. The ionic radii for ions in this arrangement are shown below.

Ion	Ionic radius / pm
Mg²⁺	72
Ca²⁺	100
O²⁻	140

(a)

Explain why solid CaO does not conduct electricity but molten CaO does.

[2]

(b)

Use the data to predict which oxide has the larger magnitude of lattice enthalpy. Explain your prediction.

[2]

(c)

Hence, identify which oxide is expected to be less volatile. Explain your answer.

[2]

Question 3

Paper 2Hard11 marks

A controlled preparation is used to investigate the ionic composition of an iron sulfide solid. Each batch contains $12.0\times10^{-3}\ \mathrm{mol}$ of iron ions of unknown charge. Different amounts of sulfide ions, $\mathrm{S^{2-}}$ are added. Assume that the iron ions keep the same charge throughout, no redox occurs, and the only solid formed contains iron and sulfide ions.

Figure 1 shows the measured mass of dried solid for each amount of sulfide added. A preliminary report identifies the solid as iron(III) sulfide, $\mathrm{Fe_2S_3}$

(a)

Describe the pattern in the graph as more sulfide ions are added, and identify the ion that limits formation of the solid beyond the plateau.

[2]

(b)

Use Figure 1 to assess the preliminary report. Deduce the simplest formula and name of the solid.

[5]

(c)

Write balanced equations showing the electron transfer from a neutral iron atom and to a neutral sulfur atom for the ions in the solid identified in (b).

[2]

(d)

Explain the nature of the bonding between the ions in the solid identified in (b).

[2]

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