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The Ionic Model · Paper 2 — Free Chemistry SL Practice Questions

What this topic covers

Every property question IB throws at you — why does this melt higher, why does that conduct here but not there — traces back to one of three bonding models: ionic, covalent, or metallic. This topic asks you to build each model from its electrostatic basics, then use it to predict and explain real behaviour: melting point, conductivity in different states, shape, solubility, malleability. The exam almost never tests the model in isolation — it tests whether you can name the right structural detail (coordination number, electron domain count, number of delocalized electrons) and explain why that detail produces the observed property. Vocabulary without mechanism gets you nowhere on the mark scheme.

Read the full Structure: Models of Bonding and Structure lesson notes →

1ChallengeSAQ-LProperties of ionic compounds7 marksPaper 2~11 min

Sodium chloride (NaCl) and magnesium oxide (MgO) are both ionic compounds with the same face-centred cubic lattice arrangement. The melting point of NaCl is

801\,^\circ\text{C}

and that of MgO is

2852\,^\circ\text{C}

. The lattice enthalpies are: NaCl

= -788\,\text{kJ mol}^{-1}

; MgO

= -3795\,\text{kJ mol}^{-1}

. Ionic radii:

\text{Na}^+ = 102\,\text{pm}

\text{Cl}^- = 181\,\text{pm}

\text{Mg}^{2+} = 72\,\text{pm}

\text{O}^{2-} = 140\,\text{pm}

(a)

State the charge on each ion in MgO and in NaCl. [1 mark]

(b)

Calculate the product of ionic charges for MgO and for NaCl. [1 mark]

(c)

Using your answer to (b), the ionic radii given, and the concept of electrostatic force, explain why the melting point of MgO is significantly higher than that of NaCl. [3 marks]

(d)

The ratio of the lattice enthalpies

\left(\dfrac{3795}{788} \approx 4.8\right)

is greater than the ratio predicted from charge products alone

\left(\dfrac{4}{1} = 4\right)

. Deduce one reason, other than ionic charge, why the lattice enthalpy of MgO is more negative than the charge-product ratio predicts, and explain the origin of this additional contribution. [2 marks]

Solutions

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