IB Chemistry HL Structure 2: Models of bonding and structure 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

Multiple choiceEasy1 mark

Four molecules are compared to assess how bond polarity and molecular geometry affect molecular polarity. Which molecule has a net dipole moment?

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Answer: A. A molecule has a net dipole when its bond dipoles do not cancel. H $_2$ O is bent, so its two O–H bond dipoles do not cancel.

B: Treats polar C=O bonds as sufficient for molecular polarity, overlooking cancellation in linear CO $_2$ C: Treats polar B–F bonds as sufficient, overlooking cancellation in trigonal-planar BF $_3$ D: Treats polar C–Cl bonds as sufficient, overlooking cancellation in tetrahedral CCl $_4$

Remember: molecular polarity depends on both bond polarity and geometry.

Question 4

Paper 2Medium6 marks

Figure 1 shows reference bond angles for methane, ammonia and water. Each central atom has four electron domains.

(a)

Use Figure 1 to state the bond angle in water and compare the bond angles in ammonia and water.

[2]

(b)

Explain the difference between the bond angles in ammonia and water in terms of the electron domains around their central atoms.

[2]

(c)

Ammonia reacts with a hydrogen ion to form an ammonium ion:

\ce{NH3 + H+ -> NH4+}

Identify the atom that donates the electron pair to $\ce{H+}$ and explain why the N–H bond formed is a coordination bond.

[2]

Question 5

Multiple choiceEasy1 mark

A piece of aluminium can be hammered into a thin sheet without fragmenting. Which feature of its metallic structure explains this malleability?

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Answer: C. Metallic bonding is the electrostatic attraction between lattice cations and delocalized electrons. When layers of cations shift during hammering, the electrons still attract the cations in their new positions, so the structure remains bonded.

A: Treats electrons as localized pairs linking particular cations, rather than as delocalized electrons. B: Assigns movement to cations passing through stationary electrons. D: Reverses the electrostatic interaction: like-charged cations repel rather than attract.

Remember: In metallic malleability, layers can shift while attraction to the delocalized electrons is maintained.

Question 6

Multiple choiceEasy1 mark

The electronegativity values, χ, of the elements in four binary compounds are shown.

Element	Mg	O	Na	Cl	Ca	F	C
χ	1.2	3.5	0.9	3.0	1.0	4.0	2.5

Which compound is predicted to have the greatest ionic contribution to its bonding?

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Answer: C [1]. A larger electronegativity difference indicates a greater ionic contribution. The differences are MgO: |3.5 − 1.2| = 2.3; NaCl: |3.0 − 0.9| = 2.1; CaF₂: |4.0 − 1.0| = 3.0; CF₄: |4.0 − 2.5| = 1.5. CaF₂ has the largest difference.

A: MgO has a substantial difference, but 2.3 is less than 3.0.
B: NaCl is ionic, but its difference of 2.1 is less than 3.0.
D: A high mean electronegativity alone does not indicate a greater covalent contribution; CF₄ is less ionic here because its electronegativity difference is smaller.

Remember: electronegativity difference indicates the ionic contribution to bonding.

Total: [1]

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