IB Chemistry HL Structure 2: Models of bonding and structure questions

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Type

Multiple choice Written

Difficulty

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Question 1

Paper 2Medium6 marks

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 2

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]

Question 3

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 4

Paper 2Hard8 marks

A materials team is screening pure chromium, iron and copper for a component. The specification requires a melting point above 1400 °C; among metals that meet this limit, the electrical conductivity at 20 °C should be as high as possible. Approximate values are shown.

Metal	Melting point / °C	Electrical conductivity at 20 °C / S m⁻¹
Chromium	1907	7.9 × 10⁶
Iron	1538	1.00 × 10⁷
Copper	1085	5.96 × 10⁷

(a)

Describe the metallic model for a transition element, including the role of d-electrons.

[2]

(b)

Explain how this model accounts for electrical conductivity.

[2]

(c)

A trainee claims: “The metal with the greatest electrical conductivity must have the strongest metallic bonding and the highest melting point.” Assess this claim using the data and the metallic model, and identify the metal that meets the specification.

[4]

Question 5

Paper 2Hard8 marks

A chemistry student uses the metallic model to explain properties of lithium and to assess claims about Group 1 metals. Figure 1 shows a simplified model of lithium before and during shear. The table gives first ionization energies.

Element	First ionization energy / kJ mol⁻¹
Li	520
Na	496
K	419

A student argues: “The decrease in first ionization energy from Li to K explains the increase in Group 1 reactivity. It also proves that metallic bonding becomes stronger, so potassium must conduct electricity better than lithium.”

(a)

State the nature of the metallic bond in terms of the particles involved.

[1]

(b)

Use Figure 1 to explain how the metallic model accounts for malleability.

[2]

(c)

Explain how the metallic model accounts for electrical conductivity.

[1]

(d)

Evaluate both inferences made by the student, using Figure 1 and the table.

[4]

Question 6

Paper 2Hard11 marks

A furnace-lining manufacturer is comparing three binary compounds for a component that must remain solid at 1500 °C and limit electrical conduction. Use the electronegativity values in the Data Booklet for Mg, O, Si and C; for the third compound, use χ(Ni) = 1.9 and χ(Al) = 1.5.

In the qualitative bonding triangle, increasing the electronegativity difference, Δχ, moves a compound towards ionic bonding. When Δχ is small, a higher mean electronegativity favours covalent character, while a lower mean electronegativity favours metallic character.

Tests on the three samples gave the following results:

Compound	Remained solid at 1500 °C	Electrical conductivity in the test
MgO	Yes	Below the detection limit
SiC	Yes	Measurable
NiAl	Yes	High

(a)

For MgO and SiC, calculate Δχ and the mean electronegativity, χ, to two decimal places.

[4]

(b)

Using your results from (a) and the electronegativities given for NiAl, state which bonding contribution is greatest for each compound: ionic, covalent or metallic.

[3]

(c)

An engineer claims: “SiC must be a better electrical insulator than MgO because its smaller electronegativity difference places it closer to covalent bonding; covalent-dominant compounds do not conduct electricity.” Evaluate this claim and recommend one of the three compounds for the furnace lining, using the bonding model and the test results.

[4]

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