Long free-response question

Ionic solids, network solids and alloys

Units 1, 2 and 3
10 points
About 23 minutes

You can use a calculator on this question, just like on exam day.

A multipart question, often built around a lab setup or a data table, that mixes several topics. You calculate, explain and justify your answers across many lettered parts. On the exam: 3 questions (Questions 1–3), part of the 1-hour-45-minute free-response section; calculator allowed.

The question and its sources

A materials-science class compares several solids. Use the information in the tables to answer the questions.

Table 1. Melting points of four solids

Substance	Melting point (°C)
Sodium fluoride, NaF	993
Magnesium oxide, MgO	2852
Silicon dioxide (quartz), SiO₂	about 1700
Carbon dioxide, CO₂	sublimes at −78

Source: Standard reference values, rounded

Table 2. Radii of some atoms and ions

Particle	Radius (pm)
Na⁺	102
Mg²⁺	72
F⁻	133
O²⁻	140
Fe atom	126
Cu atom	128
Zn atom	134
C atom	77

Source: Standard reference values, rounded

Suggested time: 23 minutes

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Part (a)

2 points

Magnesium reacts with nitrogen gas to form an ionic compound. (i) Predict the formula of the compound, and justify your prediction using the number of valence electrons in each atom. (ii) Calculate the mass percent of nitrogen in the compound.

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Part (b)

1 point

Using Coulomb's law and the data in Tables 1 and 2, explain why MgO has a much higher melting point than NaF.

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Part (c)

1 point

Describe the graph of potential energy versus internuclear distance for two hydrogen atoms that form an H₂ molecule. Include what happens to the potential energy at very large distances, at the bond length, and at very short distances, and explain why the potential energy rises at very short distances.

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Part (d)

1 point

The bond length in HCl is 127 pm, compared with 74 pm in H₂. Explain why the bond length in HCl is greater.

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Part (e)

1 point

Brass is an alloy of copper and zinc. Steel is an alloy of iron and a small amount of carbon. Using Table 2, explain why brass is a substitutional alloy but steel is an interstitial alloy.

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Part (f)

1 point

Pure iron is soft and easily bent, but steel is harder and less malleable. Explain this difference in terms of the structure of the two solids.

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Part (g)

1 point

Solid NaF does not conduct electricity, but molten NaF does. Explain this difference.

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Part (h)

1 point

Both CO₂ and SiO₂ are compounds of a group 14 element with oxygen. Using Table 1, explain the very large difference between their melting (or sublimation) points in terms of the structure of each solid.

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Part (i)

1 point

A sample of brass is 67.0% copper and 33.0% zinc by mass. Calculate the ratio of the number of Cu atoms to the number of Zn atoms in the alloy.

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