AP Chemistry Unit 2: Compound Structure and Properties

Explore AP Chemistry Unit 2 questions on bond types, electronegativity, ionic and metallic solids, Lewis diagrams, resonance, formal charge, VSEPR, and hybridization.

Covered syllabus points

2.1 Types of Chemical Bonds 2.2 Intramolecular Force and Potential Energy 2.3 Structure of Ionic Solids 2.4 Structure of Metals and Alloys 2.5 Lewis Diagrams 2.6 Resonance and Formal Charge 2.7 VSEPR and Hybridization

[Maximum number: 2]

A student learns that ionic compounds have significant covalent character when a cation has a polarizing effect on a large anion. As a result, the student hypothesizes that salts composed of small cations and large anions should have relatively low melting points.

Select two compounds from the table and explain how the data support the student's hypothesis.

[Maximum number: 5]

In the gas phase, $\mathrm{AlCl}_{3}$ is a molecular substance. A reaction of gaseous $\mathrm{AlCl}_{3}$ at high temperature is represented by the following balanced equation.

\text { Reaction 1: } \mathrm{AlCl}_{3}(g) \rightarrow \mathrm{Al}(g)+3 \mathrm{Cl}(g) \quad \Delta H_{1}^{\circ}=\text { ? }

(a)

A potential energy diagram for $\mathrm{Cl}_{2}$ is shown in the following graph.

[ 3 ]

(i)

Based on the graph, what is the bond length, in picometers, for $\mathrm{Cl}_{2}$ ? -

[ 1 ]

(ii)

A student finds that the average Al-Cl bond length is 220 picometers and the average bond energy is $425 \mathrm{~kJ} / \mathrm{mol}$ . Draw the potential energy curve for the average Al-Cl bond on the preceding graph.

[ 2 ]

For a curve with a minimum at an internuclear distance of 220 ± 10 pm: 1 point
See sample curve below
For a curve with a minimum energy value of $-425 \pm 20 \mathrm{~kJ} / \mathrm{mol}$ that approaches zero as the 1 point internuclear distance approaches 500 pm:

Figure for Answer to Question (ii) — AP Chemistry

Total for part (c) 3 points

(b)

Three proposed Lewis diagrams for the $\mathrm{AlCl}_{3}(g)$ molecule are shown.

Diagram 1

Diagram 2

Diagram 3

[ 2 ]

(i)

The $\mathrm{AlCl}_{3}(g)$ molecule has a trigonal planar geometry. Which diagram (1,2, or 3) can be eliminated based on geometry? Justify your choice based on VSEPR theory.

[ 1 ]

(ii)

Which of the three diagrams is the best representation for the bonding in $\mathrm{AlCl}_{3}$ ? Justify your choice based on formal charges.

$\mathrm{AlCl}_{3}$ is known to dimerize reversibly in the gas phase. The dimerization equilibrium is represented by the following equation.

2 \mathrm{AlCl}_{3}(g) \rightleftarrows \mathrm{Al}_{2} \mathrm{Cl}_{6}(g)

[ 1 ]

[Maximum number: 2]

A student investigates the enthalpy of solution, $\Delta H_{\text {soln }}$ , for two alkali metal halides, LiCl and NaCl. In addition to the salts, the student has access to a calorimeter, a balance with a precision of ±0.1 g, and a thermometer with a precision of ±0.1°C.

(a)

Which salt, LiCl or NaCl, has the greater lattice enthalpy? Justify your answer.

[ 1 ]

(b)

Below is a representation of a portion of a crystal of LiCl. Identify the ions in the representation by writing the appropriate formulas ( $\mathrm{Li}^{+}$ or $\mathrm{Cl}^{-}$ ) in the boxes below.

[ 1 ]

[Maximum number: 1]

Sterling silver is an alloy that is commonly used to make jewelry and consists of 92.5\% silver and 7.5\% other metals, such as copper, by mass. Over time, the alloy can form a tarnish of $\mathrm{Ag}_{2} \mathrm{~S}(s)$ when it reacts with hydrogen sulfide, as represented by the following equation.

2 \mathrm{Ag}(s)+\mathrm{H}_{2} \mathrm{~S}(g) \rightarrow \mathrm{Ag}_{2} \mathrm{~S}(s)+\mathrm{H}_{2}(g)

The following table contains the atomic radii for silver and copper.

Table for Question 2 Compound Structure and Properties question 4 — AP Chemistry

Explain why sterling silver is better classified as a substitutional alloy than as an interstitial alloy.

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Covered syllabus points

2 Compound Structure and Properties question 1

2 Compound Structure and Properties question 2

Question (a)

Question (i)

Question (ii)

Question (b)

Question (i)

Question (ii)

2 Compound Structure and Properties question 3

Question (a)

Question (b)

2 Compound Structure and Properties question 4