AP Chemistry 2.6: Resonance and Formal Charge

Practice AP Chemistry 2.6 questions on resonance, formal charge, equivalent Lewis structures, octet-rule limits, and molecular representations.

Exam points

Use equilibrium expressions and reaction quotients to predict direction of change and relative amounts of species.
Relate equilibrium constants to ΔG° and concentration data when evaluating reaction favorability.

Covered syllabus points

2.6.A—Represent a molecule with a Lewis diagram that accounts for resonance between equivalent structures or that uses…

[Maximum number: 1]

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 { ? }

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

Diagram 1

Diagram 2

Diagram 3

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)

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Covered syllabus points

2.6 Resonance and Formal Charge question 1