AP Chemistry 2.6.A: Resonance and Formal Charge

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

Exam points

Explain how chemical reactions transfer energy through bonds, particles and surroundings.
Apply energy diagrams and thermodynamic relationships to predict heat flow and reaction feasibility.

[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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2.6.A—Represent a molecule with a Lewis diagram that accounts for resonance between equivalent structures or that uses… question 1