AP Chemistry Unit 7.9: Le Châtelier Principle

Practice AP Chemistry Unit 7.9 questions on predicting equilibrium responses to concentration, temperature, pressure, volume, and dilution changes.

Exam points

Use electrochemical equations and reduction potentials to predict redox direction, cell voltage, and electrodes.
Relate electron transfer, stoichiometry, and charge balance to products and electrochemical observations.

Covered syllabus points

7.9.A—Identify the response of a system at equilibrium to an external stress, using Le Châtelier's principle

[Maximum number: 1]

White phosphorus is composed of $\mathrm{P}_{4}$ molecules with a tetrahedral structure, as shown in the

diagram on the left. Each P atom is bonded to the other three P atoms by single bonds, as shown

in the incomplete Lewis diagram on the right.

Figure for Question 7.9 Introduction to Le Châtelier’s Principle question 1 — AP Chemistry

Equation 4 for the reaction that occurs is shown.

\text { Equation 4: } \mathrm{PCl}_{3}(g)+\mathrm{Cl}_{2}(g) \rightleftarrows \mathrm{PCl}_{5}(g) \quad \Delta H_{2}^{\circ}=-88 \mathrm{~kJ} / \mathrm{mol}_{r x n}

Does the value of $K_{p}$ increase, decrease, or remain the same when the temperature is

increased to 596 K? Justify your answer based on $\Delta H_{2}^{\circ}$ .

(ii) For the correct answer and a valid justification:

Decrease. The negative value of $\Delta H_{2}^{\circ}$ indicates that the reaction is exothermic. Because

exothermic reactions favor reactant formation at higher temperature, the value of $K_{p}$

decreases.

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Covered syllabus points

7.9 Introduction to Le Châtelier’s Principle question 1