AP Chemistry 7.9: Le Châtelier Principle

Practice AP Chemistry questions on predicting equilibrium responses to concentration, temperature, pressure, volume, and dilution changes using Le Châtelier reasoning.

Exam points

Explain how reaction rates depend on collisions, activation energy and molecular pathways.
Apply rate laws, catalysts and concentration data to predict reaction speed and mechanism.

[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.A—Identify the response of a system at equilibrium to an external stress, using 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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7.9.A—Identify the response of a system at equilibrium to an external stress, using Le Châtelier's principle question 1