AP Chemistry Unit 7.3: Q, K, and Equilibrium

Practice AP Chemistry Unit 7.3 questions on writing K and Q expressions for concentration or pressure data and completing particulate equilibrium representations.

Exam points

Use thermodynamic and equilibrium relationships to predict reaction direction, energy changes, and temperature.
Calculate or interpret ΔG°, ΔH°, ΔS°, K, and concentrations from balanced reaction data.

Covered syllabus points

7.3.A—Represent the reaction quotient Qc or Qp, for a reversible reaction, and the corresponding equilibrium…

[Maximum number: 2]

Answer the following questions about magnesium.

(a)

The dissolution of magnesium hydroxide is represented by the following equation.

\mathrm{Mg}(\mathrm{OH})_{2}(s) \rightleftarrows \mathrm{Mg}^{2+}(a q)+2 \mathrm{OH}^{-}(a q) \quad K_{s p}=5.61 \times 10^{-12}

[ 2 ]

(i)

Write the expression for the solubility product constant, $K_{s p}$ .

[ 1 ]

E (i) For the correct expression:

$K_{s p}=\left[\mathrm{Mg}^{2+}\right]\left[\mathrm{OH}^{-}\right]^{2}$

(ii)

After the two solutions are combined in beaker 3 as described in part D, but before any

reaction takes place, $\left[\mathrm{OH}^{-}\right]=1.65 \times 10^{-4} M$ . Using your answer to part D, calculate the

value of the reaction quotient, Q.

[ 1 ]

(ii) For the correct calculated value, consistent with part D and part E (i):

$Q=\left[\mathrm{Mg}^{2+}\right]\left[\mathrm{OH}^{-}\right]^{2}=\left(7.62 \times 10^{-4}\right)\left(1.65 \times 10^{-4}\right)^{2}=2.07 \times 10^{-11}$

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Covered syllabus points

7.3 Reaction Quotient and Equilibrium Constant question 1

Question (a)

Question (i)

Question (ii)