Equilibrium Free-Response Questions for AP Chemistry
Review the following concepts if necessary:
- Le Chatelier's Principle for AP Chemistry
- Acid–Base Equilibrium for AP Chemistry
- Ka, Kw, Kb - The Acid, Water, and Base Dissociation Constant for AP Chemistry
- Buffers for AP Chemistry
- Titration Equilibria for AP Chemistry
- Solubility Equilibria for AP Chemistry
You have 15 minutes. You may use a calculator and the tables at the back of the book.
An aqueous solution is prepared that is initially 0.100 M in CdI42–. After equilibrium is established, the solution is found to be 0.013 M in Cd2+.
- Derive the expression for the dissociation constant, Kd, and determine the value of the constant.
- What will be the cadmium ion concentration arising when 0.400 mol of KI is added to 1.00 L of the solution in part a?
- A solution is prepared by mixing 0.500 L of the solution from part b and 0.500 L of 2.0 × 10–5 M NaOH. Will cadmium hydroxide, Cd(OH)2, precipitate? The Ksp for cadmium hydroxide is 2.2 × 10–14.
- When the initial solution is heated, the cadmium ion concentration increases. Is the equilibrium an exothermic or an endothermic process? Explain how you arrived at your conclusion.
Answers and Explanations
- Kd + [Cd2+][I–]M4/[CdI42–] Give yourself 1 point for this expression. Using the following table:
The value of [Cd2+] is given (= 0.013), and this is x. This changes the last line of the table to:
Entering these values into the Kd expression gives: (0.013)(0.052)4/(0.087) = 1.1 × 10–6. Give yourself 1 point for this answer. You can also get 1 point if you correctly put your values into the wrong equation.
Give yourself 1 point for the correct setup and 1 point for the correct answer. If you got the wrong K in part a, you can still get one or both points for using it correctly.
This value is less than Ksp, so no precipitate will form.
Give yourself 1 point for a correct calculation, and another point for the correct conclusion. If you correctly use a wrong [Cd2+] you calculated in part b, you can still get both points.
Give yourself 1 point for endothermic. Give yourself 1 point for mentioning Le Châtelier's principle.
Your score is based on a total of 8 points. Subtract one point if any answer has an incorrect number of significant figures.