17.1.3 The Common Ion Effect

Question 1

The Common Ion Effect

Answer 1

• The common ion effect is an extension of Le Châtelier’s principle that deals with the effect of ions in a weak acid or weak base solution.
• When Q = K a , a system is in equilibrium.

Question 2

note

Answer 2

• Le Châtelier’s principle states that if a stress is applied to a system in equilibrium, the system will shift in such a way as to relieve the stress. If C were added to a system, the equilibrium would shift to reactants, resulting in more A and B and less D.
• If a weak acid (HA) is dissociated in water and a small
  amount of the conjugate base (A – ) of the weak acid is added, the equilibrium will shift, giving more weak acid and less H + . This will cause an increase in pH.
• At any point on the curve corresponding to Q = K a , where Q is the reaction quotient, the system is in equilibrium.
• If A – is added to the system, the system will move along the curve to reestablish equilibrium, and the H + concentration will decrease.
• The pH of a weak acid (HA) dissolved in pure water is lower than the pH of a weak acid (HA) dissolved in a solution containing its conjugate base (A – ). This rise in pH is due to a decrease in H + concentration.

Question 3

HA(aq) <==> H+(aq) + A-(aq)
HA is a weak acid in solution. A- is the conjugate base. Suppose salt is added to the aqueous solution to yield an initial concentration for A- of [A -] = 0.3M. 1.00 M of HA is then added. K for the reaction = 1.0*10^-10. Which statement about this reaction is not correct?

Answer 3

At equilibrium the solution is acidic.

Question 4

Which statement about Le Châtelier’s principle is not true?

Answer 4

The final equilibrium constant for the reaction will increase if you increase the concentration of H + in solution.

Question 5

When carbon dioxide dissolves in water, it reacts slightly with the water to form carbonic acid, H2CO3.
CO2(aq) + H2O (aq) <==> H2CO3(aq)
Our bodies maintain low amounts of H2CO3 in our blood by controlling the amount of CO2 in the blood. Assuming all of this is true, which of the following statement about this process is correct?

Answer 5

Adding more H2CO3 to the system will cause the equilibrium to shift in the direction of the reactants.

Question 6

Which statement about the common ion effect is not true?

Answer 6

When a salt is added to a weak base or acid, it provides a common ion for the reaction. This causes the pH of the final solution to increase.

Question 7

Which of the following statements about LeChâtelier’s principle is not true?

Answer 7

The equilibrium constant will change as you change the concentration of a component in a system.

Question 8

What does LeChâtelier’s principle state?

Answer 8

If a system is in equilibrium and a stress is applied to the system, the system will shift in such a way as to relieve the stress.

Question 9

Look at the following reaction:
PCl5(g) <==> PCl3(g) + Cl2(g)
Which of the following events would not cause the equilibrium to shit to the left?

Answer 9

Remove some Cl2(g)

Question 10

Look at the equilibrium curve for the reaction of HA(aq) <==> H+(aq) + A-(aq). For this reaction, Ka = 1*10^-10. Which statement about the system at Point 1 is not true

Answer 10

[H + ] [A− ] / [HA] = 1.0 × 10^−10

Question 11

Suppose that you have a saturated solution of silver chloride (AgCl(s)). At equilibrium, AgCl(s) <==> Ag+(aq) = Cl-(aq) (where K = 1.8*10^-10)
The addition of silver nitrate to the solution causes an increase in the amount of Ag+ in this system. Which statement about this system is not correct?

Answer 11

Because K = 1.8 × 10−10 = [Ag+ ] [Cl − ] / [AgCl], K will increase if more AgCl is formed.