Prelab for Le Chatelier's Principle and Solubility Product SD Flashcards

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1

Lead sulfate is a mostly insoluble salt. The equation for the minimal dissolving of this salt is:

PbSO4 (s) ⇄ Pb2+ (a q) + SO4 2 minus (a q)

solid ⇄ dissolved

Initially, it appears as a white solid sitting in water. A solution of N a2SO4 is added to the mixture. What is observed to happen and why?

  • N a2SO4 increases the SO4 2 minus (a q) in solution. The solid dissolves. Reaction shifts to the left.
  • N a2SO4 increases the SO4 2 minus (a q) in solution. The solid remains. Reaction shifts to the left.
  • N a2SO4 increases the SO4 2 minus (a q) in solution. The solid remains. Reaction shifts to the right.
  • N a2SO4 decreases the SO4 2 minus (a q) in solution. The solid remains. Reaction shifts to the left.
  • N a2SO4 increases the SO4 2 minus (a q) in solution. The solid dissolves. Reaction shifts to the right.
  • N a2SO4 decreases the SO4 2 minus (a q) in solution. The solid dissolves. Reaction shifts to the left.

N a2SO4 increases the SO4 2 minus (a q) in solution. The solid remains. Reaction shifts to the left.

Ex.

Lead sulfate is a sparingly soluble salt in water. And its equilibrium in a solution is given as-
PbSO4 (s) <> Pb^2 (aq) + SO4 ^2- (aq)

Where, the PbSO4 in solid form at left is the undissolved and the ions at right are the dissolved ions.

Now, sodium sulfate is a very soluble salt in water and it makes solution in water quite easily. So adding Na2SO4 will increase the
ion in the solution and it will give rise to the common ion effect, which in turn will reduce the lead sullfate solubility further in the solution.

Another Ex.

The dissociation equation for PbSO4:

PbSO4 (s) ⇄ Pb2+ (a q) + SO4 2 minus (a q)

When N a2SO4 is added to this solution, dissociation of N a2SO4
(which is a strong electrolyte).

N a2SO4(aq) -> 2 Na+ (aq) + SO4 2- (aq)

Upon the addition of N a2SO4, the concentration of SO4 2- ions increases.

This will disturb the established equilibrium and to retain equilibrium some SO4 2- ions will combine with Pb2+ ions to form a solid.

Explanation:
This situation is based on Le Chatelier's Principle.
The principle states that "To any system at equilibrium, if some sort of stress is applied the equilibrium will re-establish itself by shifting in the direction that reverts the effect of applied stress."

This means that the solubility of PbSO4 decreases upon the addition of N a2SO4, this phenomenon is called the Common Ion Effect.

The only way it can do this is by shifting to the left direction (consuming SO4 2- ions).

This situation is based on Le Chatelier's Principle.
The principle states that "To any system at equilibrium, if some sort of stress is applied the equilibrium will re-establish itself by shifting in the direction that reverts the effect of applied stress."

2

The equation for the dissolving of lead (II) chloride (including heat) is:

PbCl2 (s) + heat ⇄ Pb2+ (a q) + 2 Cl minus (a q)

solid ⇄ dissolved

Which statement is true?

  • When PbCl2 (s) is heated in water, the solid remains.
  • When PbCl2 (s) is heated in water, the solid dissolves.

When PbCl2 (s) is heated in water, the solid dissolves.

Ex.

On reactant side we have lead(II) chloride plus heat and on product side we have ions of lead and chloride. If we give heat to solid lead chlride then it will get converted into ions. Given reaction is endothermic because heat is given, so when PbCl2(S) is heated in water, the solid dissolves.

3

The equation for the dissolving of magnesium hydroxide is:

Mg(O H)2 (s) + heat ⇄ Mg2+ (a q) + 2 O H minus (a q)

solid ⇄ dissolved

What happens when N a O H solution is added?

  • Concentration of O H minus increases.
  • Concentration of Mg2+ increases.
  • Concentration of O H minus decreases.

Concentration of O H minus increases.

Ex.

Mg(OH)2 is a strong base, so dissociate completely.

NaOH is also strong base, so dissociate completely.

No reaction takes place in both. Both dissociate completely to realease OH- ions.

Thus, the concentration of OH- increase.

4

The equation for the dissolving of magnesium hydroxide is:

Mg(O H)2 (s) + heat ⇄ Mg2+ (a q) + 2 O H minus (a q)

solid ⇄ dissolved

What happens when a solution containing H+ is added?

  • The concentration of O H minus decreases.
  • The concentration of O H minus increases.
  • The concentration of Mg2+ decreases.

The concentration of O H minus decreases.

Ex.

When a solution containing H+ ions is added to the magnesium hydroxide solution, the H+ ions react with the OH- ions in the solution to form water, according to the chemical equation:

H+ + OH- -> H2O

This reaction reduces the concentration of OH- ions in the solution. As a result, the equilibrium of the dissolution reaction is shifted to the right, to compensate for the decrease in OH- ions. This means that more Mg2+ ions and OH- ions are produced from the solid Mg(OH)2, leading to an increase in their concentrations in the solution.

Therefore, the correct answer is:

The concentration of OH- decreases, while the concentration of Mg2+ increases.