Question

The reactions below are all at equlibrium simultaneously in solution:

1. Ca^2+(aq) + C2O4^2- (aq) --> <-- CaC2O4 (s)

2. H2C2O4 (aq) --> <-- H+ (aq) + HC2O4 -^ (aq)

3. HC2O4^- (aq) --> <-- H^+ (aq) + C2O4 ^2- (aq)

a. If potassium hydroxide is added to this solution, would you expect to see more or less of the precipitate (CaC2O4) formed in the solution? Explain your reasoning by showing the effect of the addition of potassium hydroxide on the equilibrium for this reaction.

b. If a weak acid, such as formic acid (HCOOH) is added to the above solution, would you expect to see more or less precipitate (CaC2O4) formed than when hydrochloric acid is added to the solution. Explain your reasoning by showing the effect of the addition of acetic acid on the equilibrium for the reaction and how this differs from the effect of hydrochloric acid on the equilibrium.

Answer 1

First, let us state the Le Chatelier principle which deals with changes in an equilibrium:

The statement is as follows:

If any equilibrium is disturbed, that is, change in conditions such as P,T, concentration, partial pressure, etc.., the system will counterbalance such change in order to favour the system's equilbirium.

• Change in concentration
• Change in pressure
• Change in temperature

if KOH is added, note that OH- is a base

will react with H+ to form H+OH- = H2O

this will shift all equilbiriums of acid towards even more production ofions

if C2O4-2 ion increases, then, the solubility of CaC2O4 decreases,

therefore,

More CaC2O4(s) must be formed

b)

if weak acid is added....

Note that the weak acid will be in equilbirium

wherease the strong acid will simply add H+ to solution, which shifts toward even more H2C2O4 and HC2O4-

additio nof H+ will favour strongly the solubility of CaC2O4(s)

that is, HCl will dissolve CaC2O4(s)

the same is true for the weak acid, HCOOH which will donate H+ but not that storngly as HCl

It will favour some dissolution of the salt

but not as HCl

CaC2O4(s) decreases when HCl is added strongly than HCOOH addition