Question

The three reactions below are all at equilibrium simultaneously in solution.
Ca2+ (aq) + C2O42- (aq) « CaC2O4 (s)

H2C2O4(aq)«H+(aq)+HC2O4-(aq)

HC2O4- (aq) « H + (aq) + C2O42- (aq)
a. If HNO3 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 HNO3 on the equilibrium for the reaction.

Answer 1

Ca²⁺(aq) + C₂O₄^2-(aq) CaC₂O₄(s)

H₂C₂O₄(aq) H⁺(aq)+HC₂O₄^-(aq)

HC₂O₄^- (aq) H⁺(aq) + C₂O₄^2- (aq)

Net reaction can be written by summing up all three equation.

Net reaction:

Ca²⁺(aq) + H₂C₂O₄(aq) CaC₂O₄(s) + 2H⁺(aq)

HNO₃ is a strong acid. Hence, if HNO₃ is added then the concentration of H⁺ in the solution will increase. This will cause the reaction to move away from the equilibrium. According to Le Chatelier's principle, the system will reattain the equilibrium by shifting the reaction in the direction in which the concentration of H⁺ is decreased i.e. in the reverse direction. This means that CaC₂O₄ will react with H⁺ and thereby dissolve in aqueous solution as H₂C₂O₄. Thus, if HNO₃ is added then less of the precipitate of CaC₂O₄ will form.