Question

Write a net ionic equation starting with this equation : [Co(NH₃)₅(OH₂)]³⁺ + 3HCl à [Co(NH₃)₅Cl]Cl₂ + H₂O + 3H⁺

and explain the common-ion effect on reducing solubility of a salt.

Answer 1

The ionic equation is

[Co(NH3)5(H2O)] ⁺³ + 3H+ + 3Cl- -------------> [Co(NH3)Cl]⁺² + 2Cl- + 3H+ + H2O

Removing the spectator ions of H+ and Cl-

the net ionic equation is

[Co(NH3)5(H2O)] ⁺³ +Cl- --------->  [Co( NH3)Cl]⁺² + H2O

b) Whenever a soluble salt having a common ion is added to a solution of sparinlgy soluble salt , the solubility of sparingly salt decreases . It is because we are adding product of the reaction to the equilibrium mixture , according to Lechatlier principle when the product concentration is increased, the equilibrium favors left hand side or backward reaction is favored.

This decrease in solubility of salt with the addition of common ion is called common ion effect .

Let us take a sparingly soluble salt AB

AB (s) <--------> A+ (aq) + B-(aq)

This is the solubility equillibrium and s is the molar solubility of salt AB.

AB (s) <--------> A+ (aq) + B-(aq)

a 0 0 initial moles

a-s s s equilibrium moles

thus Ksp = [A+]{B-]

= sxs

Now as we add a common ionsoluble salt , AP of concentration C

AP -----------> A+ + P-

0 c c after dissolution

Thus our equilibrium

AB (s) <--------> A+ (aq) + B-(aq)

a 0 0 initial moles

a-s s s equilibrium moles

a- x x+ c x after common ion addition

Now Ksp = x(x+C) as Ksp is independent of initial concentrations and varies only with temperature.

as C >> x

Ksp = C x .and x <<s (solubility in the absence of common ion)