Common Ion Effect: pH of LiF, KI & NH4Cl Solutions

[tribalman100]

Homework Statement

does the ph increase , decrease or remain the same on addition of each of the folling
a) LiF to an HF Solution
B) KI to an HI solution
C) NH4Cl to an NH3 solution

Homework Equations

The Attempt at a Solution

i got the equations and the answers to these i just want to know the reason so i can do it next time on why Lif ph increases, Ki's remains the same and that NH4Cl's decreases . it would be a great help and this explanation might save my chemistry grade. thanks for the help guys

 

[Kushal]

well, think in terms of equillibria.

HF <------> H+ + F-
LiF <------> Li+ + F-

when you add LiF, the total concentration of F- increases in solution. the dynamic equilibirum of HF is disturbed. according to LCP the backward reaction is momentarily favoured to decrease F- concentration. at the same time, the concentration of H+ also decreases. hence the pH increases. (more alkaline)

try to figure out the others using the same logic.

 

[Blueshift5]

The common ion effect is a phenomenon in which the presence of a common ion in a solution affects the solubility and pH of a compound. In the case of LiF, KI, and NH4Cl solutions, the common ion is the fluoride ion (F-), iodide ion (I-), and ammonium ion (NH4+), respectively.

When LiF is added to an HF solution, the fluoride ion is already present in the solution as a common ion. This leads to a decrease in the solubility of LiF, causing the pH to increase. This is because the common ion (F-) decreases the concentration of F- ions in the solution, which shifts the equilibrium of the reaction to the left, resulting in a lower concentration of H+ ions and a higher pH.

When KI is added to an HI solution, the iodide ion is already present in the solution as a common ion. However, in this case, the common ion (I-) does not have a significant effect on the solubility of KI. Therefore, the pH of the solution remains relatively unchanged.

When NH4Cl is added to an NH3 solution, the ammonium ion (NH4+) is converted to ammonia (NH3) through a dissociation reaction. This leads to an increase in the concentration of ammonia, which is a weak base. As a result, the pH of the solution decreases as the ammonia reacts with water to form ammonium hydroxide, which is a weak acid. This decrease in pH is due to the common ion (NH4+) increasing the concentration of a weak acid in the solution.