Question

Cation Group III Quantitative Analysis Lab?

We just completed our unknown lab, where we are given a test tube with some of the Group III cations (Zn, Cr, Mn, Fe, Al, Ni, Co, and we dont know which are present) and we need to write a discussion about certain chemical concepts, and I am stuck on some.

http://web.cerritos.edu/cshimazu/SitePages/Chem%20112/112%20Experiments/5%20Cation%20III%20Part%20B/Cation%20III%20Part%20B.pdf

(That is the lab handout in case I don't make sense)

So we need to write about equilibrium, and explain it using Le Chatalier's principle and give examples

We can talk about complex ions, like why do some form complex ions and others do not?

Whey do cation III group ions form a hydroxide precipitate in the first place?

Discuss buffers. The buffers we used were NaBiO3, we used it when the solution was acidic , how do they work and why are they important?

Answer 1

When ammonium hydroxide is added to these ions, they form hydroxides as precipitate.

chemical equilibrium: At equilibrium the rate of formation of hydroxide is equal to the rate of dissociation of the hydroxide into respective ions.

According to Le Chatlier's principle, if the concentration of hydroxide is increased, concentration of metal hydroxide also increases.

If the ionic product [M^x+][OH^-] > K_sp, the solubility product, then the salt precipitates (it becomes insoluble). The solubility product for hydroxides of group III cations is small. When ammonium hydroxide is added to a solution containing group III cations, the product of ions can easily exceed the K_sp value. Hence, the hydroxide precipitates out.

The metals which have vacant d-orbitals that can accomodate extra electrons from ligands, can form complexes. The first and second period metals which do not have d-orbitals, can't form complexes.

A buffer solution contains an acid and its conjugate base or a base and its conjugate acid. Buffers help in decreasing the concentration of salt formed by shifting the equilibrium to left. This is also referred to as commom ion effect or buffering effect. For precipitating group III ions as hydroxides, ammonium chloride is added along with ammonium hydroxide. Due to the presence of common ion(ammonium ion), the equilibrium for dissociation of ammonium hydroxide (to ammonium ion and hydroxide ion) shifts to left. Consequently, concentration of hydroxide ion is less. Due to low concentration of OH^-, only group III ions are precipitated as hydroxides and not any other metal ion. (group III hydroxides have small K_sp value compared to others and ionic product can easily exceed solubility product resulting in the formation of insoluble precipitates).