1. Formation of complex compounds

Put 4-5 drops of silver nitrate into a test tube. Add ammonium hydroxide until the precipitate is formed. Add excess of NH₄OH and observe the formation of a complex compound.

Repeat the procedure with CuSO₄, NiCl₂, Zn(NO₃)₂. Write down the reactions. Find the values of overall instability constants and compare stabilities of the above complex compounds.

Notice: (a) instead of Cu(OH)₂, (CuOH)₂SO₄ precipitate is formed;

(b) coordination number of silver ion is 2; for Cu²⁺ and Zn²⁺ it is 4; for Ni²⁺ - 6.

2. Destruction of complex compounds

Take solutions of chloride diammine silver (I) and sulphate tetraammine copper (II), obtained from the previous experiment, add dropwise diluted (1:1) nitric acid until the complex compounds are destroyed.

3. Formation and properties of Cd²⁺ and Hg²⁺ complexes

Take two test tubes filled with solutions of Cd²⁺ and Hg²⁺ salts respectively. Add a saturated solution of Na₂SO₃. Observe the formation of sulfide precipitates and their further dissolution with the formation of complex compounds Na₂[M(SO₃)₂], where M= Cd²⁺or Hg²⁺.

Add some drops of NaOH to the solutions of complex compounds. Do metal hydroxides precipitate? Why?

4. Dissociation of complex compounds

a). Fill 3 test tubes with 3-5 drops of a double salt NH₄Fe(SO₄)₂. Add 3-4 drops of NaOH into the first test tube and heat the solution. Smell the vapors. Write down your observations.

Add 2-3 drops of BaCl₂ to the second test tube and 2-3 drops of KSCN to the third one. Make your conclusion about the dissociation of double salts in aqueous solutions. Write down the reactions of dissociation of a double salt and reactions of ionic exchange.

b). Fill 2 test tubes with 3-5 drops of K₃[Fe(CN)₆]. Add 2-3 drops of Na₃[Co(NO₂)₆] and 2-3 drops of KSCN to the second one. Write down your observations and the reactions of dissociation of a complex compound and ionic exchange. (Notice: in presence of K⁺ ions, the yellow precipitate of K₂Na[Co(NO₂)₆] is formed.)

Questions and problems

1. For the following complex compounds indicate: (a) inner and outer spheres; (b) central ion, its charge and coordinating number; (c) ligands ; (d) write the reaction of dissociation of complex compounds and complex ions; (e) express overall instability constants; (f) name the following complex compounds.

[Cd(NH₃)₄]Cl₂ ; K₂[Cd(CN)₄]

2. Which of the above mentioned complexes is more stable?

3. Calculate the concentration of the Ag⁺ ions in a 0.1 M solution containing an excess of 1 mole  l^-1 of NH₃.

4. In which case will a reaction occur between solutions of the electrolytes indicated below (exchange of ligands)? Write the equations of these reactions in molecular and net ionic forms and calculate their equilibrium constants:

(a) K₂[HgI₄] + KCN =

(b) K[Ag(CN)₂] + NH₃ =

Experiment 6

S-ELEMENTS

(Alkaline and alkaline earth metals)

Alkaline metals Li, Na, K, Rb, Cs, Fr are situated in the IA group of the periodic table. The outer spheres of these atoms consist of only one electron. So atoms of alkaline metals have a tendency to lose their valence electrons to be transformed into positively charged ions. Their oxidation number is +1. The strength of the attraction of the outer electron to the atom can be valued with the help of the ionization potential which determines the energy of removal of one electron from a neutral atom. In the IA group from Li to Fr the value of ionization potential decreases, the chemical activity of metals increases. Alkaline metals are strong reducing agents.

In air alkaline metals are easily oxidized, that is why they are stored under oil.

Alkaline metals are more active than hydrogen (they have negative values of redox-potentials) so they can replace hydrogen both from acids and water:

2M + HCl = 2MCl + H₂

2M + 2H₂O = 2MOH + H₂

In aqueous solutions metal hydroxides behave as strong electrolytes and are fully dissociated:

MOH M⁺ + OH

Almost all salts of alkaline metals are water soluble. Solutions of salts containing anions of weak acids undergo hydrolysis; they are basic.

Pure alkaline metals are produced by electrolysis of their melted salts.