Experiment 8 elements of va and via groups

(P-ELEMENTS)

Nitrogen and phosphor are elements of VA group of the periodic table. They have 5 electrons in the outer shell (valence electrons). They may lose electrons and form positively charged ions (oxidation numbers from +1 to +5) or gain electrons and form negatively charged ions (oxidation number -3).

Hydrogen compounds of nitrogen and phosphor are ammonia NH₃ and phosphine (PH₃). The presence of a lone electron pair of nitrogen and phosphor leads to the possibility of formation of a coordinate bond with a proton. In aqueous solutions ammonia interacts with water molecules to form ammonium hydroxide which possesses weak basic properties:

NH₃ + H₂O NH₄OH NH₄⁺ + OH

Ammonium ion has almost same properties as metallic ions, for example it can form salts. Ammonium salts decompose while heating:

NH₄Cl NH₃ + HCl

In oxides nitrogen has various oxidation states from +1 to +5. Nitric acid HNO₃ is one of the strongest acids with a high oxidizing strength. Depending on the nature of a reducing agent and the concentration of the acid, the NO₃ group can gain from 1 to 8 electrons and transfer into NO₂, NO, N₂O, N₂ or NH₄⁺.

Salts of nitric acid (nitrates) are water soluble.

Nitrous acid HNO₂ is a weak acid with redox duality. It exists only in diluted solutions and decomposes at high concentrations:

3HNO₂ HNO₃ + 2NO + H₂O

In contrast with nitric acid, phosphoric acid has no oxidizing properties. Phosphates form soluble complexes with a lot of metal ions.

Sulfur is situated in the VIA group of the periodic table and has 6 valence electrons. Its oxidation numbers are +4 , +6 and -2. Sulfuric acid H₂SO₄ (conc.) is a strong oxidizing agent and hydrogen sulfide and its salts (sulfides) are good reducing agents. Sulfurous acid H₂SO₃ and its salts (sulfites) have redox duality.

The strength of acids containing sulfur increases with the increase in oxidation number of sulfur.

Biological importance of sulfur

Sulfur is a composite of most important aminoacids. Metal sulfates are used in medicine: CaSO₄ as a stuff for plasters, BaSO₄ in rontgenoscopy of stomach, MgSO₄10H₂O as purgative. Some antibiotics have sulfur compounds as composites.

EXPERIMENTAL PART

1. Formation and properties of ammonia

Take equal quantities of dry ammonium chloride and calcium hydroxide and mix them in a porcelain mortar. Transfer the mixture in a dry test tube, cover with a cork with a tube and heat. Collect ammonia in a dry test tube holding it with its bottom upwards. Study properties of ammonia: a) Immerse the test tube filled with ammonia into a cup with water with 2 drops of phenolphtalein (hold it with the bottom upwards). Write down your observations.

b) Take some anhydrous copper sulphate in a dry test tube and pass ammonia from the tube. Observe the formation of a complex compound of tetraamminecopper (II) sulphate.

c) Hold a stirring rod moistened with hydrochloric acid in front of the test tube filled with ammonia. Observe the formation of a “smoke” of ammonium chloride.