THE NITROGEN CYCLE

SECOND TERM E-LEARNING NOTES

SUBJECT: CHEMISTRY

CLASS: SS 2

SCHEME OF WORK

WEEK 6

TOPIC: NITROGEN

CONTENTS:

1. GENERAL PROPERTIES OF NITROGEN–GROUP VA ELEMENTS, LABORATORY AND INDUSTRIAL PREPARATION OF NITROGEN,
2. PROPERTIES AND USES OF NITROGEN
3. COMPOUND OF NITROGEN – AMMONIA- LABORATORY AND INDUSTRIAL PREPARATION
4. PROPERTIES AND USES, TEST FOR AMMONIA, FOUNTAIN EXPERIMENT

PERIOD 1: GENERAL PROPERTIES OF NITROGEN – GROUP V ELEMENTS

Nitrogen and other group VA elements are non-metals and show two common valences of 3 and 5. They are electron acceptors and both form several acidic oxides. They also form similar hydrides and chloride e.g. Nitrogen and phosphorus form N₂O₃ and P₄O₆, N₂O₅ and P₄O₁₀, NH₃, PH₃, NCl₃ and PCl₃

Properties of VA elements

W = white, gr = grey, p = 28 atmospheres, r = red, s = sublimes

LABORATORY AND INDUSTRIAL PREPARATION OF NITROGEN

1. Laboratory preparation of nitrogen from air

Atmospheric air is passed through aqueous NaOH, in order to absorb carbon (iv) oxide.

2NaOH_(aq) + CO_2(g) Na₂CO_3(aq) + H₂O

It is then passed over red-hot copper metal in a combustion tube in order to remove oxygen.

Cu(s) + O_2(g) 2CuO_(s)

The residual gas collected over water is Nitrogen contaminated with small quantity of noble gases. Other methods of preparing nitrogen in the laboratory include:

Preparation from Ammonia gas.

CuO_(s) + NH_3(g) 3Cu_(s) + H₂O_(g) + N_2(g)

From ammoniumdioxonitrate (iii)

NaNO_2(aq) + NH₄Cl_(aq) NH₄NO_2(aq) + NaCl_(aq)

1. Industrial preparation of nitrogen

Nitrogen is produced commercially from fractional distillation of liquefied air.

PROCEDURE: Carbon (iv) oxide is removed from air. This air is liquefied by subjecting it to succession compression and cooling processes.

Nitrogen is collected first after distillation before oxygen because it has a lower boiling point than oxygen.

Nitrogen is stored in steel in steel cylinders and sold as liquid nitrogen or as the compressed gas.

EVALUATION

1. Describe how nitrogen can be obtained from ammonia.[mediator_tech]
2. Name the reagents necessary for the laboratory preparation of nitrogen from air.
3. How is nitrogen obtained commercially?

PERIOD 2 : PROPERTIES AND USES OF NITROGEN

PHYSICAL PROPERTIES

1. Pure nitrogen is colourless, odourless and tasteless
2. It is insoluble in water
3. It is lighter than air.
4. It has no effect on litmus paper

CHEMICAL PROPERTIES

1. Combustion: Nitrogen gas is generally unreactive at ordinary temperatures and pressures. It does not burn and does not support burning. It extinguishes a lighted splint.
2. Reaction with metals. Magnesium ribbon burns in air to produce a mixture of magnesium oxide and magnesium nitricide, a white solid.

2Mg_(s) + O_2(g) MgO_(s)

3Mg_(s) + N_2(g) Mg₃N_2(s)

Magnesium nitride decomposes on addition of water to liberate ammonia gas

Mg₃N_2(s) + 6H₂O_(l) 3Mg(OH)_2(aq) + 2NH_3(g)

1. (a) Reaction with non-metals: (a) under conditions of high temperatures and pressures, and in the presence of finely divided iron as catalyst, nitrogen combines with hydrogen, to produce ammonia. The reaction in reversible.

N_2(g) + 3H_2(g) 2NH_3(g)

(b) Nitrogen combines with oxygen at high temperature and pressure to produce Nitrogen (ii) oxide, an unstable colourless and odourless gas. The reaction is reversible.

N_2(g) + O_2(g) ⇌ 2NO_(g)

USES OF NITROGEN

1. In the Haber process in production of ammonia.
2. Liquid nitrogen is used as a refrigerant.
3. Due to its inertness and because it does not support combustion it is used for the following purposes:

(a) For prevention of fire

(b) As a diluents: to reduce combustion (nitrogen is responsible for the low heating value of producer gas, and the low combustion rate of atmospheric oxygen.

(c) Nitrogen provides inert atmosphere for food processing and packaging, and during some chemical reactions.

NITROGEN CYCLE

The continuous circulation of nitrogen between the free gaseous nitrogen in the atmosphere and the combined nitrogen in the proteins of the living tissues is known as the Nitrogen circle.

Nitrogen is an essential element in the formation of proteins in plants and animals. Unfortunately, it cannot be used directly in its free gaseous state, except in the form of trioxonitrate (v) salts (Nitrate). There are various processes through which atmospheric nitrogen is converted into trioxonitrate (v) salts in the soil, the process are called FIXATION OF FIXING OF NITROGEN.

1. When there is a thunder storm or lightning electric discharges are produced, during which atmospheric nitrogen and oxygen combine to form unstable and colourless nitrogen (ii) oxide

N_2(g) + O_2(g) 2NO_(g)

The nitrogen (II) oxide readily combines with more oxygen, to form brown fumes of nitrogen (iv) oxide.

2NO_(g) + O_2(g) 2NO_(g)

The nitrogen (iv) oxide dissolves in rain water in the presence of oxygen, to produce a very dilute solution of trioxonitrate (v) acid, HNO₃, which falls as acid rain.

4NO_2(g) + 0_2(g) 2NO_2(g)

1. Plants absorb trioxnitrate (v) salts through the roots for the formation of flesh, and the excess is excreted as urea-in urine and faeces. When plants and animal die and decay, the bacteria in the soil decompose the proteins into organic compounds of nitrogen which are then converted into ammonia and ammonium salts, by PUTREFYING bacteria. A portion of the ammonium salts, in alkali medium, is oxidized into trioxonitrate(v) salt by NITRIFYING bacteria, while the remaining portion in acidic medium is oxidized to nitrogen, by DENTRIFYING bacteria. The nitrogen eventually goes back into the atmosphere. This is the process that usually compensate for the lost of atmospheric nitrogen.
2. Manure and nitrogeneous fertilizers are being added to the soil periodically, in order to replace nitrogen that has been used up by crops or plants. The nitrogen cycle operates in nature such that the percentage of nitrogen in the atmosphere remains constant.

THE NITROGEN CYCLE

EVALUATION

1. Name one process, which converts atmospheric nitrogen to nitrate (v) in the soil.
2. Explain the formation of trioxonitrate (v) acid in the nitrogen cycle.