ICSE Class 10 Chemistry Chapter 09 Study of Compounds Ammonia

Study Of Compounds - Ammonia

Syllabus

Ammonia: Its laboratory preparation from ammonium chloride and collection; ammonia from nitrides like Mg3N2, and AlN and ammonium salts Manufacture by Haber's Process; density and solubility of ammonia (fountain experiment); aqueous solution of ammonia; its reactions with hydrogen chloride and with hot copper (II) oxide and chlorine; the burning of ammonia in oxygen; uses of ammonia.

Laboratory preparation from ammonium chloride and collection (the preparation can be studied in terms of setting of the apparatus and diagram, procedure, observation, collection and identification).

Manufacture of ammonia on a large scale - reference should be made to Haber Process for the manufacture of ammonia; Ammonia from nitrides like Mg3N2 and AlN and ammonium salts; the reactions can be studied in terms of reactant, product, condition, equation.

Density and solubility of ammonia (fountain experiment); the property can be learnt in terms of setting of the apparatus, procedure and observation and inference.

Aqueous solution of ammonia - reaction with sulphuric acid, nitric acid, hydrochloric acid and solutions of iron (III) chloride, iron (II) sulphate, lead nitrate, zinc nitrate and copper sulphate.

Its reaction with: hydrogen chloride, hot copper (II) oxide, with chlorine in excess and ammonia in excess, burning of ammonia in oxygen; all these reactions may be studied in terms of reactants, products, condition, equation and observation; reference should be made to preparation of nitrogen from ammonium nitrite.

Uses of ammonia - manufacture of fertilisers, explosives, nitric acid, refrigerans gas (Chlorofluoro carbon - and its suitable alternatives which are non-ozone depleting), cleansing agents, source of hydrogen.

The catalytic oxidation of ammonia, as the source of nitric acid (refer to Ostwald process): simple diagram for a catalytic oxidation of ammonia in the laboratory (with conditions and reactions only).

Self-explanatory

Molecular Formula And Structure

Molecular formula: NH3

Relative molecular mass: 17

The Lewis diagram or dot diagram shows the nitrogen atom bonded to three hydrogen atoms, with a lone pair of electrons on the nitrogen. This represents covalent bonding in ammonia. The nitrogen atom has one lone pair which is important for its chemical properties.

Ammonia and ammonium compounds being highly soluble in water, do not occur as minerals.

Note: The pungent smell which one encounters near a decaying heap of organic matter or a toilet is due to ammonia.

It is produced due to bacterial decomposition of urea (NH2CONH2), present in the urine.

\[CO(NH_2)_2 + 2H_2O \rightarrow 2NH_3 + H_2O + CO_2\]

Forms Of Ammonia

Ammonia is used commonly in the following forms:

(i) Gaseous ammonia (dry ammonia gas)

(ii) Liquid ammonia. When dry ammonia is liquefied by applying high pressure it is called liquid ammonia.

(iii) Liquor ammonia fortis is a saturated solution of ammonia in water. It is called 880 ammonia. It has a relative density of 0.880. It is stored in tightly stoppered bottles in a cold place.

(iv) Laboratory bench reagent: Dilute solution of liquor ammonia is used as a laboratory reagent.

9.1 Occurrence

Free state: Ammonia is present in small amounts in air and in traces in natural water.

Combined state: In nature, ammonia occurs in combined form in many compounds such as ammonium chloride (Hindi: Nausadar), ammonium sulphate, etc.

9.2 Preparation Of Ammonia Gas

9.2.1 General Methods Of Preparation

Ammonia gas can be prepared by warming an ammonium salt with caustic alkali, such as slaked lime or caustic soda or caustic potash.

9.2.2 Laboratory Preparation

(1) From Ammonium Chloride

Reactants: Ammonium chloride (NH4Cl) and calcium hydroxide [Ca(OH)2] in the ratio of 2:3 by weight.

Reaction:

\[2NH_4Cl + Ca(OH)_2 \rightarrow CaCl_2 + 2H_2O + 2NH_3\]

Procedure: Reactants are finely ground and taken in a round-bottom flask fitted in a slanting position, mouth downwards as shown in Fig. 9.1. On heating the mixture, ammonia gas is evolved.

Precautions: The flask is tilted in such a way that the water formed in the reaction does not trickle back into the heated flask and thus break it.

Drying of ammonia gas: In order to get dry ammonia, the gas is passed through a drying tower containing lumps of quicklime (CaO).

Unsuitability of other drying agents: Other drying agents like conc. sulphuric acid, phosphorous pentoxide and anhydrous calcium chloride are not used, as ammonia being basic, reacts with them.

\[2NH_3 + H_2SO_4 \rightarrow (NH_4)_2SO_4\]

\[6NH_3 + P_2O_5 + 3H_2O \rightarrow 2(NH_4)_3PO_4\text{ ammonia phosphate}\]

\[CaCl_2 + 4NH_3 \rightarrow CaCl_2 \cdot 4NH_3\text{ (addition compound)}\]

Collection

Ammonia gas is collected in inverted gas jars by the downward displacement of air because it is:

(i) lighter than air (V.D. of NH3, 8-5; that of air, 14-4),

(ii) highly soluble in water and therefore, it cannot be collected over water.

Note: 1. The reactants NH4Cl and Ca(OH)2 are taken in the ratio of 2:3 by weight, higher ratio by weight of the alkali may counteract the loss by sublimation of NH4Cl.

2. Calcium hydroxide is used, as it is cheap and not deliquescent like other alkalties.

3. Though all ammonium salts, on heating with alkaties give NH3, yet NH4NO3 (explosive in nature) and NH4NO2 on warming with alkaties do not produce ammonia because they decompose on heating.

\[NH_4NO_3 \xrightarrow{\Delta} N_2O + 2H_2O\]

\[NH_4NO_2 \xrightarrow{\Delta} N_2 + 2H_2O\]

(2) From Metal Nitrides

Ammonia can also be obtained by the action of warm water on nitrides of metals like magnesium or Aluminium. (This method is costly)

Reaction:

\[Mg_3N_2 + 6H_2O \rightarrow 3Mg(OH)_2 + 2NH_3\]

or

\[AlN + 3H_2O \rightarrow Al(OH)_3 + NH_3\]

Procedure: Magnesium nitride or Aluminium nitride is kept in a round bottomed flask fitted with a thistle funnel and a delivery tube, as shown in Fig 9.2. Warm water is added very slowly.

Ammonia gas is liberated and magnesium hydroxide or aluminium hydroxide is also formed, but it remains in the flask and the liberated ammonia gas escapes through the delivery tube.

The ammonia gas formed is collected by holding the jar with its mouth downwards over the delivery tube.

The gas can be dried by passing it through quicklime packed in a drying tower.

9.3 Preparation Of Aqueous Ammonia

An aqueous solution of ammonia is prepared by dissolving ammonia in water. The rate of dissolution of ammonia in water is very high, therefore, back suction of water is possible. To avoid this, a funnel is attached to the outer end of the delivery tube with rubber tubing (the process is similar to the method used to change HCl gas into hydrochloric acid).

The funnel provides a larger surface area for dissolution of ammonia in water and prevents back suction of water into the hot flask.

Procedure: Water is taken in a container and only a small portion of the mouth of the funnel is dipped in water.

As ammonia dissolves in water at a higher rate than its production in the flask, the pressure in the funnel above water level decreases for a moment, and water rushes into the funnel. As a result, the rim of the funnel loses its contact with water. Since, ammonia produces pushes the water down, the funnel comes in contact with water again. In this way, ammonia dissolves in water without back suction of water.

9.4 Manufacture Of Ammonia (Haber's Process)

Reactants: Nitrogen and hydrogen in the ratio of 1:3 by volume.

Sources Of Reactants

Nitrogen gas is obtained by fractional distillation of liquid air.

Hydrogen gas is obtained from water gas (Bosch process) or from natural gas.

Reaction:

\[N_2 + 3H_2 \rightleftharpoons 2NH_3 + \text{heat}\]

The reaction is reversible, exothermic and proceeds with a decrease in volume.

Favourable Conditions

Temperature: Optimum temperature is 450-500 °C.

Pressure: Above 200 atm.

Catalyst: Finely divided iron.

Promoter: Traces of molybdenum or Al2O3.

Rate of conversion: About 15% of the reacting gases get converted to ammonia.

Recovery Of Ammonia

Ammonia is separated from the unreacted nitrogen and hydrogen by:

1. Liquefaction - Ammonia has a much higher boiling point as compared to N2 and H2 so it condenses easily.

2. Absorbing in water because NH3 is highly soluble in water, while N2 and H2 are very slightly soluble.

Process: Nitrogen and hydrogen are dried, purified and then mixed in the ratio of 1:3 respectively, at a relatively high pressure. This mixture is passed in an electrically heated catalytic chamber containing finely divided iron with aluminium oxide or a little amount of molybdenum at a temperature of 500°C. Mixture of ammonia formed along with residual nitrogen and hydrogen exchanges their heat with the incoming unreacted nitrogen and hydrogen mixture.

The hot mixture of the outgoing gases contains nearly 15% ammonia. The mixture is passed through the cooling pipes (condenser).

Ammonia liquefies first, whereas nitrogen and hydrogen do not liquefy easily.

The unchanged nitrogen and hydrogen are recirculated through the plant to get more ammonia. By re-circulating in this way, an eventual yield of 98% can be achieved. The ammonia produced is stored as liquid under pressure.

Note:

(i) The reaction is exothermic, hence low temperature will favour the synthesis. However, at low temperature, the reaction proceeds slowly. In practice, the optimum temperature has been found to be in the range of 450°C to 500°C.

(ii) Four volumes of reactants produce two volumes of product, hence high pressure favours the formation of ammonia.

The optimum pressure is found to be 200 to 900 atm. Practically, a pressure of about 250 atm. is used.

(iii) Catalyst and promoter: The speed of the reaction can be improved by using a catalyst, which is finely divided iron, obtained by the reduction of iron oxide.

A promoter molybdenum or Al2O3 is used to increase the efficiency of the catalyst.

Rate of conversion: About 15% of the reacting gases get converted to ammonia.

(iv) Since the reaction is exothermic, the heat evolved further maintains the temperature. External heating is, therefore, not required after the reactants are initially heated.

Note: The purification of nitrogen and hydrogen is necessary as impurities like carbon dioxide, carbon monoxide and traces of sulphur compounds (H2S) poisons the catalyst. The removal of these catalytic poisons from nitrogen and hydrogen is very essential.

Teacher's Note

The Haber Process demonstrates how understanding gas equilibrium and industrial chemistry creates fertilizers that feed billions of people worldwide, showing how chemistry directly impacts global agriculture and food security.

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