Study of Compounds
Hydrogen chloride (HCl)
Physical properties of ammonia are given below:
- Ammonia is a colourless gas with a pungent odour.
- Freezing point of ammonia = 198.4 K
Boiling point of ammonia = 239.7 K
- Its vapour density is 8.5 and thus, it is lighter than air.
- Ammonia is extremely soluble in water. It is described using fountain experiment.
Take ammonia gas in a round-bottomed flask and fix a two-holed stopper in its mouth. Through one hole, pass a long jet tube such that the jet is close to the base of the flask. Through the other hole, pass a dropper half filled with water. Clamp the flask with iron stand. Place a beaker containing red litmus solution under the jet tube.
Press the bulb of the dropper. You will notice that red litmus solution rises up in the jet tube and comes out of the nozzle with a great force and forms a blue coloured fountain.
The fountain is formed because when the water is introduced from the dropper, it completely absorbs ammonia gas, thus creating a pressure within the flask. To make up for this, red litmus solution rises up. The colour change is due to its basic character.
- In solids and liquids, ammonia is associated with hydrogen bonds and hence, it has higher melting and boiling points.
- Ammonia decomposes at high temperatures or by electric sparks into hydrogen and nitrogen gas.
- Ammonia forms ammonium salts with acids. (Example: NH4Cl, (NH4)2 SO4, NH4NO3 etc.)
- An aqueous solution of ammonia reacts with hydrochloric acid, sulphuric acid and nitric acid to form ammonium chloride, ammonium sulphate and ammonium nitrate respectively.
- Oxidation of ammonia in excess of oxygen without catalyst
Set up the apparatus as shown. Pass ammonia gas and try to light it. You will observe that the gas does not burn.
Stop passing ammonia and pour oxygen from tube B. Pass ammonia gas and try to light it. You will observe that the gas burns with a yellow flame.
- Catalysed oxidation
Pass the electric current through the platinum filament and pass the mixture of ammonia and oxygen gas.
You will observe red-brown fumes due to oxidation of nitric oxide gas.
Stop passing current. You will observe that the platinum filament keeps glowing and red-brown fumes keep forming.
Here, platinum acts as a catalyst.
- Reducing property of ammonia gas
Reduction of chlorine
When chlorine is in excess as compared to ammonia, the ammonia reduces chlorine to hydrochloric acid. However, if ammonia is in excess as compared to chlorine, then ammonium chloride is formed.
Reduction of metal oxides
Ammonia reduces oxides of less active metal to metal, water, and nitrogen.
- Aqueous ammonia
All soluble salts of metals react with aqueous ammonia to form their respective insoluble hydroxide and ammonium salts.
Being a weak base, it precipitates the hydroxides of many metals to form their salt solutions.
- The Lone pair of electrons on the N-atom makes the NH3 molecule a Lewis base.
- It donates the electron pair and forms linkage with metal ions.
- Reaction with carbon dioxide: Ammonia forms urea on reaction with carbon dioxide at 150 oC and 150 atm.
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