Ammonia

Ammonia is prepared in the laboratory by heating any ammonium salt with an alkali. In most cases, ammonium chloride and calcium hydroxide (the cheaper alkali) are used. Both are solid so they must be thoroughly ground first to give a very fine mixture so that the reaction can occur efficiently.

$\text{Ca(OH)}_{2(s)} + 2\text{NH}_{4\text{Cl}_{(s)}} \rightarrow \text{CaCl}_{2(s)} + 2\text{H}_{2\text{O}_{(l)}} + 2\text{NH}_{3(g)}$

Calcium hydroxide reacts with ammonium chloride to produce ammonia gas, calcium chloride and water. The flask is tilted to prevent any condensed water formed from running back into the hot flask, which might break it.

Concentrated sulphuric acid is acidic and would absorb the gas forming a salt e.g. $2\text{NH}_{3(g)} + \text{H}_{2\text{SO}_{4(l)}} \rightarrow (\text{NH}_4)_2\text{SO}_{4(s)}$

It reacts with calcium chloride, forming solid complex compounds, e.g. $8\text{NH}_{3(g)} + \text{CaCl}_{2(s)} \rightarrow \text{CaCl}_2\cdot8\text{NH}_{3(s)}$

1. Ammonia is a colourless gas with a choking and characteristic smell of urine
2. It turns moist red litmus paper blue
3. It is less dense than air
4. It is very soluble in water

Reaction with air and oxygen

Ammonia will not normally burn in air but can be made to do so in two ways:

1. Burning ammonia in an oxygen-rich atmosphere; and
2. Use of a catalyst.

Burning ammonia in oxygen-rich air

When ammonia is burned in air enriched with oxygen, the products formed are nitrogen and water.

$4\text{NH}_{3(g)} + 3\text{O}_{2(g)} \rightarrow 2\text{N}_{2(g)} + 6\text{H}_{2\text{O}_{(l)}}$

Using a catalyst

The source of ammonia gas in this experiment is concentrated ammonia solution which gives off fumes of ammonia gas. If some concentrated ammonia solution is left in a stoppered flask for a few minutes, the flask will quickly become full of ammonia gas by diffusion.

The catalyst used is the metal platinum. The platinum coil is heated in a bunsen flame until it is red hot. It is then lowered into a flask containing ammonia. Near the top of the flask brown fumes can be seen.

The products of this reaction are nitrogen monoxide (nitrogen (II) oxide), which is a colourless gas, and water.

$4\text{NH}_{3(g)} + 5\text{O}_{2(g)} \rightarrow 4\text{NO}_{(g)} + 6\text{H}_{2\text{O}_{(g)}}$

It reacts with the oxygen in the air to form nitrogen dioxide gas which is brown in colour. Hence, the brown fumes are seen at the neck of the flask.

$2\text{NO}_{(g)} + \text{O}_{2(g)} \rightarrow 2\text{NO}_{2(g)}$

Reaction with copper

When ammonia gas is passed over heated copper (II) oxide, the gas is oxidized to nitrogen and water by the hot copper (II) oxide while the oxide is reduced to copper.

$3\text{CuO}_{(s)} + 2\text{NH}_{3(g)} \rightarrow 3\text{Cu}_{(s)} + 3\text{H}_{2\text{O}_{(l)}} + \text{N}_{2(g)}$

Copper (II) oxide is black. During the reaction it is oxidized to copper which is reddish brown in colour.

Reaction with hydrogen chloride gas

When ammonia and hydrogen chloride are mixed, dense white fumes of ammonium chloride are formed.

$\text{NH}_{3(g)} + \text{HCl}_{(g)} \rightarrow \text{NH}_{4\text{Cl}_{(s)}}$

This test is simply performed by dipping a glass rod into concentrated hydrochloric acid then holding the glass rod at the mouth of a gas jar containing ammonia gas.

1. Ammonia gas is used in the manufacture of nitric acid
2. Liquid ammonia is used in refrigeration
3. Ammonia gas is used in the manufacture of fertilizer such as urea
4. Ammonia solution is used in laundry works to remove temporary hardness of water
5. Ammonia solution is used in smelting of salts. These salts have stimulating effects on the brain and they prevent fainting
6. It is used to make explosive