Laboratory preparation of gases
YOU SHOULD BE ABLE TO:
I Describe the laboratory preparation of gases I Explain the use of gases, based on their properties
In the preparation of oxygen from hydrogen peroxide, manganese (IV) oxide is added as a catalyst to speed up the decomposition process.
When the catalyst is added, the hydrogen peroxide breaks down to produce oxygen and water. The equation for the reaction is given below: 2H2O2 (l) ? 2H2O (l) + O2 (g)
The oxygen produced is collected by the downward displacement of water in a gas tube, and is wet. To dry the gas, a drying agent such as concentrated sulphuric acid can be used to pass the gas through. Oxygen gas is colourless, odourless, tasteless and slightly soluble in water. It is, therefore, essential for breathing, especially for aquatic animals. It supports combustion and is important for the burning of fuels. The test for oxygen is that it relights or rekindles a glowing splint.
In the laboratory, carbon dioxide is produced from the action of an acid on a carbonate. Reacting calcium carbonate (marble chips) with hydrochloric acid produces calcium chloride, water and the gas carbon dioxide. The wet gas is collected by the downward displacement of water.
Carbon dioxide is also slightly soluble in water and produces a weakly acidic solution, carbonic acid. CO2 (g) + H2O (l) ? H2CO3 (aq).
Carbon dioxide does not support combustion so it is used in fire extinguishers. The gas, when cooled, forms dry ice (solid carbon dioxide) and is used as a refrigerant. The test for carbon dioxide is that it turns calcium hydroxide (lime water) milky or cloudy.
Ammonia is produced by the action of an alkali on an ammonium salt. When ammonium chloride and calcium hydroxide are heated, ammonia, calcium chloride and water are produced. The equation is given below: 2NH4Cl (s) + Ca(OH)2 (s) ? 2NH3 (g) + CaCl2 (s) + 2H2O (g)
NOTE: To dry the gas, calcium oxide is used instead of concentrated sulphuric acid or calcium chloride. Why? Ammonia is an alkaline gas, and would react with the sulphuric acid. 2NH3 (g) + H2SO4 (l) ? (NH4)2SO4 (aq)
Ammonia also reacts with calcium chloride. Ammonia is very soluble in water and is also lighter than air. So, notice that it is collected by the method of upward delivery. Being alkaline, it will turn damp red litmus to blue.
Ammonia dissolves in water to form a weak alkali, ammonium hydroxide. It is important in the formation of ammonium fertilisers and in the production of nitric acid. Industrially, ammonia is made by the Haber Process.
THE HABER PROCESS
Nitrogen and hydrogen are reacted in the presence of iron catalyst to produce ammonia. N2 (g) + 3 H2 (g) ? 2 NH3 (g)
This is a reversible reaction with a low yield (15%). The iron is used as a catalyst to speed up the reaction under reaction conditions of 450¡C and about 200 atmospheres (pressure). The unreacted gases are recycled.
The ammonia produced is used to make fertilisers. For example, ammonium sulphate can be made by reacting ammonia with sulphuric acid. 2NH3 (aq) + H2SO4 (aq) ? (NH4)2SO4 (aq) Ammonia is also used to make nitric acid, an important chemical.
Step 1: Ammonia is reacted with oxygen (oxidised) in the presence of mixed catalyst of platinum and rhodium at 900¡C. 4NH3 (g) + 5O2 (g) = 4NO (g) + 6H2O (l)
Step 2: The nitrogen monoxide is mixed with air to form nitrogen dioxide 2NO (g) + O2 (g) = 2 NO2 (g) Step 3: In the final step, nitrogen dioxide and oxygen and water react to form nitric acid. 4NO2 (g) + O2 (g) + 2H2O (l) = 4HNO3 (aq) Other nitrogen fertilisers that can be made include ammonium nitrate. Fertilisers add nutrients that are needed for plant growth to the soil. Nitrogen is important in the formation of proteins in leaves and stalks. Yellowing of leaves shows that nitrogen is lacking.
REVIEW
1. How would you test for the gases carbon dioxide and ammonia?
2. What property of oxygen makes it suitable for breathing for aquatic animals?
3. Why is concentrated sulphuric acid not used to dry ammonia gas?
4. Why is ammonia an important chemical?