Hydrogen
ICSE Class 8 · Chemistry · Chapter 7
What this chapter covers
- Hydrogen: the lightest element
- Preparing hydrogen from water by electrolysis
- Laboratory preparation of hydrogen
- Choosing the metal and the acid
- The Bosch process: hydrogen for industry
- Properties of hydrogen
- Uses of hydrogen
- Oxidation and reduction
Key definitions
Electrolysis is the process of breaking down a compound into simpler substances by passing an electric current through it when it is melted or dissolved in water.
Granulated zinc is zinc that has been melted and poured into cold water, so that it sets into small, rough lumps with a large surface.
Water gas is a mixture of carbon monoxide and hydrogen in roughly equal volumes, made by passing steam over white-hot coke.
Hydrogenation is the addition of hydrogen to a substance, such as the addition of hydrogen to a vegetable oil to turn it into a solid fat.
Oxidation is a chemical reaction in which a substance gains oxygen or loses hydrogen.
Reduction is a chemical reaction in which a substance loses oxygen or gains hydrogen.
Key points
- Hydrogen is the lightest element; it is found mainly in water, acids and organic compounds.
- Electrolysis of acidified water gives hydrogen at the cathode and oxygen at the anode, in the ratio 2 : 1 by volume.
- An electrolyte conducts electricity and is broken down by it; electrodes carry the current in (anode, positive) and out (cathode, negative).
- In the laboratory, hydrogen is made from granulated zinc and dilute hydrochloric or sulphuric acid and collected over water.
- Zinc is preferred because it reacts at a safe, steady rate and is cheap; dilute nitric acid is avoided because it oxidises hydrogen.
- In the Bosch process, water gas and steam pass over a heated iron oxide catalyst; carbon dioxide is then dissolved out in water under pressure.
- Hydrogen is colourless, odourless, almost insoluble and much lighter than air; it burns with a pale blue flame to form water but does not support burning.
- Hydrogen is used to make ammonia, to harden oils into vanaspati, as a clean fuel, in the oxy-hydrogen flame and to obtain metals from their oxides.
- Oxidation is the gain of oxygen or loss of hydrogen; reduction is the loss of oxygen or gain of hydrogen; the two always happen together.