Question

When hydrogen burns in oxygen, water is formed; when electricity is passed through water, hydrogen and oxygen are given out. Name the type of chemical change involved in the two cases.

Answer 1

Part 1: Reaction of hydrogen and oxygen:

• When hydrogen burns in oxygen, water is formed.
• $\underset{\mathrm{Hydrogen}}{2{\mathrm{H}}_2\left(\mathrm{g}\right)}+\underset{\mathrm{Oxygen}}{{\mathrm{O}}_2\left(\mathrm{g}\right)}\to \underset{\mathrm{Water}}{2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}$
• The above reaction is a type of combination reaction because two or more simple substances combine to form a single product.

Part 2: Passing of electricity through water:

• When electricity is passed through water, hydrogen and oxygen are formed.
• $\underset{}{\underset{\mathrm{Water}}{2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}\stackrel{\mathrm{Electric}\mathrm{current}}{\to }}\underset{\mathrm{Hydrogen}}{2{\mathrm{H}}_2\left(\mathrm{g}\right)}+\underset{\mathrm{Oxygen}}{{\mathrm{O}}_2\left(\mathrm{g}\right)}$
• The above reaction is a type of decomposition reaction in which the activation energy for decomposition is provided in the form of electrical energy.