Properties of Hydrogen Gas

Physical properties

1. Hydrogen is a neutral, colourless and odourless gas.

When mixed with air it has a characteristic pungent choking smell

2. It is insoluble in water thus can be collected over water.
3. It is the lightest known gas. It can be transferred by inverting one gas jar over another.

Chemical properties.

Burning

1. Hydrogen does not support burning/combustion. When a burning splint is inserted into a gas jar containing Hydrogen, the flame is extinguished/put off.
2. Pure dry hydrogen burn with a blue quiet flame to form water. When a stream of pure dry hydrogen is ignited, it catches fire and continues to burn with a blue flame.

III. Impure(air mixed with) hydrogen burns with an explosion. Small amount/volume of air mixed with hydrogen in a test tube produce a small explosion as a “pop” sound.

This is the confirmatory test for the presence of Hydrogen gas. A gas that burns with a “pop” sound is confirmed to be Hydrogen.

Redox in terms of Hydrogen transfer

Redox can also be defined in terms of Hydrogen transfer.

1. Oxidation is removal of Hydrogen
2. Reduction is addition of Hydrogen
3. Redox is simultaneous addition and removal of Hydrogen

Example

When a stream of dry hydrogen gas is passed through black copper (II) oxide, hydrogen gas gains the oxygen from copper(II)oxide.

Black copper (II) oxide is reduced to brown copper metal.

Black copper(II)oxide os thus the Oxidizing agent.

Hydrogen gas is oxidized to Water. Hydrogen is the Reducing agent.

Set up of apparatus

Chemical equation

1. In glass tube Copper(II)Oxide + Hydrogen ->Copper + Hydrogen gas

(oxidizing agent) (reducing agent)

(black) (brown)

CuO (s) + H₂(g) -> Cu(s) + H₂O(l)

2. When excess Hydrogen is burning.

Oxygen + Hydrogen -> Water

O₂(g) + 2H₂(g) ->2H_2O(l)

Chemical equation

1. In glass tube

Lead(II)Oxide + Hydrogen ->Lead + Hydrogen gas

(oxidizing agent) (reducing agent)

(brown when hot/ (grey)

yellow when cool)

PbO (s) + H₂(g) ->Pb(s) + H₂O(l)

2. when excess Hydrogen is burning.

Oxygen + Hydrogen -> Water

O_{2(g) + 2H2(g) -> 2H2O(l)}

Chemical equation

1. In glass tube

Iron(III)Oxide + Hydrogen ->Iron + Hydrogen gas

(oxidizing agent)

(reducing agent)

(Dark grey)

(grey)

Fe_{2O3 (s) + 3H2(g) ->Fe(s) + 3H2O(l)}

2. When excess Hydrogen is burning.

Oxygen + Hydrogen -> Water

O₂(g) + 2H₂(g) -> 2H₂O(l)

3. Water as an Oxide as Hydrogen

Burning is a reaction of an element with Oxygen. The substance formed when an element burn in air is the oxide of the element.

When hydrogen burns, it reacts/combines with Oxygen to form the oxide of Hydrogen. The oxide of Hydrogen is called water.

Hydrogen is first dried because a mixture of Hydrogen and air explode. The gas is then ignited. The products condense on a cold surface/flask containing a freezing mixture. A freezing mixture is a mixture of water and ice. The condensed products are collected in a receiver as a colourless liquid.

Tests

(a) When about 1g of white anhydrous copper(II)sulphate(VI)is added to a sample of the liquid ,it turns to blue. This confirms the liquid formed is water.

(b) When blue anhydrous cobalt (II)chloride paper is dipped in a sample of the liquid ,it turns to pink. This confirms the liquid formed is water.

(c) When the liquid is heated to boil, its boiling point is 100oC at sea level/one atmosphere pressure. This confirms the liquid is pure water.