Form 1:
Water and Hydrogen

Menu|Table of content

Properties of Hydrogen gas

(a)Physical properties

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

When mixed with air it has a characteristic pungent choking smell

2. It is insoluble in water thus can be collected over water.

It is the lightest known gas.

It can be transferred by inverting one gas jar over another.

(b)Chemical properties

(i)Burning

I. Hydrogen does not support burning/combustion.

When a burning splint is inserted into a gas jar containing Hydrogen, the flame is extinguished /put off.

II. 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.

(ii)Redox in terms of Hydrogen transfer. Redox can also be defined in terms of Hydrogen transfer.

(i)Oxidation is removal of Hydrogen

(ii)Reduction is addition of Hydrogen

(iii)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 thus the Oxidizing agent.

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

Set up of apparatus
(a)Chemical equation

(i) In glass tube

Copper (II) Oxide + Hydrogen -> Copper + Hydrogen gas (oxidizing agent) (reducing agent) (black) (brown)
CuO (s) + H2(g) -> Cu(s) + H2O(l)

(ii) when excess Hydrogen is burning.

Oxygen + Hydrogen -> Water
O2 (g) + 2H2 (g) -> 2H2O (l)

(b)Chemical equation

(i) In glass tube

Lead (II) Oxide (oxidizing agent)+ Hydrogen (reducing agent)-> Lead + Hydrogen gas

PbO (s) + H2 (g) -> Pb(s) + H2O (l)

(ii) when excess Hydrogen is burning.

Oxygen + Hydrogen -> Water
O2 (g) + 2H2 (g) -> 2H2O(l)

(c)Chemical equation

(i) In glass tube

Iron (III) Oxide (oxidizing agent)+ Hydrogen (reducing agent)-> Iron + Hydrogen gas

Fe2O3 (s) + 3H2 (g) -> Fe(s) + 3H2O (l)

(ii) when excess Hydrogen is burning.

Oxygen + Hydrogen -> Water
O2 (g) + 2H2 (g) -> 2H2O (l)

iii) 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.

<< Hydrogen | Uses of Hydrogen gas >>