ICSE Class 8 Chemistry Chapter 2 Hydrogen

Hydrogen

Symbol of hydrogen: H, Formula of hydrogen: H₂, Valency: 1, Atomic number: 1, Atomic mass: 1.00794 amu.

Introduction

Hydrogen is the lightest of all elements known to mankind. This gas was first prepared by Robert Boyle in 1672. Henry Cavendish studied the properties of hydrogen in 1776 and called it the inflammable gas due to its combustible nature. In 1783, Lavoisier named the gas "hydrogen" meaning water producer.

Occurrence

In free state: Hydrogen is the most abundant element in the universe. The outer atmospheres of the sun and the stars consist largely of hydrogen. The outermost layer of the earth's atmosphere contains hydrogen, but in traces. On the earth, free hydrogen occurs in very small quantities, mainly in volcanic gases.

In combined state: Hydrogen is much more common in the form of compounds. The most important compound of hydrogen is water, which covers 70% of the earth's surface. Acids, alkalis, petroleum and natural gas, all are compounds containing hydrogen. Hydrogen is one of the main constituents of the compounds which form the body of the animal and vegetable matter.

Today hydrogen is widely seen as the mass fuel of the future, when there will be no petroleum or coal or gases acting as fuel.

There would have been no sunlight or heat if there were no hydrogen in the universe.

Preparation Of Hydrogen

The principal sources of hydrogen on the earth are water, acids and alkalis. These compounds liberate the gas when they react with chemically active metals.

1. By the action of water or steam on metals

(a) From cold water: The very active metals like sodium, potassium and calcium liberate hydrogen gas from cold water, the other product being a metallic hydroxide. Sodium and potassium react violently with water, while calcium reacts comparatively slowly.

Metal + Cold water - Metallic + Hydrogen hydroxide

\[2Na + 2H_2O \xrightarrow{\text{Fast}} 2NaOH + H_2\]

(Sodium hydroxide)

\[2K + 2H_2O \xrightarrow{\text{Fast}} 2KOH + H_2\]

(Potassium hydroxide)

\[Ca + 2H_2O \xrightarrow{\text{Slow}} Ca(OH)_2 + H_2\]

(Calcium hydroxide)

(b) From hot water: Magnesium liberates hydrogen from boiling water. However, the reaction is a slow one.

\[Mg + H_2O \xrightarrow{\text{Slow Hot water}} Mg(OH)_2 + H_2\]

(Calcium hydroxide)

(c) From steam: When steam is passed over heated metals like magnesium, zinc, aluminium and iron, hydrogen is liberated. The other product is a metallic oxide.

Hot metal + Steam - Metallic + Hydrogen oxide

\[Mg + H_2O \xrightarrow{\text{(steam)}} MgO + H_2\]

\[Zn + H_2O \xrightarrow{\text{(steam)}} ZnO + H_2\]

\[2Al + 3H_2O \xrightarrow{\text{(steam)}} Al_2O_3 + 3H_2\]

\[3Fe + 4H_2O \xrightarrow{\text{(red hot (steam)}} Fe_3O_4 + 4H_2\]

Reaction between iron and steam is reversible.

2. By the action of dilute acids on active metals

Metals like magnesium, zinc and iron react with dilute hydrochloric (or dilute sulphuric) acid to liberate hydrogen gas and their respective salts.

Metal + Dilute acid - Salt + Hydrogen

\[Mg + H_2SO_4 \rightarrow MgSO_4 + H_2\]

(Magnesium sulphate)

\[Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2\]

(Zinc sulphate)

\[Zn + 2HCl \rightarrow ZnCl_2 + H_2\]

(Zinc chloride)

\[Fe + 2HCl \rightarrow FeCl_2 + H_2\]

(Ferrous chloride)

The metals are arranged in a series in the descending order on the basis of their reactivity known as metal activity series. The most reactive metal is placed at the top and the least reactive one is placed at the bottom of the series. Though hydrogen is not a metal, it is placed in the series as many of its chemical reactions are similar to those of metals.

Metals such as copper, silver and gold, which are below hydrogen in the activity series, do not displace hydrogen from water.

3. By the action of alkalis on metals

Metals like zinc, aluminium, lead, etc., react with hot alkali solutions to liberate hydrogen gas.

Metal + Alkali - Salt + Hydrogen

\[Zn + 2NaOH \rightarrow Na_2ZnO_2 + H_2\]

(sodium hydroxide) (sodium zincate)

\[2Al + 2NaOH + 2H_2O \rightarrow 2NaAlO_2 + 3H_2\]

(sodium aluminate)

\[Pb + 2NaOH \rightarrow Na_2PbO_2 + H_2\]

(sodium hydroxide) (sodium plumbate)

Zinc, aluminium and lead are amphoteric in nature i.e. they can react with acid as well as with base to produce hydrogen gas.

4. By the electrolysis of water

When an electric current is passed through acidulated water, it dissociates into hydrogen and oxygen.

\[2H_2O \xrightarrow{\text{Electric Current}} 2H_2 + O_2\]

(Acidulated water) (Hydrogen) (Oxygen)

Hydrogen is collected at the cathode (negative electrode) and oxygen is collected at the anode (positive electrode). The ratio of hydrogen and oxygen thus collected is 2 - 1 by volume.

Laboratory Preparation Of Hydrogen

Hydrogen is prepared in the laboratory by the action of dilute hydrochloric acid or dilute sulphuric acid on granulated zinc.

Balanced equation for the reaction:

Metal + Dilute acid - Salt + Hydrogen

\[Zn + 2HCl \rightarrow ZnCl_2 + H_2(g)\]

\[Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2(g)\]

Granulated zinc contains an impurity, copper, which acts as a positive catalyst for the reaction. This is why granulated zinc is preferred over pure zinc for laboratory preparation of hydrogen.

The set-up for the experiment is as shown in Fig. 2.2.

Procedure: Take a few pieces of granulated zinc in a round bottom flask. Fix a two-holed air tight stopper in the mouth of the flask. Pass a thistle funnel through a hole and pass one end of a delivery tube through the other hole. Place the other end of the delivery tube inside a beehive shelf kept in a trough containing water.

Now pour some dilute hydrochloric acid through the thistle funnel. A brisk effervescence occurs. This indicates the liberation of hydrogen gas. Allow the first few bubbles of gas to escape. Then invert over the beehive shelf a gas jar completely filled with water. Hydrogen displaces water and gets collected in the gas jar. This method of collecting gas is known as "downward displacement of water".

Precautions!

Do not collect the first few bubbles of hydrogen gas, since they contain air too.

Do not bring a flame near the apparatus, since hydrogen is inflammable.

Note:

Hydrogen is sparingly soluble in water. Therefore, as it is collected over water, it picks up moisture. Dry hydrogen gas is obtained when it is collected over mercury.

Since hydrogen is lighter than air, it is possible to collect the gas by downward displacement of air. But it is not safe to do so since a mixture of hydrogen and air can lead to an explosion.

Catalyst is a substance (an element or a compound) that modifies the rate of a chemical reaction, but does not itself undergo any change in its composition during the reaction. A positive catalyst increases the rate of a reaction.

Teacher's Note

Laboratory preparation of hydrogen demonstrates the reactivity of metals with acids - a principle used in everyday applications like metal cleaning and pickling processes in industries.

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