ICSE Class 8 Chemistry Chapter 01 Hydrogen

Hydrogen

Hydrogen is the lightest element known. It was first prepared by Robert Boyle by the action of dilute sulphuric acid on iron nails. Cavendish studied the gas and called it inflammable air, as the gas burns when kindled.

The stars (including our sun) are mainly composed of hydrogen. In fact, hydrogen is the most abundant element in the universe. On the earth, however, hydrogen occurs in very small amounts in the free state-in the air, in volcanic gases and in the earth's crust.

But, there is plenty of hydrogen on earth in combination with other elements, i.e., as part of compounds. Water is an important source of hydrogen. Every nine parts by mass of water contains one part of hydrogen. In combination with carbon, hydrogen is present in natural gas and petroleum, as well as, in all living things. Acids and alkalis also contain hydrogen.

Occurrence and Preparation of Hydrogen

Occurrence of Hydrogen

The stars (including our sun) are mainly composed of hydrogen. In fact, hydrogen is the most abundant element in the universe. On the earth, however, hydrogen occurs in very small amounts in the free state-in the air, in volcanic gases and in the earth's crust.

But, there is plenty of hydrogen on earth in combination with other elements, i.e., as part of compounds. Water is an important source of hydrogen. Every nine parts by mass of water contains one part of hydrogen. In combination with carbon, hydrogen is present in natural gas and petroleum, as well as, in all living things. Acids and alkalis also contain hydrogen.

Preparing Hydrogen

Since hydrogen is available as part of compounds, it can be obtained by displacement from a compound by a more active element. You have learnt in the previous class that in a displacement reaction, one element displaces another from its compound, and a new compound is formed.

A reactive metal like sodium, magnesium, zinc or iron displaces hydrogen from water, an acid (like dilute hydrochloric or sulphuric acid) or an alkali.

\[2Na + 2H_2O \rightarrow 2NaOH + H_2\]

\[Mg + H_2O \rightarrow MgO + H_2\]
(steam)

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

\[Fe + H_2SO_4 \rightarrow FeSO_4 + H_2\]

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

Hydrogen is liberated when an electric current is passed through water containing a small amount of sulphuric acid.

\[2H_2O \xrightarrow{\text{electric current}} 2H_2 + O_2\]

The Laboratory Method

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

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

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

As the gas is almost insoluble in water, it is collected by the displacement of water.

Procedure: A conical flask is fitted with a thistle funnel and a delivery tube. The other end of the delivery tube passes through a beehive shelf placed in a water trough. A gas jar is inverted over the beehive shelf. Some granulated zinc is placed in the conical flask. Dilute hydrochloric or sulphuric acid is added through the thistle funnel till the lower end of the funnel dips in the liquid. Hydrogen then begins to evolve. The gas is collected by the downward displacement of water. It is not collected by the downward displacement of air since a mixture of hydrogen and air is explosive.

Initially, the air inside the flask and the delivery tube is driven out. So, whatever is collected in the first one or two jars is air, and is rejected. The gas collected afterwards is hydrogen.

Teacher's Note

Hydrogen displacement reactions demonstrate how reactive metals can push less reactive elements out of compounds, a principle used in metallurgy and industrial processes for extracting and purifying metals.

Properties of Hydrogen

Physical Properties

1. Hydrogen is a colourless and odourless (having no smell) gas.

2. It is the lightest element and the lightest gas known. It is 14.6 times lighter than air.

3. It is almost insoluble in water.

Chemical Properties

1. Reaction with oxygen (or air): When kindled, hydrogen burns in air or oxygen to form water.

\[2H_2 + O_2 \rightarrow 2H_2O\]

A large amount of heat is produced in this reaction. So, a mixture of hydrogen and oxygen may explode.

2. Reaction with chlorine: When kindled, hydrogen burns in chlorine to form hydrogen chloride gas. Also, a mixture of hydrogen and chlorine, when placed in sunlight, explodes to form the same product.

\[H_2 + Cl_2 \rightarrow 2HCl \uparrow\]
(hydrogen chloride)

Hydrogen chloride gas can be dissolved in water to obtain hydrochloric acid.

3. Reaction with sulphur: Hydrogen reacts with molten sulphur to give hydrogen sulphide gas, which smells like rotten eggs.

\[H_2 + S \rightarrow H_2S \uparrow\]
(molten) (hydrogen sulphide)

4. Reaction with nitrogen: Hydrogen reacts with nitrogen, only under special conditions, to form an appreciable amount of ammonia.

\[3H_2 + N_2 \xrightarrow[{\text{Fe catalyst}}]{500°C/200 \text{ atm}} 2NH_3 \uparrow\]
(ammonia)

The ammonia gas formed, in turn, decomposes back to hydrogen and nitrogen. So, this reaction occurs both ways. Such reactions are called reversible reactions.

Teacher's Note

Hydrogen's reactions with non-metals like oxygen and chlorine show why proper safety precautions are needed when handling hydrogen, and why controlled conditions are essential for industrial synthesis reactions like ammonia production.

5. Reactions with some metal oxides: When passed over some hot metal oxides like zinc oxide, lead oxide, copper oxide and iron oxides, hydrogen gas converts them into the corresponding metals.

\[ZnO + H_2 \xrightarrow{\text{heat}} Zn + H_2O\]

\[PbO + H_2 \xrightarrow{\text{heat}} Pb + H_2O\]

\[CuO + H_2 \xrightarrow{\text{heat}} Cu + H_2O\]

\[FeO + H_2 \xrightarrow{\text{heat}} Fe + H_2O\]

\[Fe_2O_3 + 3H_2 \xrightarrow{\text{heat}} 2Fe + 3H_2O\]

The oxides of metals like potassium, calcium, sodium and magnesium are not converted into the metals.

Hydrogen is a Reducing Agent

The addition of hydrogen to or the removal of oxygen from a substance is called reduction. On the other hand, the addition of oxygen to or the removal of hydrogen from a substance is called oxidation. Also, a substance causing reduction is known as a reducing agent and one causing oxidation is called an oxidising agent.

You have just seen that hydrogen adds itself to oxygen, chlorine, sulphur and nitrogen, and removes oxygen from some metal oxides.

Thus, hydrogen is a reducing agent. It reduces:

O to H2O
Cl to HCl
S to H2S
N to NH3
by adding H
ZnO to Zn
PbO to Pb
CuO to Cu
FeO to Fe
Fe2O3 to Fe
by removing O

Teacher's Note

Understanding reduction and oxidation helps explain why hydrogen is valuable in industry for purifying metals and in chemical manufacturing, making these fundamental concepts applicable to real-world metallurgical processes.

Reactions of Metals with Water

All metals do not have similar reactivities. Some are more reactive than others. Metals may, therefore, be arranged in order of activity, and the series is called the activity series of metals. As many of the chemical reactions of hydrogen are similar to those of metals, hydrogen is also included in the series. The activity of metals decreases as one goes down the series.

A more active metal displaces a less active metal from its compound. Similarly, metals that are more active than hydrogen, i.e., higher than hydrogen in the activity series, can displace hydrogen from water. Also, the greater the activity of the metal, the more vigorously does it react with water.

Potassium, sodium and calcium react with water in cold conditions, displacing hydrogen from it and forming their hydroxides.

\[2K + 2H_2O \rightarrow 2KOH + H_2 \uparrow\]
(potassium hydroxide)

\[2Na + 2H_2O \rightarrow 2NaOH + H_2 \uparrow\]
(sodium hydroxide)

\[Ca + 2H_2O \rightarrow Ca(OH)_2 + H_2 \uparrow\]
(calcium hydroxide)

Magnesium reacts with steam to form magnesium oxide and hydrogen.

\[Mg + H_2O \rightarrow MgO + H_2 \uparrow\]
(steam) (magnesium oxide)

When aluminium comes in contact with water, it forms a protective layer of aluminium hydroxide over itself. The aluminium hydroxide thus formed does not allow the metal to react further with water.

Zinc and iron displace hydrogen from water when steam is passed over the red-hot metal.

\[Zn + H_2O \rightarrow ZnO + H_2 \uparrow\]
(red hot) (steam) (zinc oxide)

\[3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2 \uparrow\]
(red hot) (steam) (ferroserric oxide)

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

Teacher's Note

The activity series of metals explains why some metals like sodium react explosively with water while others like copper do not, helping us understand why certain metals are chosen for specific applications based on their reactivity.

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