Maharashtra Board Class 11 Chemistry Chapter 8 Elements of Groups 1 and Group 2 PDF Download

Elements of Groups 1 and Group 2

We have seen in chapter 7 that the element of group 1 and group 2 belong to the s-block of the modern periodic table.

8.1 Hydrogen

Hydrogen is the first element in the periodic table. Hydrogen appears at the top of group 1 of the alkali metals. But it differs from alkali metals in a number of respects and, therefore, is studied separately.

Hydrogen has the simplest atomic structure of all the elements. A hydrogen atom consists of a nucleus of charge +1 and one extranuclear electron. Hydrogen has a little tendency to lose this electron however can pair with the other electron easily forming a covalent bond. It exists in diatomic form as H₂ molecule; therefore it is often called dihydrogen.

8.1.1 Occurrence

In the free state hydrogen exists as dihydrogen gas. Hydrogen is most abundant element in the universe; it makes 70% of the total mass of the universe. Hydrogen is also the principal element in the solar system. On the earth, hydrogen is the tenth most abundant element on mass basis and the third most abundant element on atom basis.

8.1.2 Position of Hydrogen in the Periodic Table

Position of hydrogen in the periodic table has been a subject of discussion.

Electronic configuration of hydrogen is \(1s^1\) which is similar to \(ns^1\) which is the outer electronic configuration of alkali metals belonging to the group 1. But \(1s^1\) also resembles the outer electronic configuration of group 17 elements, which is \(ns^2np^5\). By adding one electron to H we get electronic configuration of the inert gas He which is \(1s^2\) and by adding one electron to \(ns^2np^5\) we get \(ns^2np^6\) which is the outer electronic configuration of the remaining inert gases. Some chemical properties of hydrogen are similar to those of alkali metals while some resemble halogens. The uniqueness of hydrogen is that H⁺ formed by loss of the electron from hydrogen atom does not exist freely. It is always associated with other molecules. For example:

\[H^+ + H_2O \longrightarrow H_3O^+\]

This is because H⁺ is nothing else but a proton. Hydrogen is, therefore, placed separately above the group 1. It may be noted, here, that metastable metallic hydrogen was discovered at Harvard university, USA, in January 2017.

8.1.3 Isotopes of Hydrogen

If different atoms of the same element have different mass numbers they are called isotopes of each other. (Refer to chapter 4). Hydrogen has three isotopes with mass numbers 1, 2 and 3. They all contain one proton and one electron but different number of neutrons in the nucleus.

Teacher's Note

Hydrogen is the simplest element. It has only one electron. Just like water boils at different temperatures in the mountains and plains, hydrogen also shows different properties based on how it is prepared.

Exam Trick

Remember the three isotopes: H-1 (no neutrons), H-2 or Deuterium (one neutron), H-3 or Tritium (two neutrons). Think: more neutrons = heavier isotope.

Points to Remember

Hydrogen is the first element in the periodic table.
Hydrogen exists as H₂ molecule in free state.
Hydrogen has three isotopes with different numbers of neutrons.
Only tritium is radioactive among the three isotopes.
Hydrogen can show both metallic and non-metallic properties.

8.1.4 Preparation of Dihydrogen

Hydrogen can be prepared using many methods.

A. Laboratory Methods

i. Dihydrogen is prepared in laboratory by the action of dilute hydrochloric acid on zinc granules.

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

ii. Dihydrogen can be prepared by the action of aqueous solution of sodium hydroxide on zinc.

\[Zn(s) + 2NaOH(aq) \rightarrow Na_2ZnO_2(aq) + H_2(g)\]

B. Industrial Methods

i. By electrolysis of pure water: Pure water is a poor conductor of electricity. Therefore a dilute aqueous solution of acid or alkali is used to prepare dihydrogen by electrolysis. For example, electrolysis of dilute aqueous solution of sulphuric acid yields two volumes of hydrogen at cathode and one volume of oxygen at anode.

\[2H_2O(l) \xrightarrow{\text{electrolysis, trace of acid or alkali}} 2H_2\uparrow + O_2\uparrow\]

Pure dihydrogen (> 99.5% purity) gas is obtained by electrolysis of warm solution of barium hydroxide between nickel electrodes.

ii. From carbon or hydrocarbon: Three stages are involved in this industrial process of preparation of dihydrogen.

Stage 1: Reaction of steam on hydrocarbon or coke (C) at 1270 K temperature in presence of nickel catalyst, gives water-gas, which is a mixture of carbon monoxide and hydrogen.

\[CH_4(g) + H_2O(g) \xrightarrow{1270 \text{ K}, \text{Ni}} CO(g) + 3H_2(g)\]

This is called water-gas.

\[C(s) + H_2O(g) \xrightarrow{1270 \text{ K}} CO(g) + H_2(g)\]

Sawdust, scrapwood, etc. can also be used in place of carbon.

As water-gas is used for synthesis of CH₃OH and a number of hydrocarbons, it is also called 'syngas'. Production of syngas is also the first stage of gasification of coal.

Stage 2: Water-gas shift reaction: The carbon monoxide in the watergas is transformed into carbondioxide by reacting with steam in presence of iron chromate as catalyst. This is called water-gas shift reaction.

\[CO(g) + H_2O(g) \xrightarrow{673 \text{ K}, \text{iron chromate catalyst}} CO_2(g) + H_2(g)\]

Stage 3: Carbon dioxide is removed by scrubbing with sodium arsenite solution.

Today major industrial production of dihydrogen is from petrochemicals (about 77%), about 18% from coal, about 4% by electrolytic methods and about 1% by other methods.

Teacher's Note

Hydrogen is made in factories in large amounts. Just like a baker uses different methods to make different types of bread, scientists use different methods to make hydrogen for different uses.

Exam Trick

Remember the water-gas shift reaction: it changes CO into CO₂ using steam and a catalyst. Think: CO + H₂O = CO₂ + H₂ (at 673 K).

Points to Remember

Zinc and dilute HCl is the easiest lab method to make hydrogen.
Industrial hydrogen comes mostly from petrochemicals and coal.
Water-gas is a mixture of CO and H₂.
The water-gas shift reaction converts CO to CO₂.
Electrolysis of water gives pure hydrogen.

8.1.5 Properties of Dihydrogen

A. Physical Properties

Dihydrogen is colourless, tasteless and odourless gas. It burns with a pale blue flame. It is a nonpolar water insoluble gas, lighter than air.

B. Chemical Properties

i. Reaction with metals: Dihydrogen combines with all the reactive metals including alkali metals, calcium, strontium and barium at high temperature, to form metal hydrides. For example:

\[2Na(s) + H_2(g) \rightarrow 2NaH(s)\]

(In this respect dihydrogen is similar to halogens which also react with metals and form metal halides.)

ii. Reaction with dioxygen: Dihydrogen reacts with dioxygen in the presence of catalyst or by heating to form water. This reaction is highly exothermic.

\[2H_2(g) + O_2(g) \xrightarrow{\text{catalyst, on heating}} 2H_2O(l); \Delta H = -235 \text{ kJ mol}^{-1}\]

iii. Reaction with halogens: Dihydrogen inflames with fluorine even at -250°C in dark, whereas it requires catalyst to react with iodine. The vigour of reaction of dihydrogen decreases with increasing atomic number of halogen.

\[H_2(g) + X_2(g) \rightarrow 2HX(g)\]

(In this respect dihydrogen resembles alkali metals which also react with halogens to form halides.)

iv. Reducing nature of dihydrogen: Dihydrogen reduces oxides and ions of metals those are less reactive than iron, to the corresponding metals at moderate temperature. For example:

\[CuO(s) + H_2(g) \rightarrow Cu(s) + H_2O(l)\]

\[Fe_3O_4(s) + 4H_2(g) \rightarrow 3Fe(s) + 4H_2O(l)\]

The bond dissociation energy of H-H bond is high, which is 436 kJ mol^-1. Therefore reactions of dihydrogen take place at high temperature and/or in the presence of catalyst.

b. Hydrogenation of unsaturated organic compound: The hydrogenation of unsaturated organic compounds such as oil using nickel catalyst gives saturated organic compounds such as solid fat (vanaspati Ghee).

c. Hydroformylation of olefins and subsequent reduction of aldehyde to form alcohol

\[H_2 + CO + R-CH=CH_2 \rightarrow R-CH_2-CH_2-CHO\]

\[H_2 + R-CH_2-CH_2-CHO \rightarrow R-CH_2-CH_2-CH_2OH\]

7.1.6 Uses of Dihydrogen

i. Largest use of dihydrogen is in production of ammonia.

ii. Dihydrogen is used in formation of vanaspati ghee by catalytic hydrogenation of oils.

iii. Liquid dihydrogen is used as a rocket fuel.

iv. Dihydrogen is used in preparation of important organic compounds like methanol in bulk quantity.

\[2H_2(g) + CO(g) \xrightarrow{\text{Cobalt catalyst}} CH_3OH(l)\]

v. Dihydrogen is used for preparation of hydrogen chloride (HCl) and metal hydrides.

Teacher's Note

Hydrogen is very useful. Most ammonia for fertilizers is made using hydrogen. Hydrogen is also used to make ghee soft from hard oil in our kitchens.

Exam Trick

Remember: H₂ is the reducing agent (it removes oxygen). It reacts with metal oxides to give metals. Think: CuO + H₂ = Cu + H₂O (copper gets back).

Points to Remember

Dihydrogen is colourless, odourless, and very light.
It burns with a pale blue flame in air.
Main use is in making ammonia for fertilizers.
It is used to harden oils into ghee.
It acts as a reducing agent for metal oxides.

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