s-BLOCK ELEMENTS

1. Hydrogen position in Periodic Table and its physical and chemical properties and its uses.

2. Compounds of Hydrogen

Introduction

Hydrogen, alkali metals (like sodium and potassium) and alkaline earth metals (like magnesium and calcium) are the essential parts of the world we live in.

Example :

- Hydrogen is used in making vanaspati.

- Yellow glow of street light is due to sodium.

- Sodium hydroxide sold under the name of caustic soda is used in the manufacture of soap.

- Plaster of paris, a compound of calcium is used as a building material as well as by doctors in setting of bone fracture.

1) Hydrogen Position in Periodic Table & its Physical and Chemical Properties and its Uses.

Hydrogen is the first element of the periodic table.

Hydrogen has the simplest atomic structure and consists of a nucleus containing one proton with a charge +1 and one orbital electron.

The electronic structure may be written as 1s1

1.1) Position in the Periodic Table

Where is hydrogen placed in periodic table?

Elements are place in the periodic table according to their outermost electronic configuration.

So hydrogen (1s1) may be placed with alkali metals (ns1).

But hydrogen attains noble gas configuration of helium atom (1s2) by gaining one electron.

It forms the hydride ion H-(1s2) like halogens (ns2np5) by gaining one electron.

On electrolysis of used alkali hydride, hydrogen is liberated at anode just as chlorine is liberated at anode during electrolysis of sodium chloride.

Thus hydrogen ought to be placed in group 17 along with halogens.

Hydrogen also resembles group 14 elements, since both have a half filled shell of electrons. So where should hydrogen be placed?

This problem is solved by placing hydrogen neither with alkali metals nor with halogens.

It has been given a unique position in the periodic table

1.2) Isotopes of Hydrogen

If atoms of the same element have different mass numbers they are called isotopes.

This difference in mass number arises because the nucleus contains a different number of neutrons.

Naturally occurring hydrogen contains three isotopes: protium 11 H or H, deuterium 12 H or D and tritium 13 H or T

These three isotopes contain one proton and 0, 1 and 2 neutrons, respectively in the nucleus (Fig 1).

Protium is by far the most abundant.

Naturally occurring hydrogen contains 99.986% of the 11 H isotope, 0.014% of D and 7 *10 -16 €% of T, therefore the properties of hydrogen are essentially those of the lightest isotope.

Tritium is radioactive and emits low energy β particles (t1/2 = 12.33 yrs).

Inductive Effect

Inductive Effect

Due to difference in mass of different isotopes, there arise a few differences in their properties.

Example:

a) H2 is more rapidly adsorbed on the metal surface than D2.

b) H2 reacts over 13-times faster with Cl2 than does D2.

Difference in properties that arises from the difference in mass is called Isotope Effect

Since the percentage difference in the mass of isotopes of hydrogen is very large.

The difference in properties of isotopes of hydrogen is very large.

The difference in properties of compounds containing these isotopes is also large.

1.3) Physical Properties

- Hydrogen is a Diatomic Gas, H2.

- It is colourless and has no smell.

- It is lightest of all the gases known.

- It is insoluble in water, acids and most of the organic solvents.

- It is adsorbed when passed over platinum and palladium.

1.4) Chemical properties

a) Combustion:

Hydrogen is combustible and burns in air with pale blue flame.

2H2 + O2 ⟶ 2H2O

b) Reducing property:

Hydrogen reduces heated metal oxides to metals.

ZnO + H2 ⟶ Zn + H2O

CuO + H2 ⟶ Cu + H2O

c) Reaction with non-metals:

Hydrogen combines with nitrogen, carbon, oxygen and chlorine under appropriate conditions to form ammonia, methane, water and hydrogen chloride, respectively.

3H2 + N2 ⟶ 2NH2

2H2 + C ⟶ CH2

2H2 + O2 ⟶ 2H2O H2 + Cl2 ⟶ 2HCl

Reaction with metals:

Hydrogen reacts with highly electropositive metals to from the corresponding hydrides.

2Na + H2 ⟶ 2NaH

2Li + H2 ⟶ 2LiH

1.5) Uses

Hydrogen is Used:

a) For conversion of coal into synthetic petroleum.

b) In the manufacture of bulk organic chemicals, particularly methanol.

c) In the hydrogenation of oils. Vegetable oils change in to vegetable ghee when hydrogen is passed through the oils at 443K in presence of nickel as catalyst.

d) In the manufacture of ammonia, which is used in the production of fertilizers.

e) As primary fuel for heavy rockets.

f) For filling balloons.

2) Compounds of Hydrogen

Hydrogen forms a large number of compounds: here we will consider only two of them :

2.1) Water (H2 O)

Water is a covalent compound made up of two hydrogen atoms linked with one oxygen atom through covalent bonds.

Its Lewis structure and molecular structure are shown below.

This oxide of hydrogen is essential to all life.

It occurs in the form of snow, as water in rivers, lakes, sea etc. and as vapour in the atmosphere.

Because of the large electronegativity of oxygen, water molecule is highly polar.

It has partial negative charge (δ-) on the oxygen atom and partial positive charge (δ+) on the hydrogen atom.

An electrostatic attraction between H of one molecule with O of other molecule results in the formation of intermolecular hydrogen bonds.

The remarkable characteristic of water is that in solid form, it is less dense than liquid form.

Consequently an ice cube floats on water.

Water molecules are joined together in an extensive three dimensional network in which oxygen atom is bonded to four hydrogen atoms, two by hydrogen bonds and two by normal covalent bonds, in a near tetrahedral hydrogen bonded structure which has got open spaces.

This is responsible for low density.

2.2) Heavy Water and its Applications

Water containing deuterium in place of ordinary hydrogen (protium) is termed as heavy water (D2O).

Heavy water is separated from water by electrolysis.

The equilibrium constant for the dissociation of water containing protium is very high (1.0 * 10-14) as compared to water containing deuterium (3.0 * 10 -15)

H2O ⟶ H+ + OH-

D2O ⟶ D+ + OD-

O - H bonds are broken more rapidly than O - €“D bonds.

Thus when water is electrolyzed, H2 is liberated much faster than D2, and the remaining water thus becomes enriched in heavy water D2O.

In order to obtain one litre of almost pure D2O, we have to electrolyze about 30000 litres of ordinary water.

2.2.1) Uses :

a) Heavy water is used as a moderator in nuclear reactors. In this process the high speed neutrons are passed through heavy water in order to slow down their speed.

b) It is used in the study of mechanism of chemical reactions involving hydrogen.

c) It is used as the starting material for the preparation of a number of deuterium compounds,

Example:

CaC2 + 2D2 ⟶ CD2 + Ca(OD)2

SO3 + D2 ⟶ D2SO4

2.3) Hydrogen peroxide (H2O2)

Hydrogen peroxide is an important compound of hydrogen.

Its chemical formula is H2O2

2.3.1) Methods of Preparation:

Two methods of preparation of hydrogen peroxide are given below:

a) By the action of dilute mineral acids (H2SO4) on metallic peroxides (barium peroxide, sodium peroxide)

BaO2 . 8H2O + H2SO4 ⟶ BaSO4 + H2O2 + 8H2O

Na2O2 + H2SO4 ⟶ Na2SO4 + H2O2

b) By the electrolysis of H2SO4 (50% W/W) followed by distillation At cathode:

2H+ + 2e ⟶ H2

At Anode:

2SO24- ⟶ S2 O82- + 2e-1

The anodic solution which contains persulphate ions (S2 O8)2- is distilled with sulphuric acid at reduced pressure yielding H2O2

S2 O82- + 2H+ 2H2 O ⟶ 2H 2SO 4 + H2O2

2.3.2) Properties:

- Hydrogen peroxide is a clolourless syrupy liquid and has sharp odour.

- It has a boiling pointof 423K.

- It is miscible in all proportions with water, alcohol and ether.

- The oxidation state of oxygen in hydrogen peroxide is –1, a value, which lies between the oxidation state of oxygen in O2 (zero) and water (–2).

- Therefore, hydrogen peroxide acts as an oxidizingagent as well as a reducing agent in acidic and alkaline media.


2.3.2.1) Oxiding Properties:

(a) Oxidizing action in Acid Solution:

(i)PbS + 4H2O2 ⟶ PbSO4 + 4H2O

(ii)2FeSO4 + H2SO4 + H2O2 ⟶ Fe2(SO4)3 + 2H2O

(b) Oxidizing action in Alkaline Solution:

MnCl2 + H2O2 + 2KOH ⟶ 2KCl 2H2O + MnO2

(c) Reducing action in Acid Solution:

2KMnO4 + 3H2SO4 + 5H2O2 ⟶ 2MnSO4 + K2SO4 + 8H2O + 5O2

(d) Reducing action in alkaline solution:

2KMnO4 + 3H2O2 ⟶ 2MnO2 + 3O2 + 2H2O + 2KOH

Cl2 + H2O2 + 2KOH ⟶ 2KCl + 2HX2O + O2

2.3.3) Uses:

Hydrogen peroxide is used:

- for bleaching hair, leather and wool etc.

- as a germicide and disinfectant.

- as an explosive when mixed with alcohol.

- in the preparation of foam rubber.

- in pollution control e.g. treatment of drainage and sewage water for dechlorination.