${NH}_{3}$ has exceptionally high melting point and boiling point as compared to those of the hydrides of the remaining element of group $15$ . Explain.

Important Points to Remember in Chapter NA - Hydrogen from Tamil Nadu Board Chemistry Standard 11 Vol I Solutions

1. Position of hydrogen in the periodic table:

(i) Hydrogen is lightest element and it shows resemblance with halogens as well as alkali metals. It occupies the first position in the periodic table

(ii) Hydrogen is the only element which does not have neutrons in its nucleus.

(iii) At room temperature, normal hydrogen consists of about 75% ortho-form and 25% para-form.

2. Isotopes of Hydrogen:

Hydrogen has three isotopes: Protium ( ${}_{1}^{1} H$ ), deuterium ${}_{1}^{2} H$ or D) and tritium ( ${}_{1}^{3} H$ or T).

3. Dihydrogen:

(i) The term used for the mixture of diatomic molecules like $H_{2}, D_{2}, HD$ containing $H$ and $D$ atoms in the ratio of their natural abundance.

(ii) In the laboratory dihydrogen can be prepared by the action of active metals (like) with dilute sulphuric acid.

(iii) On Commercial Scale, $H_{2}$ can be prepared by the processes such as electrolysis, water gas shift reaction, etc.

(iv) Syngas is a mixture of $CO$ and $H_{2}$ formed by the reaction of steam or hydrocarbons or red hot coke.

(v) Dihydrogen oxidises active metals whereas it reduces non-metals and also some metal oxides.

4. Hydrides:

(i) Hydrides are binary compounds of metals and hydrogen.

(ii) The compounds of hydrogen and s-block elements are ionic hydrides, while the compounds of hydrogen and most of p-block elements are covalent hydrides.

(iii) The hydrides of transition metals are non- stoichiometric compounds and are called interstitial hydrides.

5. Water:

(i) Water is a universal solvent. It is amphoteric in nature and can undergo redox reactions, hydrolytic reactions, hydrate formation, etc.

(ii) In, $H_{2} O$ , $O$ is ${sp}^{3} -$ hybridized but $∠ H O H = 104 \cdot 5 °$ due to greater repulsions between lone pairs as compared to bond pair-lone pair repulsions. It forms four hydrogen bonds and has a boiling point of $373 K$ and melting point of $273 K$ .

(iii) Ice has open cage like structure with a number of vacant spaces in the crystal lattice.

6. Hard and Soft Water:

(i) Hardwater: The water containing dissolved impurities of sulphates, chloride or bicarbonates of ${Ca}^{2 +}$ and ${Mg}^{2 +}$ ions. Hard water does not produce lather readily with soap but produces lather readily with detergents.

(ii) Temporary hardness of water is due to the presence of bicarbonates of calcium and magnesium while permanent hardness is due to the presence of chlorides and sulphates of calcium and magnesium.

(iii) Temporary hardness is removed either by simply boiling hard water or by treating it with a calculated amount of quick lime (Clarke's method) while permanent hardness is removed by treating hard water with a calculated amount of $N a_{2} C O_{3}$ .

(iv) Both temporary and permanent hardness of water are removed either by lime-soda process or by ion exchange process.

(v) Heavy water is the oxide of heavy hydrogen. One part of heavy water is present in 5000 parts of ordinary water.

(vi) Zeolite is hydrated sodium aluminium silicate. It renders hard water soft by exchanging its ${Na}^{+}$ ions with ${Ca}^{2 +}$ and ${Mg}^{2 +}$ ions present in hard water.

(vii) Demineralised Water: Sample of water free from all types of dissolved minerals.

7. Hydrogen Peroxide:

(i) Hydrogen Peroxide is a binary compound of hydrogen and oxygen elements which can act as oxidising agent, reducing agent as well as a weak acid.

(ii) Hydrogen peroxide has a non-planar structure. In gas phase, dihedral angle is $111.5 °$ and in solid phase at $110 K$ , dihedral angle is $90.2 °$ .

(iii) Hydrogen peroxide can be prepared by reaction of $H_{2} S O_{4} (a q)$ on $B a O_{2} . 8 H_{2} O (s)$ or by electrolysis of acidified ${(N H_{4})}_{2} S O_{4}$ solution.

(iv) Strength of Hydrogen Peroxide is expressed in terms of number volume method. For example, x volume hydrogen peroxide means that 1 litre of liquid $H_{2} O_{2}$ will decompose to liberate $x L O_{2}$ at S.T.P.

(v) Hydrogen Economy refers to the storage and transportation of energy in the form of liquid or gaseous dihydrogen. It is the use of $H_{2}$ as a fuel in industry, power plants, homes and vehicles.

8. Hydrogen bond:

(i) The attractive forces which binds hydrogen atom of one molecule with the electronegative atom of another molecule is called as hydrogen bonding.

(ii) Hydrogen bonds can occur within a molecule (intramolecular hydrogen bonding) and between two molecules of the same type or different type (intermolecular hydrogen bonding).