Notes On Dihydrogen: Properties And Uses - CBSE Class 11 Chemistry
Physical properties: Dihydrogen is a colourless, odourless and tasteless gas. It is a non-poisonous gas. The solubility of hydrogen in water is very low. Under standard conditions, only two parts of dihydrogen dissolve in 100 parts of water by volume. Chemical properties: Dihydrogen gas is chemically inert at room temperature. The chemical behaviour of a diatomic molecule is determined by its bond dissociation enthalpy to a large extent. Bond dissociation enthalpy is the energy required to break a chemical bond. In a dihydrogen molecule, the H - H bond dissociation enthalpy is very high due to small size of the hydrogen atom. Thus, it requires an extremely high temperature to dissociate dihydrogen into hydrogen atoms. It is important to note that although a dihydrogen molecule is inert at room temperature, hydrogen atoms are very reactive, because they have only one electron in 1s orbital. Hydrogen atom reacts either by losing one electron to form H+ ions (or) by gaining one electron to form H - ions (or) by sharing electrons to form single covalent bonds. Reactions of dihydrogen: Dihydrogen gas at high temperatures reacts with highly electro-positive metals such as potassium, sodium and calcium. It combines to form their respective hydrides. 2K + H2  $\underset{\to }{\text{High Temperature}}$ 2K+H- 2Na + H2  $\underset{\to }{\text{High Temperature}}$ 2Na+H- Ca + H2  $\underset{\to }{\text{High Temperature}}$Ca+H2- Dihydrogen burns in oxygen at high temperature in the presence of platinum as a catalyst, to form water and releases energy. 2H2(g) + O2(g)        $\stackrel{\to }{\text{Platinum}}$          2H2O(l) Di hydrogen combines with halogens to form hydrogen halides.  H2(g) + X2(g) → 2HX(g) H2(g) + Cl2(g) → 2HCl(g) When hydrogen is passed through boiling sulphur, it produces hydrogen sulphide. H2(g) +          S(l)          →               H2S(g) Hydrogen     Boiling sulphur           Hydrogen Sulphate Dihydrogen reacts with nitrogen at 673K and 200 atmosphere pressure in the presence of iron as a catalyst to form ammonia.                                              673 Kelvin                                               200Atm N2(g)        +     3H2(g)           →              2NH3(g) Nitrogen            Hydrogen        Fe(Iron)    Ammonia Dihydrogen reacts with vegetable oils in the presence of finely divided nickel as a catalyst to give hydrogenated products such as edible fats. Uses: It is used as rocket fuel because of its high calorific value. Oxy-hydrogen blow torch is used for cutting and welding purposes uses dihydrogen and pure oxygen. The reaction produces temperatures of up to 4000K. Dihydrogen is also used in the synthesis of ammonia by Haber's process to manufacture nitric acid and nitrogenous fertilisers. Dihydrogen is used in the manufacture of vanaspati. Commercially, it is used in the preparation of hydrogen chloride.

#### Summary

Physical properties: Dihydrogen is a colourless, odourless and tasteless gas. It is a non-poisonous gas. The solubility of hydrogen in water is very low. Under standard conditions, only two parts of dihydrogen dissolve in 100 parts of water by volume. Chemical properties: Dihydrogen gas is chemically inert at room temperature. The chemical behaviour of a diatomic molecule is determined by its bond dissociation enthalpy to a large extent. Bond dissociation enthalpy is the energy required to break a chemical bond. In a dihydrogen molecule, the H - H bond dissociation enthalpy is very high due to small size of the hydrogen atom. Thus, it requires an extremely high temperature to dissociate dihydrogen into hydrogen atoms. It is important to note that although a dihydrogen molecule is inert at room temperature, hydrogen atoms are very reactive, because they have only one electron in 1s orbital. Hydrogen atom reacts either by losing one electron to form H+ ions (or) by gaining one electron to form H - ions (or) by sharing electrons to form single covalent bonds. Reactions of dihydrogen: Dihydrogen gas at high temperatures reacts with highly electro-positive metals such as potassium, sodium and calcium. It combines to form their respective hydrides. 2K + H2  $\underset{\to }{\text{High Temperature}}$ 2K+H- 2Na + H2  $\underset{\to }{\text{High Temperature}}$ 2Na+H- Ca + H2  $\underset{\to }{\text{High Temperature}}$Ca+H2- Dihydrogen burns in oxygen at high temperature in the presence of platinum as a catalyst, to form water and releases energy. 2H2(g) + O2(g)        $\stackrel{\to }{\text{Platinum}}$          2H2O(l) Di hydrogen combines with halogens to form hydrogen halides.  H2(g) + X2(g) → 2HX(g) H2(g) + Cl2(g) → 2HCl(g) When hydrogen is passed through boiling sulphur, it produces hydrogen sulphide. H2(g) +          S(l)          →               H2S(g) Hydrogen     Boiling sulphur           Hydrogen Sulphate Dihydrogen reacts with nitrogen at 673K and 200 atmosphere pressure in the presence of iron as a catalyst to form ammonia.                                              673 Kelvin                                               200Atm N2(g)        +     3H2(g)           →              2NH3(g) Nitrogen            Hydrogen        Fe(Iron)    Ammonia Dihydrogen reacts with vegetable oils in the presence of finely divided nickel as a catalyst to give hydrogenated products such as edible fats. Uses: It is used as rocket fuel because of its high calorific value. Oxy-hydrogen blow torch is used for cutting and welding purposes uses dihydrogen and pure oxygen. The reaction produces temperatures of up to 4000K. Dihydrogen is also used in the synthesis of ammonia by Haber's process to manufacture nitric acid and nitrogenous fertilisers. Dihydrogen is used in the manufacture of vanaspati. Commercially, it is used in the preparation of hydrogen chloride.

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