Gravity Classes Your Search for Best IIT-JEE/AIEEE Coaching Ends Here....
AboutGravity Classes
Nurtured by two enterprising graduates of the Department of Chemical Engineering & Bio-Chemical Engineering, IIT Delhi, GRAVITY CLASSEScame into existence in January 2007.
It is a colourless, tasteless and odourless gas. It is slightly soluble in water. It is highly combustible. The Physical constants of atomic hydrogen are,
Atomic radius (pm) – 37
Ionic radius of H– ion (pm) – 210
Ionisation energy (kJ mol–1) – 1312
Electron affinity (kJ mol–1) –72.8
Electronegativity – 2.1
Chemical properties of hydrogen:200k
Hydrogen is quite stable and dissociates into hydrogen atoms only when heated above 2000 K, H2 ————→ H + H. Its bond dissociation energy is very high, H2 → H + H; ?H = 435.9 kJ mol–1. Due to its high bond dissociation energy, it is not very reactive. However, it combines with many elements or compounds.
(i) Action with metals: Heat Heat
To forms corresponding hydrides. 2Na + H2————→ 2NaH; Ca + H2————→ CaH2.
With transition metals (elements of d – block) such as Pd, Ni, Pt etc. hydrogen forms interstitial hydrides in which the small molecules of hydrogen occupy the interstitial sites in the crystal lattices of these hydrides. As a result of formation of interstitial hydrides, these metals adsorb large volume of hydrogen on their surface. This property of adsorption of a gas by a metal is called occlusion. The occluded hydrogen can be liberated from the metals by strong heating.
970K Fe, Mo
2H2 + O2 ————→ 2H2O N2 + 3H2 ——————→ 2NH3
Dark 750K, Pressure
H2 + F2 ———→ 2HF
Sunlight
H2 + Cl2 ——————→ 2HCl
673K, Pressure
H2 + Br2 → 2HBr
673K
H2 + I2 ————→ 2HI Pt
The reactivity of halogen towards hydrogen decreases as, F2 > Cl2 > Br2 > I2
As a result, F2 reacts in dark, Cl2 in the presence of sunlight, Br2 reacts only upon heating while the reaction with I2 occurs in the presence of a catalyst.
(iii) Reaction with unsaturated hydrocarbons : H2 reacts with unsaturated hydrocarbons such as ethylene and acetylene to give saturated hydrocarbons.
Ni or Pt or Pd Ni or Pt or Pd
H2C = CH2 + H2 ————————→CH3 – CH3 HC ≡ CH + 2H2 ————————→ CH3 – CH3 Ethylene 473 K Ethane Acetylene 473 K Ethane
This reaction is used in the hydrogenation or hardening of oils. The vegetable oils such as groundnut oil or cotton-seed oil are unsaturated in nature because they contain at least one double bond in their molecules. Hydrogen is passed through the oils at about 473 K in the presence of catalyst to form solid fats. The vegetable ghee such as Dalda, Rath, etc. are usually prepared by this process.
Ni
Vegetable oil + ————→ Fat
(liquid) 473K (Solid)
(6) Uses of hydrogen
(i) As a reducing agent
(ii) In the hydrogenation of vegetable oils
(iii) As a rocket fuel in the form of liquid H2
(iv) In the manufacture of synthetic petrol
(v) In the preparation of many compounds such as NH3, CH3OH, Urea etc.
Home