`color{green}(⧫)` `color{green}("Occurrence :")`

`color{green}(★)` Dihydrogen is the most abundant element in the universe (70% of the total mass of the universe) and is the principal element in the solar atmosphere.

`color{green}(★)` The giant planets Jupiter and Saturn consist mostly of hydrogen.

`color{green}(★)` However, due to its light nature, it is much less abundant (0.15% by mass) in the earth’s atmosphere.

`color{green}(★)` In the combined form it constitutes 15.4% of the earth's crust and the oceans.

`color{green}(★)` In the combined form besides in water, it occurs in plant and animal tissues, carbohydrates, proteins, hydrides including hydrocarbons and many other compounds.

`color{green}(⧫)` `color{green}("Isotopes of Hydrogen :")`

`color{green}(★)` Hydrogen has three isotopes: protium, `color{red}(text()_(1)^( 1)H)`, deuterium, `color{red}(text()_(1)^(2) H)` or `color{red}(D)` and tritium, `color{red}(text()_(1)^(3) H)` or `color{red}(T)`.

`color{green}(★)` These isotopes differ from one another in respect of the presence of neutrons.

`color{green}(★)` Ordinary hydrogen, protium, has no neutrons, deuterium (also known as heavy hydrogen) has one and tritium has two neutrons in the nucleus.

`color{purple}♣ color{Violet} " Just for Curious"`

In the year 1934, an American scientist, Harold C. Urey, got Nobel Prize for separating hydrogen isotope of mass number 2 by physical methods.

`color{green}(⧫)` The predominant form is protium.

`color{green}(⧫)` Terrestrial hydrogen contains `color{red}(0.0156%)` of deuterium mostly in the form of `color{red}(HD)`.

`color{green}(⧫)` The tritium concentration is about one atom per `color{red}(10^(18))` atoms of protium.

`color{green}(⧫)` Of these isotopes, only tritium is radioactive and emits low energy `color{red}(β^–)` particles (`color{red}(t_(1/2), 12.33)` years).

`color{green}(★)` Since the isotopes have the same electronic configuration, they have almost the same chemical properties. The only difference is in their rates of reactions, mainly due to their different enthalpy of bond dissociation (Table 9.1). However, in physical properties these isotopes differ considerably due to their large mass differences.

`color{green}(★)` There are a number of methods for preparing dihydrogen from metals and metal hydrides.

`color{green}(⧫)` `color{green}("Physical Properties :")`

`color{green}(★)` Dihydrogen is a colourless, odourless, tasteless, combustible gas.

`color{green}(★)` It is lighter than air and insoluble in water.

`color{green}(★)` Its other physical properties alongwith those of deuterium are given in Table 9.1.

`color{green}(⧫)` `color{green}("Chemical Properties :")`

`color{green}(★)` The chemical behaviour of dihydrogen (and for that matter any molecule) is determined, to a large extent, by bond dissociation enthalpy. The `color{red}(H–H)` bond dissociation enthalpy is the highest for a single bond between two atoms of any element.

`color{green}(★)` It is because of this factor that the dissociation of dihydrogen into its atoms is only `color{red}(~0.081%)` around `color{red}(2000K)` which increases to `color{red}(95.5%)` at `color{red}(5000K.)`

`color{green}(★)` Also, it is relatively inert at room temperature due to the high `color{red}(H–H)` bond enthalpy.

`color{green}(★)` Thus, the atomic hydrogen is produced at a high temperature in an electric arc or under ultraviolet radiations.

`color{green}(★)` Since its orbital is incomplete with `color{red}(1s^1)` electronic configuration, it does combine with almost all the elements.

`color{green}(★)` It accomplishes reactions by (i) loss of the only electron to give `color{red}(H^+)`, (ii) gain of an electron to form `color{red}(H^–)`, and (iii) sharing electrons to form a single covalent bond.

`color{green}(★)` The chemistry of dihydrogen can be illustrated by the following reactions:

`color{green}(★)` `color{green}("Reaction with halogens :")` It reacts with halogens, `color{red}(X_2)` to give hydrogen halides, `color{red}(HX)`,

`color{red}(H_2 (g) + X_2 (g) →2HX (g ) \ \ \ \ (X = F,Cl, Br,I))`

`color{green}(★)` While the reaction with fluorine occurs even in the dark, with iodine it requires a catalyst.

`color{green}(★)` `color{green}("Reaction with dioxygen:")` It reacts with dioxygen to form water. The reaction is highly exothermic.

`color{red}(2H_2(g) +O_2(g) oversettext(catalyst or heating) → 2H_2O (I) : DeltaH^(⊖) = -285.9 kJ mol^(-1))`

`color{green}(⧫)` `color{green}("Reaction with dinitrogen :")` With dinitrogen it forms ammonia.

`color{red}(3H_2(g) +N_2(g) underset(Fe) oversettext(673K , 200 atm)→ 2NH_3 (g) : DeltaH^(⊖) = -92.6 kJ mol^(-1))`

This is the method for the manufacture of ammonia by the Haber process.

`color{green}(★)` Reactions with metals: With many metals it combines at a high temperature to yield the corresponding hydrides.

`color{red}(H_2(g) +2M(g) → 2MH(s))`; where `color{red}(M)` is an alkali metal

`color{green}(★)` Reactions with metal ions and metal oxides: It reduces some metal ions in aqueous solution and oxides of metals (less active than iron) into corresponding metals.

`color{red}(H_2(g) +Pd^(2+) (aq) → Pd(s) +2H^(+) (aq))`

`color{red}(yH_2 (g) +M_x O_y (s) → x M(s) +y H_2O (l))`

`color{green}(⧫)` `color{green}("Reactions with organic compounds:")` It reacts with many organic compounds in the presence of catalysts to give useful hydrogenated products of commercial importance. For example :

(i) Hydrogenation of vegetable oils using nickel as catalyst gives edible fats (margarine and vanaspati ghee)

(ii) Hydroformylation of olefins yields aldehydes which further undergo reduction to give alcohols

`color{red}(H_2 + CO + RCH = CH_2 → RCH_2 CH_2 CHO)`

`color{red}(H_2 + RCH_2 CH_2 CHO → RCH_2 CH_2 CH_2 OH)`

`color{green}(★)` The largest single use of dihydrogen is in the synthesis of ammonia which is used in the manufacture of nitric acid and nitrogenous fertilizers.

`color{green}(★)` Dihydrogen is used in the manufacture of vanaspati fat by the hydrogenation of polyunsaturated vegetable oils like soyabean, cotton seeds etc.

`color{green}(★)` It is used in the manufacture of bulk organic chemicals, particularly methanol.

`color{red}(CO(g) +2H_2 (g) undersettext(catalyst) oversettext(cobalt)→ CH_3OH(l))`

`color{green}(★)` It is widely used for the manufacture of metal hydrides.

`color{green}(★)` It is used for the preparation of hydrogen chloride, a highly useful chemical.

`color{green}(★)` In metallurgical processes, it is used to reduce heavy metal oxides to metals.

`color{green}(★)` Atomic hydrogen and oxy-hydrogen torches find use for cutting and welding purposes. Atomic hydrogen atoms (produced by dissociation of dihydrogen with the help of an electric arc) are allowed to recombine on the surface to be welded to generate the temperature of `color{red}(4000 K.)`

`color{green}(★)` It is used as a rocket fuel in space research.

`color{green}(★)` Dihydrogen is used in fuel cells for generating electrical energy. It has many advantages over the conventional fossil fuels and electric power. It does not produce any pollution and releases greater energy per unit mass of fuel in comparison to gasoline and other fuels.