Chemistry Preparation of dihydrogen, Properties of dihydrogen, Uses of dihydrogen

### Topics to be covered

=> Dihydrogen
=> Preparation of dihydrogen
=> Properties of dihydrogen
=> Uses of dihydrogen

### DIHYDROGEN, H_2

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.

### PREPARATION OF DIHYDROGEN, H_2

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

### Laboratory Preparation of Dihydrogen

(i) It is usually prepared by the reaction of granulated zinc with dilute hydrochloric acid.

color{red}(Zn +2H^(+) → Zn^(2+) + H_2)

(ii) It can also be prepared by the reaction of zinc with aqueous alkali.

color{red}(Zn +2NaOH → undersettext(Sodium zincate)(Na_2ZnO_2) +H_2)

### Commercial Production of Dihydrogen

color{green}("The commonly used processes are outlined below:")

(i) Electrolysis of acidified water using platinum electrodes gives hydrogen.

color{red}(2H_2O (l) undersettext(Traces of acid/ base) oversettext(Electrolysis) → 2H_2 (g) +O_2 (g))

(ii) High purity ( color{red}(>99.95%) ) dihydrogen is obtained by electrolysing warm aqueous barium hydroxide solution between nickel electrodes.

(iii) It is obtained as a byproduct in the manufacture of sodium hydroxide and chlorine by the electrolysis of brine solution. During electrolysis, the reactions that take place are:

color{green}("at anode:") color{red}(2Cl^–(aq) → Cl_2(g) + 2e^–)

color{green}("at cathode:") color{red}(2H_2O (l) + 2e^(–) → H_2(g) + 2OH^(–)(aq))

color{green}("The overall reaction is")

color{red}(2Na^(+) (aq) + 2Cl^(–)(aq) + 2H_2O(l) → Cl_2(g) +H_2(g) +2Na^(+)(aq) +2OH^(-) (aq))

(iv) Reaction of steam on hydrocarbons or coke at high temperatures in the presence of catalyst yields hydrogen.

color{red}(C_nH_(2n+2) + nH_2O underset(Ni)overset(1270K)→ nCO + (2n+1)H_2)

color{red}(e.g. CH_4 (g) +H_2O (g) underset(Ni) overset(1270K) → CO(g) +3H_2(g))

color{green}(★) The mixture of color{red}(CO) and color{red}(H_2) is called water gas. As this mixture of color{red}(CO) and color{red}(H_2) is used for the synthesis of methanol and a number of hydrocarbons, it is also called color{green}("synthesis gas or 'syngas'.") Nowadays 'syngas' is produced from sewage, saw-dust, scrap wood, newspapers etc. The process of producing 'syngas' from coal is called color{green}("'coal gasification'.")

color{red}(C(s) +H_2O(g) overset(1270K)→ CO(g) +H_2(g))

color{green}(★) The production of dihydrogen can be increased by reacting carbon monoxide of syngas mixtures with steam in the presence of iron chromate as catalyst.

color{red}(CO(g) +H_2O(g) undersettext(catalyst)overset(673K)→ CO_2 (g) +H_2(g))

color{green}(★) This is called water-gas shift reaction. Carbon dioxide is removed by scrubbing with sodium arsenite solution.

color{green}(★) Presently color{red}(~77%) of the industrial dihydrogen is produced from petro-chemicals, color{red}(18%) from coal, color{red}(4%) from electrolysis of aq. solutions and color{red}(1%) from other sources.

### PROPERTIES OF DIHYDROGEN

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)

### Uses of Dihydrogen

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.

Q 3111291120

Comment on the reactions of dihydrogen with (i) chlorine, (ii) sodium, and (iii) copper(II) oxide

Solution:

(i) Dihydrogen reduces chlorine into chloride (Cl^–) ion and itself gets oxidised to H^+ ion by chlorine to form hydrogen
chloride. An electron pair is shared between H and Cl leading to the formation of a covalent molecule.

(ii) Dihydrogen is reduced by sodium to form NaH. An electron is transferred from Na to H leading to the formation of an ionic
compound, Na^+ H^–.

(iii) Dihydrogen reduces copper(II) oxide to copper in zero oxidation state and itself gets oxidised to H_2O, which is a covalent
molecule.