Preparation of Hydrogen

Preparation of Dihydrogen

(1) From water

Dihydrogen may be obtained from water by reduction either with active metals or by electricity.

(a) By the action of water on Active Metals

(1) Cold Water : Very active metals i.e. alkali and certain alkaline earth metals like Na, K , Ca react with water at room temperature evolving dihydrogen.

2Na + 2H₂O ———–> 2NaOH + H₂

2K + 2H₂O ———–> 2KOH + H₂

Ca + 2H₂O ———–> 2Ca(OH)₂  + H₂

The reaction with alkali metal is so vigorous and exothermic that the hydrogen involved catches fire. To slow down the reaction, amalgams of these metals are generally used.

In amalgams, only a small surface area of the metal comes in contact with water ,and ,therefore the reaction is slowed down.

(2) Boiling water : Less active metals like Zn, Mg , Al decompose boiling water liberating dihydrogen.

Zn + H₂O ———–> ZnO + H₂

Mg + H₂O ———–> MgO + H₂

2Al + 3H₂O ———–> Al₂O₃+ H₂

(3) Steam : Still less active metals like Fe, Sn, Ni decompose steam at high temperature evolving dihydrogen.

3Fe + 4H₂O ———–> Fe₃O₄ + H₂

steam               magnetic oxide

Temperature: 1023-1073 K

 

(b) By Electrolysis of Water

Dihydrogen of high purity is usually obtained by the electrolysis of water in the presence of small amount of an acid or a base. During electrolysis, dihydrogen is collected at cathode while dioxygen is liberated at anode.

2H₂O (l) ———> 2H₂ (g) + O₂ (g)

Pure water is only weakly ionized and hence is a poor conductor of electricity but presence of an acid or a base makes it a better conductor of electricity.

This method is not commercially used since it is quite expensive.

(2) From Alkalis

Metals like Be, Zn, Sn react with boiling alkali solution liberating dihydrogen.

Be + 2NaOH ———–> Na₂BeO₂ + H₂

Zn + 2NaOH ———–> Na₂ZnO₂ + H₂

Sn + 2NaOH + H₂O———–> Na₂SnO3 + H₂

2Al + 2NaOH + 2H₂O ———-> 2NaAlO₂ + 3H₂

(3) From acids

Metals which are more electropositive than hydrogen (lie above hydrogen in the electrochemical series) such as Zn, Fe, Mg react with dilute mineral acids to liberate dihydrogen gas.

Zn + H₂SO₄ ———-> ZnSO₄ + H₂

Fe +2HCl ———–> FeCl₂ + H₂

Metals like Copper ,silver ,Mercury which are less electropositive than hydrogen (lie below hydrogen in the electrochemical series) do not liberate hydrogen from acids.

Laboratory preparation of dihydrogen

 

In laboratory ,dihydrogen is prepared by reaction of dil H₂SO₄ on granulated zinc.

Zn + H₂SO₄ ——–> ZnSO₄ + H₂

Granulated pieces of zinc are placed in a Woulfe’s bottle and are covered with water. The bottle is fitted with a thistle funnel and a delivery tube.

Conc H₂SO₄ is poured slowly through the thistle funnel. As the acid falls in the Woulfe’s bottle ,it gets diluted and then reacts with zinc evolving dihydrogen gas. It is collected by downward displacement of water.

Sometimes the bubbles of dihydrogen produced stick to the surface of the zinc metal preventing the further reaction of the acid on the metal. Such a situation can be avoided by adding few crystal of copper sulphate to the reaction mixture.

Preparation of Pure Dihydrogen Gas

(1) By the action of pure sulphuric acid on magnesium ribbon

Mg + H₂SO₄ (dil) ——-> MgSO₄ + H₂

(2) By the electrolysis of a warm solution of barium hydroxide using platinum or Nickel electrodes.

(3) By the action of water on sodium hydride

NaH + H₂O ——–> NaOH + H₂

(4) By the action of KOH on scrap aluminium

2Al + 2KOH  + 2H₂O———-> 2KAlO₂ + 3H₂

Commercial preparation of Dihydrogen

(1) By the electrolysis of water

A small quantity of acid or alkali is added to water to make it a good conductor and electrolysed in a cell. In this cell ,iron sheet is used as a cathode while nickel plated iron sheet act as anode.

The two electrodes are separated from each other by an asbestos diaphragm  which prevents mixing of dihydrogen and dioxygen. On passing electric current, dihydrogen gas is collected at cathode while dioxygen at anode.

 

When 20% NaOH solution is used for electrolysis, the decomposition of water takes place as follows:

At cathode

H⁺ + e‾ ———–> H•

H• + H• ———> H₂

At anode

4OH‾ ———> 4OH + e‾

4 OH‾ ——–> 2H₂O + O₂

(2) From Syngas – Bosch process

When super heated steam is passed over red hot coke or coal at 1270K in the presence of nickel catalyst, a mixture of carbon monoxide and dihydrogen is produced.

C(s) + H₂O (g) ———-> CO + H₂ -121.3 KJ

A mixture 1:1 of CO and H₂ was called water gas. All mixtures of CO and H₂ irrespective of their composition are called synthetic gas or syngas.

This process of producing syngas from coke or coal is coal gasification.

Syngas is produced from sewage ,sawdust, scrap wood ,newspaper.

It is difficult to obtain pure hydrogen from water gas or Syngas, since CO is difficult to remove. To remove CO and increase the production of dihydrogen from syngas, CO of the syngas is oxidised to CO₂ by mixing it with more steam at 673 K in presence of iron chromate as catalyst.

CO ( g) + H₂O ( g ) + H₂O (g) ————-> CO₂ (g) + 2H₂(g)      673K, FeCrO₄

         syn gas .             steam

Chemical reaction in which carbon monoxide of the syngas reacts with steam to form carbon dioxide and more dihydrogen is called water gas shift reaction.

CO₂ thus produced is removed either by scrubbing the mixture with sodium arsenide solution or bypassing the mixture through water under 30 atm pressure when carbon dioxide dissolves leaving behind dihydrogen which is collected.

(3) From Hydrocarbons

(1) Partial oxidation of hydrocarbons : A mixture of hydrocarbons is mixed with steam and passed over heated Nickel catalyst at 1270 K.

C_nH_2n+2 + nH₂O ————-> nCO + (2n+1) H₂

Natural gas may also be used.

CH₄ (g) + H₂ (g) ——-> CO(g) + 3H₂(g)

Whole process of obtaining dihydrogen from natural gas is called steam reforming process.

 

(2) Thermal cracking of natural gas

Dihydrogen may also be obtained by thermal cracking of natural gas at 1270 K in the presence of a catalyst.

CH₄ ——> C + 2H₂

(4) Lane’s Process

Dihyrogen can also be manufactured by passing alternate currents of steam and water gas over red hot iron. It consists of two stages:

(1) Oxidation Stage: Super heated steam is passed over iron filling heated to about 1023 – 1073 K when hydrogen is formed and magnetic oxide of iron is left behind.

3Fe + 4H₂O ————> Fe₃O₄ + 4H₂ + 160.7 KJ

(2) Reduction stage: When the whole of iron has being oxidised, the steam supply is cut off and a steam of water gas is passed to reduce Fe₃O₄ back to iron.

Fe₃O₄+ 4 H₂ ———–> 3Fe + 4H₂O

Fe₃O₄+ 4CO———> 3Fe + 4CO₂

By passing steam and water gas alternatively over heated iron, dihydrogen gas can be manufactured from a small quantity of iron.

(5) As a by – product

Large quantities of dihydrogen are obtained as a by product in various industries.

(1) From petroleum cracking plants

(2) In the manufacture of sodium hydroxide and chlorine by electrolysis of brine solution.

Anode     2Cl‾ (aq) ————–> Cl₂ (g) + 2 e‾

Cathode    2H₂O (l) + 2e‾ ——–> H₂(g) + OH‾ (aq)

Overall reaction:    2Cl‾ (aq) +  2H₂O (l) ———> Cl₂ (g) + H₂(g) + OH‾ (aq)

Properties of Dihydrogen

(1) It is colourless, tasteless and odourless gas.

(2)It is the lightest substance known.

(3)It is slightly soluble in water since its molecules are non polar.

(4)It can be liquefied under low temperature and high pressure.

Chemical Properties of Dihydrogen

(1) The H-H bond dissociation enthalpy is the highest for a single bond between two atoms of any element.

H₂ ( g) ———–> 2H(g)

Dihydrogen is quite stable and relatively inert at room temperature due to its high H-H .bond dissociation enthalpy. Therefore most of the reactions of dihydrogen occur at high temperature.

(2) It is neutral to Litmus.

(3) Combustibility : It is highly combustible gas and burns in air or dioxygen with a pale blue flame to form water. It is not a supporter of combustion.

2H₂ ( g ) + O₂ (g) ———> 2H₂O(l)

(4) Reaction with metals

Dihydrogen reacts with strongly electropositive metals like Sodium ,Potassium ,calcium at high temperature to form salt like metal hydrides, in which the oxidation state of hydrogen is -1.

2Na + H₂ ————-> 2NaH

Ca + H₂ ————-> CaH₂  

(5) Reaction with non metals

Dihydrogen combines with many non metals at high temperature in presence of a catalyst to form covalent or molecular hydrides.

(a)With dioxygen ,dihydrogen forms water.The reaction is highly exothermic.

2H₂ (g)+ O₂ (g) ——–> 2H₂O (l)    ΔH = – 285.9 Kj mol^-1

(b) Reaction  with halogens

Dihydrogen combines with halogens to form hydrogen halides.

H₂ (g) + X₂ (g) ———–> 2HX (g) where X= F, Cl, Br, I

The reactivity of halogens towards dihydrogen decreases in the order:

F₂ > Cl₂ > Br₂ > I₂

Fluorine reacts in dark, chlorine in the presence of diffused sunlight, while the reaction with bromine and Iodine occurs on heating in the presence of a catalyst.

 

H₂ (g) + F₂ ( g ) ———–> 2HF (g) .    dark

H₂ (g) + Cl₂ ( g ) ———–> 2HCl (g) .   673 K , sunlight

H₂ (g) + Br₂ ( g ) ———–> 2HBr (g)      673 K , catalyst

H₂ (g) + I₂ ( g ) ———–> 2HI (g)          673 K , Pt catalyst

(c) With dinitrogen it forms ammonia

N₂ (g) + 3 H₂ (g) ———-> 2NH₃ ( g)     673 K , 200 atm, Fe, Mo , ΔH = -92.6 KJ/ mol

Fe act as a catalyst while Mo act as a promoter. This reaction is used for the manufacture of ammonia by the Haber’s process.

(d) With Sulphur ,it forms hydrogen sulphide.

H₂ (g) + S (l) ———-> H₂S (g) .  700K

e) With carbon

Depending upon condition it forms Methane or acetylene.

C(s) + 2H₂ (g) ———-> CH₄ (g)    1375 K

2C (s) + H₂ (g) ———–> C≡C         electric arc, 3300 K

(6) Reduction of metal oxides and ions

Dihydrogen acts as a reducing agent and hence reduces oxides of certain metals less electropositive than zinc such as those of Cu, Pb, Fe etc to corresponding metals.

CuO + H₂ ——-> Cu + H₂O

ZnO + H₂ ————–> Zn + H₂O

PbO + H₂ ———> Pb + H₂O

Dihydrogen also reduces some metal ions in aqueous solution.

Pd²⁺ (aq) + H₂ (g) ———> Pd (s) + 2 H⁺ (aq)

Cu ²⁺ (aq) + H₂ (g)———> Cu (s) + 2H⁺ (aq)

(7) Hydrogenation of unsaturated hydrocarbons

Unsaturated hydrocarbons such as alkenes and alkynes add dihydrogen in presence of a catalyst to form saturated hydrocarbon.

Hydrogenation of unsaturated organic compounds in presence of heterogeneous and homgeneous catalyst is used in many industrial processes.

(8) Hydroformylation of Olefins

Olefins reacts with carbon monoxide and dihydrogen in presence of octacarbonyl dicobalt as catalyst under high temperature and pressure to form aldehydes.

This reaction is called hydroformylation or the oxo- process. The aldehydes thus obtained on subsequent catalytic reduction gives alcohols.

RCH₂CH₂CHO + H₂ ———-> RCH₂CH₂CH₂OH      Ni , heat

(9) Hydrogenation of Oils

The vegeatable oil such as soyabean oil, cotton seed oil ,groundnut oil are called polyunsaturated oils since they contain many carbon carbon double bonds.When these oils are exposed to air for prolonged period ,the double bond present in them undergo oxidation and the oils become rancid i.e. develop unpleasant odour and taste due to formation of lower aldehydes and carboxylic acid.

Dihydrogen is bubbled through edible oils in presence of finely divided nickel at 473 K when the oils are converted into solid fats.

Vegetable oil + H₂ ——–> Vegeatble fat     473 K , Ni catalyst

This process is called hydrogenation or hardening of oil and is used in the manufacture of vegetable ghee.

Uses of Dihydrogen

Dihydrogen is used

(1) in the manufacture of ammonia which is used for the production of various fertilizers.

(2) in the hydrogenation of vegetable oils to form solid fats

(3) in manufacture of bulk chemical such as methanol

CO(g) + 2H₂ (g) ——-> CH₃OH (l)

(4) in the manufacture of hydrogen chloride

H₂ (g ) + Cl₂ (g) ——–> 2HCl (g)     sunlight

(5) in the manufacture of metal hydrides

(6) in metallurgy to reduce heavy metal oxides to metal

(7) in the atomic hydrogen and oxy-hydrogen torches for cutting and welding.

(8) Liquid hydrogen is used as a rocket fuel in space research.

(9) It is also used in fuel cells for generating electrical energy.It does not cause any pollution and produces greater energy per unit mass of fuel in comparison to gasoline and other fuels.