HYDROGEN FOR XI STD CHEMISTRY

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INTRODUCTION

Hydrogen, chemical element that exists as a gas at room temperature. When hydrogen gas burns in air, it forms water. French chemist Antoine Lavoisier named hydrogen from the Greek words for “water former.”

Hydrogen has the smallest atoms of any element. A hydrogen atom contains one proton, and only one electron . The proton is the center, or nucleus, of the hydrogen atom, and the electron travels around the nucleus.

Pure hydrogen exists as hydrogen gas, in which pairs of hydrogen atoms bond together to make molecules.

The hydrogen atomconsisting the protonin the centre or the nucleus of the hydro-gen atom and the ele-ctron travelling aroun-d the nucleus.

POSITION IN THE PERIODIC TABLE

Hydrogen is the first element in the periodic table of the elements and is represented by the symbol H.

Hydrogen, with only one proton, is the simplest element. It is usually placed in Period 1 and Group 1 of the periodic table.

Hydrogen can combine chemically with almost every other element and forms more compounds than does any other element. These compounds include water, minerals, and hydrocarbons—compounds made of hydrogen and carbon—such as petroleum and natural gas.

Position of hydrogen in the periodic table

OCCURRENCE OF DIHYDROGEN

Hydrogen is the tenth most common element on Earth. Because it is so light, though, hydrogen accounts for less than 1 percent of Earth's total mass. It is usually found in compounds. Pure hydrogen gas rarely occurs in nature, although volcanoes and some oil wells release small amounts of hydrogen gas.

Hydrogen is in nearly every compound in the human body. For example, it is in keratin, the main protein that forms our hair and skin, and in the enzymes that digest food in our intestines. Hydrogen is in the molecules in food that provide energy: fats, proteins, and carbohydrates.

Presence of hydrogen in volcanoes and in our food particles

Cont….

Hydrogen accounts for about 73 percent of the observed mass of the universe and is the most common element in the universe.

Hydrogen atoms were the first atoms to form in the early universe and that the atoms of the other elements formed later from the hydrogen atoms.

About 90 percent of the atoms in the universe are hydrogen, about 9 percent are helium, and all the other elements account for less than 1 percent.

Cont….

Common Molecules:

Many common molecules contain hydrogen. In these molecules, butane contains ten hydrogen atoms, ammonia contains three hydrogen atoms, and water contains two hydrogen atoms.

INTERIOR OF THE SUN

The Sun’s energy is

produced in the core through nuclear fusion of hydrogen atoms into helium. Gases in the core are about 150 times as dense as water and reach temperatures as high as 16 million degrees C (29 million degrees F).

ISOTOPES OF HYDROGEN

Atoms of an element that have different numbers of neutrons in their nuclei are called isotopes of that element.

Isotopes all usually share the same chemical behavior, but have different masses.

The isotopes of hydrogen are protium, deuterium, and tritium. Hydrogen always has one proton in its nucleus.

PREPARATION OF DIHYDROGEN

Laboratory preparation of dihydrogen:

1.It is usually prepared by the reaction of granulated zinc with dilute hydrochloric acid. The chemical equation for this reaction is the following:

Zn + 2HCl → ZnCl2 + H2

2.It can also be prepared by the reaction of zinc with aqueous alkali. The chemical equation for this reaction is the following:

Zn +2NaOH Na2ZnO2 + H2 (Sodium zincate)

Dihydrogen in 3D

Cont….

Commercial production of dihydrogen:

1. Electrolysis of acidified water using platinum electrodes gives hydrogen.

2 H2O   electrolysis 2H2 + O2

This chemical equation shows that two water molecules (with electricity), form two molecules of hydrogen gas and one molecule of oxygen gas.

2.High purity (>99.95%) dihydrogen is obtained by electrolysing warm aqueous barium hydroxide solution between nickel electrodes.

Commercial production of dihydrogen

Cont….

3. It is obtained as a byproduct in the manufacture of sodium hydroxide & chlorine by the electrolysis of brine solutions .

The reactions that takes place are:

At anode : 2Cl- Cl2 +2e-

At cathode: 2H2O + 2e- H2 + 2OH-

The overall reaction is 2Na+ + 2Cl- +2H2O

Cl2 + H2 + 2Na+ + 2OH-

Cont….4. Reaction of steam on hydrocarbons at high temperature in

the presence of catalyst yields hydrogen. e.g.,

CH4 + H2O 1270K CO + 3H2

Ni

The mixture of CO & H2 is called water gas.

It is used for synthesis of methanol & a number of hydrocarbons, therefore it is called synthesis gas or ‘syngas’.

The production of dihydrogen can be increased by reacting carbon monoxide with steam in the presence iron chromate as catalyst.

CO + H2O 673K CO2 + H2

Catalyst

This is called water-gas shift reaction. Carbon dioxide is removed by scrubbing with sodium arsenite solution.

PHYSICAL PROPERTIES OF H2

Dihydrogen is a : Colourless , Odourless Tasteless Combustible gas Lighter than air Insoluble in water It’s melting point – 18.73 K

& boiling point – 23.67 K

CHEMICAL PROPERTIES OF H2

Hydrogen gas does not usually react with other chemicals at room temperature, because the bond between the hydrogen atoms is very strong and can only be broken with a large amount of energy.

Since its orbital is incomplete with 1s1 electronic configuration, it does combine with almost all the elements .

It accomplishes reactions by:

1.loss of one e- to give H+

2.gain of an e- to form H-

3.sharing electrons to form a single covalent bond.

CHEMISTRY OF DIHYDROGEN

Reaction with halogens:

It reacts with halogens, X2 to give hydrogen halides, HX,

H2+X2 2HX (X= F, Cl, Br, I)

While the reaction with fluorine occurs even in the dark, with iodine it requires a catalyst.

Reaction with dioxygen:

It reacts with dioxygen to form water. The reaction is highly exothermic.

2H2 + O2 catalyst or heating 2H2O ; H = -285.9 kJ mol-1

Cont…. Reaction with dinitrogen:

With dintrogen it forms ammonia.

3H2 +N2 673K,200atm 2NH3; H=-92.6 kJ mol-1

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

Haber Process:

German chemist and Nobel laureate Fritz Haber developed an economical method of producing ammonia from air and seawater. In his process, nitrogen is separated from the other components of air through distillization. Hydrogen is obtained from seawater by passing an electric current through the water. The nitrogen and hydrogen are combined to form ammonia (NH3).

Cont…. Reaction with metals:

Hydrogen also forms ionic bonds with some metals, at a high temperature, creating a compound called a hydride.

H2 +2M 2MH

Where M is an alkali metal (e.g. lithium, sodium, potassium, rubidium, cesium, and francium.)

Reactions with metal ions & metal oxides:

It reduces some metal ions in aqueous solution & oxides of metals (less active than iron ) into corresponding metals.

H2+Pd 2+ Pd + 2H+

yH2 +MxOy xM + yH2O

Cont…. Reactions with organic compounds:

1.Hydrogenation of vegetable oils using nickel as catalyst gives edible fats. (margarine & vanaspati ghee).

2.Hydroformylation of olefins yields aldehydes which further undergo reduction to give alcohols.

H2+CO+RCH=CH2 RCH2CH2CHO

H2 +RCH2CH2CHO RCH2CH2CH2OH

USES OF DIHYDROGEN The largest use of dihydrogen is in the synthesis of

ammonia which is used in the manufacture of nitric acid & nitrogenous fertilizers.

Dihydrogen is used in the manufacture of vanaspati fat. It is used in the manufacture of bulk organic chemicals,

particularly methanol.

CO + 2H2 catalyst cobalt CH3OH

It is widely used for the manufacture of metal hydrides.

It is used for the preparation of hydrogen chloride, a highly useful chemical.

Cont……

In metallurgical processes, it is widely used to reduce heavy metal oxides to metals.

Atomic hydrogen & oxy-hydrogen torches find use for cutting & welding purposes.

It is used as a rocket fuel in space research. Dihydrogen is used in the fuel cells for

generating electrical energy. It has many advantages over the conventional fossil fuels & electric power.

Various uses of dihydrogen

HYDRIDES Dihydrogen also forms ionic bonds with some

metals, at a high temperature, creating a compound called a hydride.

If E is the symbol of an element then hydride can be expressed as EHX (e.g. MgH2) or EmHn (e.g. B2H6).

The hydrides are classified into three categories: 1.Ionic or saline or saltlike hydrides.

2.Covalent or molecular hydrides.

3.Metallic or non-stoichiometric hydrides.

Nearly all elements are able to form hydride compound

IONIC OR SALINE HYDRIDES These are stoichiometric compounds of dihydrogen

formed with most of the s-block elements which are highly electropositive in character.

Covalent character is found in the lighter metal hydrides (e.g. LiH, BeH2 & MgH2).

The ionic hydrides are crystalline, non-volatile & non-conducting in solid state.

Their melts conduct electricity & on electrolysis liberate dihydrogen gas at anode, which confirms the existence of H-ion.

2H-(melt) anode H2+2e-

Saline hydrides react violently with water producing dihydrogen gas .

NaH + H2O NaOH + H2

COVALENT OR MOLECULAR HYDRIDE Dihydrogen forms molecular compounds with most of

the p-block elements. For e.g. CH4, NH3, H2O & HF.

Hydrogen compounds of non metals have also been considered as hydrides. Being covalent they are volatile compounds.

Molecular hydrides are further classified according to the relative number of electrons & bonds in their Lewis structure into:

1.Electron-deficient

2.Electron-precise

3.Electron-rich hydrides.

ELECTRON-DEFICIENT HYDRIDES

ELECTRON-PRECISE HYDRIDES

ELECTRON-RICH HYDRIDES

Has few electrons for Lewis structure.

Elements of group 13 forms these compounds.

For e.g. Diborane (B2H6).

They act as Lewis acids i.e. electron acceptor.

Have the required number of electrons for Lewis structure.

Elements of group 14 forms these compounds.

For e.g. CH4.

Have excess electrons which are present as lone pair.

Electrons of group 15-17 forms such compounds.

For e.g.NH3- has1lonepair, H2O- has 2 lone pairs.They act as Lewis bases i.e. electron donor.

METALLIC HYDRIDES

These are formed by many d-block & f-block elements. The metals of group 7,8 & 9 do not form hydride. These hydrides conduct heat & electricity though not as

efficiently as their parent metals do. Unlike saline hydrides, they are almost non-

stoichiometric, being deficient in hydrogen. For e.g. LaH2.87 & YbH2.55.

Law of constant composition does not hold good. The property of absorption of hydrogen on transition

metal is widely used in catalytic reduction/hydrogenation reactions for the preparation of large number of compounds.

Some of the metals can accommodate a very large volume of hydrogen & can be used as its storage media.

Water

A major part of all living organisms is made up of water.

Human body has about 65% & some plants have as much as 95% water.

It is a crucial compound for the survival of all life forms.

It is a solvent of great importance.

Different uses of water

Physical properties of water

It is a colourless & tasteless liquid. The unusual properties of water in the condensed

phase (liquid & solid) are due to the presence of extensive hydrogen bonding between water molecules.

Water has a higher specific heat, thermal conductivity, surface tension, dipole moment & dielectric constant when compared to other liquids.

It is an excellent solvent for transportation of ions & molecules required for plant & animal metabolism.

Due to hydrogen bonding with polar molecules, even covalent compounds like alcohol & carbohydrates dissolve in water.

STRUCTURE OF WATER

In the gas phase water is a bent molecule with a bond angle of 104.50 ,

and O-H bond length of 95.7 pm.

It is a highly polar molecule .

In the liquid phase water molecules are associated together by hydrogen bonds.

Density of water is more than that of ice.

Hydrogen Bonding in Water:

Hydrogen bonds are chemical bonds that form between molecules containing a hydrogen atom bonded to a strongly electronegative atom . Because the electronegative atom pulls the electron from the hydrogen atom, the atoms form a very polar molecule, meaning one end is negatively charged and the other end is positively charged. Hydrogen bonds form between these molecules because the negative ends of the molecules are attracted to the positive ends of other molecules, and vice versa. Hydrogen bonding makes water form a liquid at room temperature.

STRUCTURE OF ICE:

Ice has a highly ordered three dimensional hydrogen bonded structure.

Examination of ice crystals with x-rays shows that each oxygen atom is surrounded tetrahedrally by four other oxygen atoms a distance of 276pm.

Hydrogen bonding gives ice a rather open type structure with wide holes. These holes can hold some other molecules of appropriate size interstitially.

Structure of ice

Chemical properties of water

Amphoteric nature: it has the ability to act as an acid as well as a base i.e., it behaves as an amphoteric substance.

In the Bronsted sense it acts an acid with NH3 and a base with H2S.

H2O+ NH3 OH- + NH4+

H2O+ H2S H3O++ HS-

The auto-protolysis (self- ionization) of water takes place as follows:

H2O +H2O H3O+ +OH-

acid-1 base-2 acid-2 base-1

REDOX REACTIONS INVOLVING WATER

Water can be easily reduced to dihydrogen by highly electropositive metals.

2H2O +2Na 2NaOH +H2

Thus ,it is a great source of dihydrogen.

Water is oxidised to O2 during photosynthesis.

6CO2 +12H2O C6H12O6 + 6H2O +6O2

With fluorine also it is oxidised toO2.

2F2 + 2H2O 4H+ + 4F- +O2

Hydrolysis reaction

Due to high dielectric constant, it has a very strong hydrating tendency. It dissolves many ionic compounds. However, certain covalent& some ionic compounds are hydrolysed in water.

P4O10 +6H2O 4H3PO4

SiCl4 +2H2O SiO2 + 4HCl

N3- + 3H2O NH3 +3OH-

A hydrolysis process generally involves water

HYDRATES FORMATION From aqueous solutions many salts can be

crystallised as hydrated salts. Such an association of water is of different types viz.,

(i) coordinated water e.g.,

[Cr(H2O)6 ]3+ 3Cl-

(ii) interstitial water e.g.,

BaCl2.2H2O

(iii) hydrogen-bonded water e.g.,

[Cu(H2O)4]2+SO42-.H2O in CuSO4.5H2O

HARD & SOFT WATER:

The presence of calcium & magnesium salts in the form of hydrogen carbonate, chloride & sulphate in water makes water ‘hard’.

Hard water does not give lather with soap.

Water free from soluble salts of calcium & magnesium is called soft water.

It gives lather with soap easily.

Cont…. Hard water forms scum/precipitate with soap. Soap containing sodium stearate( C17H35COONa) reacts with

hard water to precipitate out Ca/Mg stearate.

2C17H35COONa(aq)+M2+(aq) M(C17H35COO)2 +2Na+(aq);

M is Ca/Mg

Hard water is harmful for boilers, because of deposition of salts in the form of scale. This reduces the efficiency of the boiler.

The hardness of water is of two types: 1.Temporary hardness. 2.Permanent hardness.

TEMPORARY HARDNESS:

Temporary hardness is due to the presence magnesium & calcium hydrogen carbonates. It can be removed by:

1.Boiling: During boiling, the soluble Mg(HCO3)2 is converted into insoluble Mg (OH)2 & Ca(HCO3)2 is changed to insoluble CaCO3. It is because of high solubility product of Mg(OH)2 as compared to that of MgCO3, that Mg(OH)2 is precipitated. These precipitates can be removed by filtration. Filtrate thus obtained will be soft water.

Mg(HCO3)2 heating Mg(OH)2 + 2CO2

Ca(HCO3)2 heating CaCO3 + H2O + CO2

Cont….

2.Clark’s method: In this method calculated amount of line is added to hard water. It precipitates out calcium carbonate & magnesium hydroxide which can be filtered off.

Ca(HCO3)2+Ca(OH)2 2CaCO3 +2H2O

Mg(HCO3)2+2Ca(OH)2 2CaCO3 +Mg(OH)2 +2H2O

Ways to reduce temporary hardness

PERMANENT HARDNESS:

It is due to the presence of soluble salts of magnesium & calcium in the form of chlorides & sulphates in water.

Permanent hardness is not removed by boiling. It can be removed by the following methods:

1.Treatment with washing soda (sodium carbonate): Washing soda reacts with soluble calcium & magnesium chlorides & sulphates in hard water to form insoluble carbonates.

MCl2 + Na2CO3 MCO3 + 2NaCl (M=Mg, Ca)

MSO4+Na2CO3 MCO3 + Na2SO4

Cont….2.Calgon’s method: Sodium hexametaphosphate (Na6P6O18),

commercially called ‘calgon’, when added to hard water ,the following reactions takes place.

Na6P6O18 2Na+Na4P6O182- (M=Mg, Ca)

M2++Na4P6O182- [Na2MP6O18]2-+2Na+

The complex anion keeps the Mg2+ & Ca2+ions in solution.

3.Ion-exchange method: This method is also called zeolite /permutit process. Hydrated sodium aluminium silicate is zeolite. Sodium aluminium silicate (NaAlSiO4) can be written as NaZ. When this is added to hard water, exchange reactions take place.

2NaZ(s)+M2+(aq) MZ2(s)+2Na+(aq) (M=Mg, Ca)

Cont…. Permutit/zeolite is said to be exhausted when all the sodium in

it is used up. It is generated for further use by treating with an aqueous sodium chloride solution.

MZ2(s)+2NaCl(aq) 2NaZ(s)+MCl2(aq)

4.Synthetic resins method: This method is more efficient than zeolite process. Cation exchange resins contain large -organic molecule with-SO3H group & are water insoluble. Ion exchange resin (RSO3H) is changed to RNa by treating it with NaCl. The resin exchanges Na+ ions with Ca2+ & Mg2+ ions present in hard water to make the water soft. Here R is resin anion.

2RNa+M2+ R2M+2Na+

The resin can be regenerated by adding aqueous NaCl solution.

Cont….

Pure de-mineralised (de-ionised) water free from all soluble mineral salts is obtained by passing water successively through a cation exchange (in the H+ form) & an anion-exchange ( in the OH- form) resins:

2RH+M2+ MR2+2H+

In this cation exchange process, H+ exchanges for Na+,Ca2+, Mg2+ & other cations present in water.

This process results in proton release & thus makes the water acidic.

Cont….

In the anion exchange process:

RNH2 + H2O RNH3+.OH-

RNH3+.OH- + X- RNH3

+.X- + OH-

OH- exchanges for anions like Cl-,HCO3-, SO4

2- etc. present in water. OH- ions liberated neutralise the H+ ions set free in the cation exchange.

H+ + OH- H2O

The exhausted cation & anion exchange resin beds are regenerated by treatment with dilute acid & alkali solutions respectively.

Methods for reducing permanent hardness

HYDROGEN PEROXIDE:

Hydrogen peroxide is an important chemical used in pollution control treatment of domestic & industrial effluents.

PREPARATION: It can be prepared by the following methods:

1.Acidifying barium peroxide & removing excess water by evaporation under reduced pressure gives hydrogen peroxide.

BaO2.8H2O+H2SO4 BaSO4+H2O2+8H2O

2.Preoxodisulphate, obtained by electrolytic oxidation of acidified sulphate solutions at high current density, on hydrolysis yields hydrogen peroxide.

2HSO4- electrolysis HO3SOOSO3H hydrolysis 2HSO4

-+2H++H2O2

Cont….

This method is now used for the laboratory preparation of D2O2.

K2S2O8+2D2O 2KDSO4+D2O2

3.Industially it is prepared by the auto-oxidation of 2-alklylanthraquinols.

2-ethylanthraquinol H2O2+(oxidised product)

In this case 1% H2O2 is formed. It is extracted with water &

concentrated to 30% (by mass) by distillation under reduced pressure. It can be further concentrated to 85% by careful distillation under low pressure. The remaining water can be frozen out to obtain pure H2O2.

Preparation of hydrogen peroxide

PHYSICAL PROPERTIES:

The pure state H2O2 is an almost

colourless liquid Meting point - 272.4K. Boiling point - 423K Vapour pressure (298K) – 1.9mmHg. H2O2 is miscible with water in all proportions & forms

a hydrate H2O2.H2O.

A 30% solution of H2O2 is marketed as ‘100V’ hydrogen peroxide. It means that 1ml of 30% H2O2 solution will give 100V of oxygen at STP.

Hydrogen peroxide has a non-planar structure.

CHEMICAL PROPERTIES:

It acts as an oxidising as well as reducing agent in both acidic & alkaline media.

1.Oxidising action in acidic medium:

2Fe2+ +2H+ +H2O2 2Fe3+ +2H2O

PbS +4H2O2 PbSO4+4H2O

2.Reducing action in acidic medium:

2MnO4- +6H+ +5H2O2 2Mn2+ +8H2O+5O2

HOCl +H2O2 H3O+ +Cl- +O2

Cont….

3.Oxidising action in basic medium:

2Fe2+ +H2O2 2Fe3+ +2OH-

Mn2+ +H2O2 Mn4+ +2OH-

4.Reducing action in basic medium:

I2+H2O2+2OH- 2I-+2H2O+O2

2MnO4-+3H2O2 2MnO2+3O2+2H2O+2OH-

STORAGE

H2O2 decomposes slowly on exposure to light.

2H2O2 2H2O+O2

In the presence of metal surfaces or traces of alkali, the above reaction is catalysed. It is, therefore stored in wax-lined glass or plastic vessels in dark.

It is kept away from dust because dust can induce explosive decomposition of the compound.

USES:

It is used as hair bleach & as a mild disinfectant. As an antiseptic it is sold in the market as perhydrol.

It is used to manufacture chemicals like sodium perborate & per-carbonate, which are used in high quality detergents.

It is used in the synthesis of hydroquinone, tartaric acid & certain food products & pharmaceuticals etc.

It is employed in the industries as bleaching agent for textiles, paper pulp, leather, oils, fats etc.

It is also used in environmental chemistry.

HEAVY WATER,D2O

It is extensively used as a moderator in nuclear reactors & in exchange reactions for the study of reaction mechanisms.

It can be prepared by exhaustive electrolysis of water or as a by-product in some fertilizer industries.

PHYSICAL PROPERTIES: Molecular mass: 20.0276 g/mol. Melting point: 276.8K. Boiling point: 374.4K.

Cont…..

The bottom ice cubes were made with heavy water, which is water that uses deuterium hydrogen (nucleus with an extra neutron) not regular hydrogen which has no neutron.

Production of Hard water

USES:

It is used for the preparation of other deuterium compounds. For e.g.

CaC2 + 2D2O C2D2 + Ca(OD)2

SO3 + D2O D2SO4

Al4C3 + 12D2O 3CD4 + 4Al(OD)3

DIHYDROGEN AS A FUEL:

Dihydrogen releases large quantities of heat on combustion.

Dihydrogen can release more energy than petrol's. HYDROGEN ECONOMY: The basic principle of

hydrogen economy is the transportation & storage of energy in the form of liquid or gaseous dihydrogen.

Energy is transmitted in the form of dihydrogen & not as electric power.

It is also use in fuel cell for generation of electric power.

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