Hydrogen

INTRODUCTION

Hydrogen, chemical element that exists as a gas at room temperature.

When hydrogen gas burns in air, it forms water.

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.

First element in the periodic table with atomic mass of 1 u and atomic

number 1.

Discovery of Hydrogen

In 1766, Henry Cavendish was the first to recognize hydrogen gas as a discrete substance, by identifying the gas from a metal-acid reaction as flammable air. In 1781 he was the first person to find that the gas produces water when burned. This was a key experiment in disproving the Aristotelian theory of the four elements.

THE STRUCTURE OF HYDROGEN ATOM

The hydrogen atom consisting the proton in the centre or the nucleus of the hydrogen atom and the electron travelling around

POSITION OF HYDROGEN IN THE PERIODIC TABLE

RESEMBLANCE WITH OTHER ELEMENTS

• With Alkali Metals:

Hydrogen has an electronic configuration of 1s1 which is similar to the

electronic configuration of alkali metals (ns1). Hydrogen forms oxides, halides

and sulphides like alkali metals. But it gains an electron to become an anion. It has high ionization enthalpy and forms covalent bonds.

• With Halogens:

In terms of ionization enthalpy, hydrogen resembles halogens. Like halogens, hydrogen forms diatomic molecule, combines with elements to form hydrides and a large number of covalent compounds. But it loses an electron to become a cationand hydrogen reacts with halogens themselves.

Preparation of Dihydrogen

There are a number of methods for preparing dihydrogen from metals and

metal hydrides:

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

Zn + 2H+ Zn+2 + H2

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

Zn + 2NaOH Na2ZnO2 + H2

COMMERCIAL PRODUCTION

• 1- Electrolysis of acidified water using platinum electrode gives

HYDROGEN.

• 2H2O ELECTROLYSIS 2H2 + O2

• 2- Electrolysis of warm aqueous barium oxide solution between

nickel electrodes.

• 3- In manufacture of sodium hydroxide and chlorine by electrolysis

of brine solution (Byproduct)

• 4- Reaction of steam on hydrocarbons or coke at high

temperatures in prescence of catalyst gives hydrogen.

• Eg. CH4 + H2O CO+ 3H2

1270K/Ni

PHYSICAL PROPERTIES

• DIHYDROGEN IS COLOURLESS, ODOURLESS AND TASTELESS.

• IT IS COMBUSTIBLE. HYDROGEN AND HELIUM IS PRESENT ON THE SURFACE OF

THE EARTH.

CHEMICAL PROPERTIES

The chemical behavior of dihydrogen, like all other molecules depends

the bond dissociation enthalpy.

The inertness of dihydrogen at room temperature is due to the

dissociation enthalpy

Since it’s orbital 1s is empty, it combines with almost all elements

It can lose 1 electron to form H+, gain one electron to form H- or share

it’s electron to form single bonds

Reactions of Hydrogen with other substances

Reactions with substance also gives us the chemical properties of hydrogen:

• Reaction with Halogen: It leads to the formation of Hydrogen Halides

H2 (g) + X2 (g) 2HX (g)

In which X is a halogen, eg. F, Cl, Br, I.

Reaction with Dioxygen: It reacts with dioxygen to form water. The reaction is

highly exothermic

2H2 + O2 2H2O

Reaction with Dinitrogen:

With Nitrogen it forms Ammonia

3H2 + N2 2NH3

Reaction With Metals:

It combines with many metals at a high temperature to yield the corresponding hydrides.

H2 + 2M 2MH (M=ALKALI METAL)

Reaction with metal ions and metal oxides:

It reduces some metal ions in aqueous state.

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

It reduces oxides of metals into their corresponding oxide:

yH2(g) + MxOy (s) xM(s) + yH2O(L)

Uses Of Dihydrogen

WATER

• Major part of the living beings is made up of water

• Humans have 65% of water while some plants have 95% of water

• It is a solvent of great importance and necessary for the survival of all organisms

• The distribution of water around the globe is not uniform

CHEMICAL PROPERTIES OF

WATERWater reacts with a number of substances

Amphoteric Nature

Water has the ability to act as an acid as well as a base

H2O + NH3 (OH-) +NH4+

H2O + H2S (H3O+) + HS+

Redox Reaction

Water can be easily reduced to dihydrogen by highly electropositive metals

2H2O + 2Na 2NaOH + H2

Water is oxidized to O2 during photosynthesis

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

Hydrolysis Reaction and Hydrates Formation

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

P4O10 + 6H2O 4H3PO4

• From aqueous solution many salts can be crystallized as hydrated salts. These are of different types: (i)coordinate water (ii) interstitial water (iii) hydrogen bonded water

• Water which contain salts of magnesium and calcium in the form of hydrogen carbonate, chloride and sulphate is called HARD WATER. It does not give lather with soap

• Water free from soluble salts of calcium and magnesium is known as SOFT WATER. It gives lather with soap easily.

Hard Water

The hardness of water is of two types:

Temporary Hardness

Permanent Hardness

REMOVAL OF TEMPORARY HARDNESSDUE TO MAGNESIUM AND CALCIUM HYDROGEN

CARBONATES

BOILING: DURING BOILING, THE SOLUBLE MG(HCO3)2 IS CONVERTED

INTO INSOLUBLE MG(OH)2 AND CA(HCO3)2 IS CHANGED TO

INSOLUBLE CACO3. THESE PRECIPITATES CAN BE REMOVED BY

FILTRATION. FILTRATE THUS OBTAINED IS A HARD WATER.

MG(HCO3)2 MG(OH)2 +

2CO2

CA(HCO3)2 CACO3 + H2O

+ CO2

Clarke’s Method: A calculated amount of lime is added to hard water which precipitates out calcium carbonate and magnesium hydroxide which can be filtered off.

Ca(HCO3) + Ca(OH)22CaCO3

+ 2H2O

REMOVAL OF PERMANENT HARDNESS

• TREATMENT WITH WASHING SODA: WASHING SODA REACTS WITH SOLUBLE CALCIUM AND MAGNESIUM

CHLORIDES AND SULPHATES TO FORM INSOLUBLE CARBONATES

• CALGON’S METHOD: SODIUM HEXAMETAPHOSPHATE(NA6P6O18) COMMERCIALLY KNOWN AS CALGON,

WHEN ADDED TO HARD WATER LEADS TO THE FORMATION COMPLEX ANIONS WHICH KEEPS MG2+ AND

CA2+ IONS IN THE SOLUTION.

• Ion exchange method: When sodium aluminium silicate (zeolite/permutit) is added to hard water, exchange reaction takes place.

• Synthetic resins method: Nowadays hard water is softened by using synthetic cation exchangers. This method is more efficient than zeolite process.

Hydrogen PeroxideAN IMPORTANT CHEMICAL USED IN POLLUTION CONTROL TREATMENT OF DOMESTIC AND INDUSTRIAL EFFLUENTS

PREPARATION ACIDYFING BARIUM PEROXIDE AND REMOVING EXCESS WATER BY

WVAPORATION UNDER REDUCED PRESSURE GIVES H2O2

PEROXODISULPHATE ON HYDROLYSIS YIELDS HYDROGEN PEROXIDE

HO3SOOSO3H HYDROLYSIS 2HSO4 + 2H + H2O2

Industrially it is prepared by the auto-oxidation of 2-alklylanthraquinols. In this case 1% H2O2 is formed. It is extracted with water and concentrated to 30% 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

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

Physical Properties

In the pure state, H2O2 is a very pale blue liquid with a melting point of 272.4 K, boiling

point of 423 K. It is miscible in water in all proportions and it form hydrates H2O2.H2O. A

30% solution of H2O2 is marketed as 100 volume hydrogen peroxide.

USES OF HYDROGEN PEROXIDE

1) USED AS HAIR BLEACH AND MILD DISINFECTANT2)USED TO MANUFACTURE CHEMICALS LIKE PER-CARBONATE3)USED IN SYNTHESSIS OF CERTAIN FOOD PRODUCTS4)USED IN INDUSTRIES AS BLEACHING AGENT5)USED IN ENVIRONMENTAL CHEMISTRY

HEAVY WATER (D2O)

IT IS USED AS A MODERATOR IN NUCLEAUR REACTORS AND IN EXCHANGE REACTIONS.

IT IS PREPARED BY EXHAUSTIVE ELECTROLYSIS OF WATER AS A BY-PRODUCTIN SOME FERTILIZER INDUSTRIES.

IT IS USED FOR PREPARATION OF OTHER DEUTERIUM COMPOUNDS:

CaC2 + 2D2O C2D2 + Ca(OD)2

SO3 + D2O 2SO4

AL4C3 + 12D2O 3CD4 + 4AL(OD)3

DIHYDROGEN AS A FUEL

HYDROGEN RELEASES LARGE QUANTITIES OF HEAT ON COMBUSTION. IT RELEASES MORE ENERGY THAN PETROL, ALMOST THRICE. THE ONLY POLLUTANTS ARE OXIDES OF NITROGEN ,WHICH CAN BE REDUCED BY INJECTING A SMALL AMOUNT OF WATER IN THE CYLINDER TO LOWER TEMERATURE.

Done By

Kevin Anil

XI-M