HYDROGEN AND ITS COMPOUND

 

HYDROGEN AND ITS COMPOUND

HYDROGEN AS AN ELEMENT

Carvendish is regarded as the discoverer of hydrogen because he was first (in 1766) to prepared it in the state, describe its properties and recognise  it as an element.

Hydrogen is the lightest of the element making up  about 1% of the earth crust. It is found uncombined only in very small amount in the atmosphere. Hydrogen is widely distributed in combination with other elements in the form of water, acid, organic substances and petroleum.

CONFIGURATION AND POSSIBLE OXIDATION NUMBER

The electronic configuration of hydrogen is 1S1. It has an oxidation number of 1 in most of its compounds except  hydrides where it is -1.

ISOTOPES OF HYDROGEN

Nuclei of the same element having same atomic number but different mass number are known as isotopes.

There are three isotopes of hydrogen

1.       Protium         ii. Deuterium                     iii. Tritium     

1.       PROTIUM: Ordinary hydrogen is known as protium. It has one electron, one proton but it has no neutron. Mass number: 1 ; charge number: 1

  : H . % in natural hydrogen: 99.98%

Structure

2.       DEUTERIUM: Heavy hydrogen is known as deuterium. It has one electron, one proton and one neutron.

Mass number :2; charge number: 1 .

Symbol: H or D. % in natural hydrogen : 0.0156%

Structure

Heavy water (D20) consists of deuterium isotopes of hydrogen.

3.       TRITIUM: It has one electron, one proton and two neutrons. Mass number: 3; charge number: 1.

Symbol: H or T. % in natural hydrogen 4x10-15% .

Tritium is a radioactive  isotopes it emits  beta-rays. It has a half-life of 12.5 years. It is present in traces.

UNIQUE POSITION OF HYDROGEN IN THE PERIODIC TABLE

Hydrogen is separated as it has only 1 electron and so exhibits some unique behaviour.

Most elements lose or gain electrons to reach a noble gas configuration. Hydrogen is unique in that it can both gain one electron to have configuration of He or it can lose one electron and just be a lone proton with no electrons. A half-filled shell(in this case the 1s orbital) is very unstable. It is better to have no electron or two electrons. Hydrogen can behave either like an alkali metal and be H+ (having loss 1 electron) or can act more like a halogen and be H- (having gain 1 electron)        

LABORATORY  PREPARATION OF HYDROGEN

There are three ways in which hydrogen can be prepared in the laboratory:

1.       Action of dilute acid on zinc- Dilute HCl or H2SO4 react with granulated metallic zinc to liberated hydrogen gas.

Zn + 2HCl               ZnSO4    +    H2; Zn + H2SO4              ZnSO4  +   H2.

The hydrogen gas is given off in a gas -jar over water. If the gas is required dry, it is passed through fixed calcium chloride or conc. H2SO4 and collected by downward displacement of air, since it is lighter than air.

Dilute HNO3 is not used for the preparation of hydrogen because it is a strong oxidizing acid and will produce hydrogen only with magnesium.

2.       Action of cold water on active metal- sodium and potassium react rapidly with cold water, liberating hydrogen. This reaction is very vigorous and should be carried out with extremely care using only a small piece of the metal.

2Na + H2O            NaOH +H2;  2K + H2O               2KOH + H2

Hydrogen gas is given off with effervescence and is collected in the test-tube inverted over the metal. Calcium react slowly with cold water to produce hydrogen gas.

Ca + 2H2O           Ca(OH)2  + H2

3.       Action of steam on iron: Iron react red heat liberates hydrogen from steam. Triirontetraoxide Fe₃O₄  is formed at the same time.

3Fe + 4H2O                Fe3O4  +  4H2

The hydrogen gas produced is collected over water. The black residue left behind in the combustion tube is the triirontetraoxide.

INDUSTRIAL PREPARATION OF HYDROGEN

1.       FROM WATER GAS (Bosch Process): on a large scale, hydrogen gas is prepared from water gas. When steam is passed over red-hot cake at about 1100^oC, a mixture of carbon(ii) oxide and hydrogen known as water gas is produced. Excess steam is then mixed with the water gas and pass over a suitable catalyst iron(iii)oxide or chromium(iii)oxide at 450^oC. The carbon(ii)oxide in the water gas is converted to carbon(iv)oxide with more hydrogen.

C + H2O                       CO + H2    (endothermic reaction)

CO +H2 + H2O             CO2 + 2H2 (exothermic reaction)

2.       FROM NATURAL GAS: Methane, ethane, propane, butane and hydrogen gas is prepared from any of the natural gases by heating at 1000^oC, the hydrocarbon in a process called thermal cracking to obtain hydrogen gas and carbon. e.g CH₄                   C + 2H₂

3.       FROM PETROLEUM (steam reforming of natural gas): Hydrocarbon are treated with steam in the presence of a suitable catalyst e.g Nickel catalyst and heated to a temperature of 800oC and 30 atmosphere to obtain carbon(iv) oxide and hydrogen. The carbon (iv) oxide produce is absorbed in alkaline solution living a pure hydrogen gas.

C3H8 + 6H2O            10H2 + 3CO2

4.       BY ELECTROLYSIS OF DILUTE ACID (brine Process): In this case hydrogen is obtained on a commercial scale from dilute solution of acid alkaline and salt to obtain larger volume of the gas compared with the smaller volume obtained in the laboratory.

PHYSICAL PROPERTIES OF HYDROGEN

1.       Pure hydrogen is a colourless, odourless and tasteless gas.

2.       It is lighter than air (less dense than air)

3.       It is neutral to litmus

4.       It is almost insoluble in water

5.       It has a very low boiling point of -235^oC

6.       It burns in a little spark to form water.

CHEMICAL PROPERTIES OF HYDROGEN GAS

1.       As a reducing agent: Hydrogen gas is reduce heavy metallic oxide to the corresponding metal with itself being oxidized to water. e.g.

CuO  + H2                         Cu  + H2O; PbO   +  H2               Pb + H2O

2.       Burning in air (oxidation reaction) : hydrogen burns in air when ignited to form water because it is inflammable. H₂ + O₂                H₂O

3.       Reaction with metal: Hydrogen react with metal directly to form hydride e.g.

2Li + H2                 2LiH; 2Na  + H2                 2NaH.

4.       Reaction with non-metallic: Hydrogen combine with non-metal to form gaseous product but with chlorine gas the reaction takes place under light and it is explosive e.g.

H2 + Cl2                          2HCl (explosive)

It is explosive because of the formation of chlorine radical.

H2 + S            H2S;       N2  + 3H2            NH3

USES OF HYDROGEN

1.       It is used in filling balloon.

2.       Used as fuel in rocket

3.       Used for synthesis of ammonia in the harber process

4.       In the manufacture of margarine i.e hydrogen of oil in the Sabatier reaction.

5.       Used in oxyhydrogen flame which is used in welding and cutting of metals at a temperature of a 4000^oC.

6.       Hydrogenation of coal in the Bergius process.

7.       Reduction of oxides to metal.

TEST FOR HYDROGEN

Introduce a lightest splint into a gas jar of hydrogen gas, it gives a pop sound, this is the case, the presence of hydrogen.

HYDRIDES

Active metal like NaK and Ca form ionic hydride with hydrogen. The ionic hydride are crystalline solid with high melting and boiling point. They conduct electricity when molten and react readily with water to form hydroxide and liberate hydrogen gas.

Aluminium and Boron form complex covalent hydride which are important reducing agent especially in organic chemistry e.g. sodium tetrahydriodoborate (iii), NaBH4 and Lithium tetrahydridoaluminate (iii), LiAlH4 are common complex hydride.

Non-metallic element like chlorine and nitrogen form simple covalent hydrides. These are volatile compounds that are gaseous at room temperature. The hydride of fluorine HF, and oxygen, H2O are exception because they are liquids due to the presence of hydrogen bonding. The hydrides of the more electronegative element like chlorine and sulphur form acidic solution when dissolve in water.