HYDROGEN

Hydrogen
❑ H2 is the most abundant element in the universe and 3 rd most
abundant element in the surface of globe.

❑ It is the lightest element.

❑ It is a colourless, odourless, tasteless, non toxic, combustible,

nonmetallic and diatomic gas.

❑ H2 was discovered by Henry Cavendish.

❑ The name ‘hydrogen’ was given by Antoine Lavoisier.

❑ H2 shows similarities with alkali metals as well as halogens, so that

the position of hydrogen in the periodic table is anomalous.
 Similarities of hydrogen with alkali metals
1. Both have same valence shell electronic configuration.

2. Both are electropositive in nature.

3. Both shows +1 oxidation state.

4. Both have high affinity for non metals.

5. Both are used as reducing agents.

 Differences between hydrogen with alkali metals
1. H2 is a non metal, while others are metals.

2. Hydrogen is diatomic, while alkali metals are monoatomic.

3. H2 has high IE, while alkali metals have low IE.

4. H2 forms covalent compounds, while they form ionic compounds.

5. H2O is amphoteric, while oxides of alkali metals are basic.

 Similarities of hydrogen with Halogens
1. Both are non metals.

2. Both are diatomic.

3. Both have high IE.

4. Both form covalent compounds.

5. Both shows -1 oxidation state when reacts with metals.

6. Both have 1 electron less than the nearest noble gas.

 Differences between hydrogen with halogens
1. H2 has only one valence electron, while halogens have

7 valence electrons.

2. Hydrogen has less tendency to form H- ion, while halogens

have greater tendency to form halide ion.

3. H2O is amphoteric, while oxides of halogens are acidic.

 Isotopes of hydrogen
▪ H2 has 3 isotopes.

PROTIUM (1H1) DEUTERIUM (1H2) TRITIUM (1H3)

It contains 1 proton, It contains 1proton, It contains 1proton, 1
1 electron but no 1 electron and 1 electron and 2 neutrons
neutron neutron
Ordinary hydrogen Heavy hydrogen Radio active hydrogen
Abundance- 99.98% Abundance- 0.015% Abundance- 10-15 %
 Allotropes of hydrogen
▪ H2 has 2 allotropes.

(a) Ortho H2 -same nuclear spin

(b) Para H2 -opposite nuclear spin

▪ At room temperature, ordinary H2 contains 75% ortho H2 and 25%

para H2 (3 : 1)

▪ As the temperature is lowered, the percentage of ortho hydrogen

in the mixture decreases while that of para hydrogen increases.

▪ At 20K, we get pure para H2.

Preparation of hydrogen
Laboratory preparation of hydrogen

1) By the reaction of Zn with dilute HCl

Zn + 2HCl → ZnCl2 +H2

2) By the reaction of Zn with aqueous alkali

Zn + 2NaOH → Na2ZnO2 + H2
 Commercial preparation of hydrogen.

(1) Electrolysis of acidified water using Pt electrodes.

2H2O→ 2H2 +O2

(2) High purity (>99.95%) H2 is obtained by the electrolysis of

warm aqueous Ba(OH)2 solution between Ni electrodes.

(3) By the reaction of steam on hydrocarbons or coke at high

temperature in presence of catalyst (Ni).
1273 K
CH4 + H2O(g) → CO + 3H2
❑ The mixture of CO & H2 is called water gas or synthesis gas or syn gas.

❑ The water gas can be prepared from coal, and this reaction is called

‘coal gasification reaction”

1273 K
❑ C + H2O(g) → CO + H2

❑ The production of H2 can be increased by reacting CO of syn gas

with steam in the presence of iron chromate as the catalyst.

Iron chromate
❑ CO + 3H2 + H2O(g) → CO2 + 4H2

❑ [CO2 can be removed by scrubbing with sodium arsenite solution]

❑ This reaction is called water gas shift reaction

❖ Presently ~77% of the industrial hydrogen is prepared from

petrochemicals, 18% from coal, 4% from electrolysis of

aqueous solutions and 1% from other sources.

 Chemical properties of hydrogen

▪ Hydrogen is chemically inert at room temperature due to high

bond strength of H-H bond.

▪ The H-H bond dissociation enthalpy is the highest for a single

bond between two atoms of any element.

▪ 1) Reaction with oxygen:

2H2 +O2 → 2H2O ΔH=-285.9 kJ/mol

2) Reaction with nitrogen:

3H2 +N2 → 2NH3 ΔH=-92.6 kJ/mol

3) Reaction with halogens

H2 +F2 → 2HF

H2 +Cl2 → 2HCl

H2 +Br2 → 2HBr

H2 +I2 → 2HI
4) Hydrogenation of vegetable oil

Vegetable oil + H2 → Vegetable ghee

5) Hydroformylation reaction (OXO PROCESS)

H2
• RCH=CH2+CO+H2 → RCH2CH2CHO → RCH2CH2CH2OH
 Hydrides
▪ They are of 3 types.
(1) Ionic or saline hydrides
▪ They are hydrides of s-block elements.
▪ They are formed by ionic bonding.
▪ They are crystalline , non volatile, and non conducting in solid
state, but they conduct in molten state or in aqueous state.
▪ Example: NaH, KH, CaH2
▪ LiH, BeH2, and MgH2 contains significant covalent character.
▪ BeH2 & MgH2 are polymeric in nature.
(2) Covalent or molecular hydrides

▪ They are hydrides of p-block elements.

▪ They are formed by covalent bonding.
▪ They are of 3 types.

(a) Electron deficient hydrides

th
They are hydrides of 13 group elements.

Here, the central atom contains less than 8 electrons.

Eg: B2H6, AlH3
(b) Electron precise hydrides
th
▪ They are hydrides of 14 group elements.
▪ Here, the central atom contains 8 electrons.
▪ Eg: CH4, SiH4

(c) Electron rich hydrides

th th th
▪ They are hydrides of 15 ,16 , and 17 group elements.
▪ Here, the central atom lone pair of electrons.
▪ Eg: NH3, H2O, HF etc
(3) Metallic or Non stoichiometric or Interstitial hydrides

▪ They are hydrides of d and f block elements

▪ Most of them are non stoichiometric in nature.

▪ Eg: CrH, LaH2.87, YbH2.55. TiH1.5-1.8, VH0.56, NiH0.6-0.7, PdH0.6-0.8 etc

▪ However the metals of group 7, 8 and 9 7 8 9

do not form hydrides. So that this Mn Fe Co
region of the periodic table is called Tc Ru Rh
hydride gap. Re Os Ir
 Water (H2O)
▪ IUPAC name---Oxidane
▪ Molecular mass---18
▪ Freezing point---273 K
▪ Boiling point---373 K
▪ Density---1 g/cm
▪ Dipole moment---1.85 D
▪ Shape---Bent or V-shape
0
▪ Bond angle---104.5
▪ Water is considered as a universal solvent.

▪ Water has very strong hydrating tendency, due to high

dielectric constant value.

▪ The crystalline form of water is ice.

▪ Ice has an open cage like structure.

▪ In ice, one water molecule can able to form 4 hydrogen

bonds.

▪ Water behaves as an acid as well as a base, ie water has an

amphoteric character..
Hydrate formation

From aqueous solution, many salts can be crystallized as

hydrated salts. Such an association of water is of different
types. They are

(a) Coordinated hydrates

Here water molecules are attached by coordinate bond.

Eg: CrCl3.6H2O OR [Cr(H2O)6]Cl3

(b) Interstitial hydrates

Here water molecules occupies in the interstitial positions.

Eg: BaCl2.2H2O.

(c) Hydrogen bonded hydrates.

Here some water molecules are attached by H2 bond.

Eg: CuSO4.5H2O OR [Cu(H2O)4]SO4.H2O

Here, four H2O molecules are attached by coordinate bond

and one water molecule is attached by H2 bond.
 Heavy water (D2O)
• D2O is mainly prepared from ordinary water by exhaustive electrolysis.

• D2O is mainly used as a moderator in nuclear reactor and in exchange

reactions for the study of reaction mechanisms.

• D2O is used for the preparation of large number of deuterium compounds.

Examples:

CaC2+2D2O→C2D2+Ca(OD)2

SO3+D2O → D2SO4

Al4C3+12D2O → 3CD4+4Al(OD)3
 Soft water and Hard water
▪ The water which form lather readily with soap solutions are
called soft water.

▪ Example- Rain water, distilled water.

▪ The water which cannot form lather readily with soap

solution are called hard water.

▪ Example- Sea water, tap water, lake water etc.

 Types of hardness

▪ There are 2 types of hardness

1). Temporary hardness:

It is due to the presence of soluble bicarbonates of Ca and Mg.

2). Permanent hardness:

It is due to the presence of soluble chlorides and sulphates of

Ca and Mg.
 Methods to remove temporary hardness of water
(i) By boiling

On boiling the hard water, the following reactions taking place.

Eg : Ca(HCO3)2 → CaCO3 ↓+ H2O + CO2↑

Mg(HCO3)2 → Mg(OH)2 ↓+ 2 CO2 ↑

(ii) By Clark’s Process

Here the hard water is treated with calculated quantity of slaked lime.
Ca(HCO3)2 + Ca(OH)2 → 2CaCO3 ↓+ 2H2O

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

Methods to remove permanent hardness of water
(i) Washing Soda Process

Eg : MSO4 + Na2CO3 → MCO3 ↓+ Na2SO4

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

(ii) Calgon’s method

• Calgon is sodium hexa meta phosphate i.e., Na6P6O18 or Na2[Na4(PO3)6].

• When calgon is added to hard water, the Ca2+ and Mg2+ ions present in
it combine with calgon to form soluble complex of Ca and Mg salts.

2CaCl2 + Na2[Na4(PO3)6] → Na2[Ca2(PO3)6] + 4NaCl

(iii) Ion Exchange Method

▪ Also called zeolite or permutit method.

▪ Hydrated, microporous, sodium aluminium silicates are called

zeolites; NaAlSiO4.x H2O.

▪ For the sake of simplicity, it is written as NaZ.

▪ In this process, when zeolite is added to hard water, the

following ion exchange reaction takes place.

▪ Ca2+ + 2NaZ → CaZ2 + 2Na+

(iv) Synthetic resin method

▪ Resins are artificially prepared organic compounds.

▪ There are 2 types of resins.

▪ (a) cation exchange resins

▪ It consists of a long chain hydrocarbon in which one end is attached to

acidic groups such as –COOH or –SO3H .

▪ (b) Anion Exchange Resins

▪ It consists of a long chain hydrocarbon chain in which one end is

attached to basic groups such as OH─ ions.
▪ In this method, the hard water is first passed through
cation exchange resins, so that the cations in hard
water are exchanged by H+ ions.

▪ Then the hard water is passed through anion exchange

resins, so that anions are exchanged with OH- ions.

▪ Finally we get pure water.

 Hydrogen peroxide(H2O2)
❑ H2O2 is a colourless liquid.

❑ Its aqueous solution (30%) is used as an antiseptic under the name

“PERHYDROL”

❑ It is miscible with water in all proportions & forms a hydrateH2O2.H2O.

❑ It slowly decomposes on exposure to light

2H2O2 (l) → 2H2O(l) +O2(g)

❑ So that it is stored in wax lined glass or plastic vessels and kept in dark.

❑ Urea is added as stabilizer to prevent the decomposition of H2O2.

 PREPARATION OF H2O2..
(1) By the reaction of barium peroxide with H2SO4

BaO2+H2SO4 →BaSO4+H2O2+8H2O

(2) By the electrolysis of H2SO4 solution

2H2SO4 electrolysis H2S2O8 +H2

H2S2O8 on complete hydrolysis gives H2O2 .

H2S2O8 + 2H2O→H2O2+ 2H2SO4.

(3) Industrially H2O2 is prepared by the auto oxidation of
2-alkyl anthraquinols.
 Structure of H2O2

▪ H2O2 has a non planar structure.

▪ It has an open book like structure.

 Strength of H2O2
▪ The concentration of H2O2 can be expressed in 2 ways.
1. Percentage strength
▪ 10% H2O2 means 10g H2O2 is present in 100 g solution
▪ 30% H2O2 means 30g H2O2 is present in 100 g solution
2. Volume strength
▪ 10 Volume H2O2 solution means, 1 volume H2O2 can produce 10
volume O2 .
▪ 30 Volume H2O2 solution means, 1 volume H2O2 can produce 30
volume O2.
• Relation between volume strength & % strength

Volume strength=10/3×% strength

• Relation between volume strength & molarity

Volume strength=11.2×molarity

• Relation between volume strength & normlity

Volume strength=5.6 × normality

 Chemical properties of H2O2
▪ H2O2 acts as an oxidising as well as reducing agent in both
acidic and basic medium.

2H2O2 (l)→ 2H2O(l) +O2(g)

▪ Oxidising action in acidic medium

H2O2 oxidises (a) Fe2+ to Fe3+ (b)PbS to PbSO4

▪ Oxidising action in basic medium

H2O2 oxidises (a) Fe2+ to Fe3+ (b)Mn2+ to Mn4+

Reducing action in acidic medium

H2O2 reduces

(a) MnO4- to Mn2+ & (b)HOCl to Cl-

Reducing action in basic medium

H2O2 reduces

(a)I2 to I- & (b) MnO4- to MnO2

1. What is the volume of “ 20 volume H2O2” required to get 5000 cm3
of oxygen at STP?

(1) 250 cm3

(2) 50 cm3

(3) 100 cm3

(4) 125 cm3

2. A dilute solution of H2O2 is labelled as 20 volume. Its percentage
strength is approximately

(1) 60 %

(2) 30 %

(3) 6 %

(4) 3 %
3. The strength (in g/L) of 10 volume solution of H2O2 is

(1) 0.3036 g/L

(2) 3.036 g/L

(3) 30.36 g/L

(4) 303.6 g/L

4. 250 mL of a sample of H2O2 on complete decomposition yield

3 L of O2 gas at STP. The volume strength of solution is about

(1) 12

(2) 20

(3) 1.07

(4) 4
 Hydrogen economy
❖ The term hydrogen economy refers to the transportation and
storage of energy in the form of liquid or gaseous hydrogen