o level chemistry notes on hydrogen,with questions and answers

Introduction to Hydrogen

 

Hydrogen is the first element of the modern periodic table and lightest among all the elements found in nature. It exists in diatomic form. Hydrogen was discovered by Henry Cavendish in 1776 A.D. The name hydrogen was given by Antoine Lavosier in 1817. Hydrogen is found in free state in very trace quantity. However, it is ninth most abundant element in earth. Hydrogen is found in the core of the sun. Hydrogen is the main cause for the nuclear fusion at the core of the sun.

              Hydrogen

Hydrogen is derived from the Greek word “hygrogene” in which hydro means water and gene means producer. Hydrogen produces water when burnt with oxygen.

Name: Hydrogen

Also called: Inflammable gas

Symbol: H

Atomic number: 1

Atomic Mass: 1.008

Molecular Formula: H₂

Electronic configuration: 1s²

Boiling Point: 20.4 Kelvin

Melting Point: 13.8 Kelvin

Isotopes: Protium, Deuterium, Tritium

Hydrogen is the first element in modern periodic table. Since hydrogen has only one electron in its valance shell, it has equal potential to gain or lose electron for attaining the stable configuration state. It has unique physical structure and shows anomalous behaviour.
Hydrogen either can gain electron to become and hydride ion or lose electron to become an hydrogen ion as shown below:

H–e−⟶H+Hydrogen ion

H+e−⟶H−Hydride ion

As hydrogen resembles to alkali as well as halogen in the periodic table, the position of hydrogen is still controversial in Mendeleev’s as well as modern periodic table.
Some of the similarities with alkalis are:
1. Electronic configuration: Both hydrogen and alkali metals have one electron in valence shell. So they have same outermost electronic configuration.
2. Non Metallic Character: Like alkali metals, hydrogen loses its one electron to form hydrogen ion.

H⟶H++e−

Li⟶Li++e−

3. Oxidation state and valency: Both hydrogen and alkali metals have oxidation state +1 and valency equals to 1.
4. Electrolysis: Hydrogen and alkali metals are liberated at cathode when their compounds are electrolyzed.
5. Affinity for non metals: Like alkali metals, hydrogen combines with non metals like halogen, oxygen and sulphur to form their respective halides, oxides, sulphide etc.
6. Reducing nature: Alkali metals and halogen are strong reducing agent.

CuO+H2−→ΔCu+H2O

CuO+Na−→ΔCu+NaO

Similarities with halogens:
1. Electronic configuration: Both hydrogen and halogens have one electron less to acquire stable configuration state. So, both of them are one electron deficient elements.
2. Non metallic characters: Like halogens, hydrogen is also shows non metallic characters.

3. Atomicity: Like halogens, hydrogen exists in diatomic state.
4. Physical state: Like halogen, hydrogen is a gas.
5. Ionization energy: The ionization energy of hydrogen is similar to that of halogens.
6. Mutual replacement: Hydrogen replaces hydrogen from its covalent compounds and vice versa .This means hydrogen can be used instead of halogen and halogen can be used instead of hydrogen in some reactions.
7. Electrolysis: In the electrolysis of metal halides like NaCl, HBr etc. halogen and hydrogen both are liberated at anode.
8. Oxidation state: Both halogens and hydrogen have oxidation state -1


Preparation of hydrogen:
The various methods for the preparation of hydrogen are given below:
1. From water: Alkali and certain alkali metals when reacted with cold water liberates hydrogen gas.

2Na+H2O−→−−−−−Cold waterNaOH+H2↑

2K+H2O−→−−−−−Cold waterKOH+H2↑

Things to know: The reaction of alkali metal is very vigorous and exothermic so that heat is evolved and may catches fire easily. So, amalgams are used in order to slow down the reaction.
2. From hot water: Less reactive metals like zinc, magnesium, aluminum gets decomposed in boiling water and liberates hydrogen.

Zn+H2O−→−−−−Hot waterZnO+H2↑

Mg+H2O−→−−−−Hot waterMgO+H2↑

2Al+3H2O−→−−−−Hot waterAl2O3+3H2↑

3. From steam: Some metals like iron, tin, nickel when passed with steam decomposes to give hydrogen gas. This reaction is reversible.

2Cu+H2OSteam⟶Fe2O3Ferrosoferic oxide Oxide+H2↑

4. From metallic (ionic) hydride: Hydrides of alkali and alkaline earth metal reacts with water to give hydrogen.

LiH+H2O⟶LiOH+H2↑

CaH2+2H2O⟶Ca(OH)2+2H2↑

5. By the electrolysis of water: When water is electrolyzed in presence of some acid or base, hydrogen gas is evolved.

2H2O−→−−−−−ElectrolysisH2↑+O2↑

Note: When water is electrolyzed without the presence of acid or base. There will be no liberation of hydrogen gas. This is because pure water is a bad conductor of heat and electricity. And when acid and base is added the degree of ionization of water increases thereby decomposes to hydrogen and oxygen.

6. From acids: Acids when treated with metals that lie above the electrochemical series can displace hydrogen from their respective mineral acids.

Fe+2HCl⟶FeCl2+H2↑

Zn+H2SO4⟶ZnSO4+H2↑

Mg+2HCl⟶MgCl2+H2↑

Physical Properties, Chemical Properties and Uses of Hydrogen

Physical properties of hydrogen:
– Hydrogen is a colorless, odourless and tasteless gas which is sparingly soluble in water.
– Hydrogen is the lightest gas.
– Density: 0.8987 gram per litre
– Boiling point: -253^oC
– Melting point: -259^oC
– It can be liquefied and solidified at low temperature and high pressure.
Chemical properties:
1. Action with indicators: It does not show any reaction with litmus and hence is neutral towards litmus paper andother indicators.
2. Combustibility: Hydrogen is a inflammable and combustible gas. It is non supporter of combustion.
It gives pale blue flame in air or oxygen to form water. So, hydrogen is also called water gas.

2H2+O2⟶H2O

3. Dissociation: Hydrogen is quiet stable to its high bond energy. It dissociates into atomic hydrogen atoms when heated above1730 ^o.

ZnSO4−→−−−−>1730∘C2ZnO+SO2+O2

4. Reaction with halogen: Hydrogen does not react with halogens at ordinary temperature. But at high temperature in the presence of catalyst, hydrogen react to give their respective halides.

H2+F2⟶2HF

H2+Cl2⟶2HCl

H2+Br2⟶2HBr

H2+I2⟶2Hl

Atomic, Molecular and Nascent Hydrogen

There are the various forms of hydrogen. Different hydrogens have different physical and chemical properties.
1. Molecular hydrogen: It is the most stable form of hydrogen. Hydrogen found in free state is always molecular hydrogen. When atomic hydrogen is kept free, it immediately revert back to molecular form producing large amount of heat.

H⋅+H⋅⟶2H+104KCal

2. Atomic hydrogen: Atomic hydrogen is formed by dissociation of hydrogen molecule into atoms when subjected to high electric discharge at low pressure. The breaking of molecular hydrogen into atomic hydrogen is endothermic process. It is represented by symbol H and  a dot (⋅) .

H2↽−−⇀2H–104KCal

3. Nascent Hydrogen: Nascent hydrogen is formed during chemical reaction. So, it also called newly born hydrogen. It has very short life span and cannot exist independently. Nascent hydrogen is more reactive than molecular hydrogen but less reactive than atomic hydrogen. It is denoted by [H].
Following reactions support the statement that nascent hydrogen is more reactive than molecular hydrogen.
– Reduction of ferric chloride: When ferric chloride is reacted with ordinary hydrogen, no reaction takes place.

FeCl3+H2⟶Noreaction

Zn+H2SO4⟶ZnSO4+2H

But when zinc is treated with acidified FeCl₃, it is changed into light green color which is ferrous chloride.

H2OYellowish brown+[H]⟶FeCl2Light green+HCl

– Reduction of potassium dichromate: Ordinary hydrogen cannot reduce potassium dichromate but when nascent hydrogen is acidified with K₂Cr₂O₇, it is reduced to chromium sulphate.

K2Cr2O7+H2SO4+H2⟶Noreaction

Zn+H2SO4⟶ZnSO4+2H

K2Cr2O7Yellowish brown+4H2SO4+6[H]⟶K2SO4Cr2(SO4)3Green+7H2O

Differences between Nascent hydrogen and atomic hydrogen:

Atomic Hydrogen	Nascent Hydrogen
Atomic hydrogen is produced by the dissociation of ordinary hydrogen at high pressure and low temperature.	Nascent hydrogen is formed during chemical reaction.
It is strong reducing agent and highly reactive than nascent hydrogen.	Nascent hydrogen is weaker reducing agent and is less reactive.
Atomic hydrogen does not exist in solution since it cannot isolated.	Nascent hydrogen exists in solution as it can be isolated.

Ortho and Para hydrogen

Concept:
When two hydrogen molecules combine to form a diatomic molecule, their electron always spin in opposite direction according to Pauli’s exclusion principle. When one electron spins in clockwise direction, another spins in anticlockwise direction and vice versa. But their nucleus may spin in same direction or different direction. When nuclei of a hydrogen molecule spin in same direction, then the molecular hydrogen is called ortho hydrogen. When the nuclei spin in opposite directions, then the molecular hydrogen is said to be para hydrogen. So, ortho and para hydrogen are the two nuclear spin isomers of molecular hydrogen.

At room temperature, ordinary hydrogen consists of 75% ortho form and 25% para form. One lowering the temperature, ortho form decreases and para form increases. At 25 K, the sample of hydrogen consists of 99% ortho form and 1% para form. The para hydrogen can be converted into ortho hydrogen by the following ways.
– Collision of para hydrogen with atomic hydrogen
– heating para hydrogen to 800^oC or more
– Passing the electric discharge.
– Mixing paramagnetic molecules like O₂, NO₂ etc.
Ortho and para hydrogen have similar chemical properties but differ in physical properties such as boiling point melting point, thermal conductivity, specific heat capacity, internal energy etc.
Isotopes of hydrogen

Ortho hydrogen	Para hydrogen
When nuclei of a hydrogen molecule spin in same direction , then the molecular hydrogen is called ortho hydrogen.	When nuclei of a hydrogen molecule spin in different directions, then the molecular hydrogen is called para hydrogen.
At room temperature, ordinary hydrogen consists of 75% ortho hydrogen.	At room temperature, ordinary hydrogen consists of 25% ortho hydrogen.
Ortho form increases with the increase in temperature.	Ortho form decreases with the decrease in temperature.
It is more stable.	It is less stable.

Isotopes of Hydrogen

Isotopes are the chemical species having same atomic number bur different atomic mass number.
Hydrogen have three isotopes namely protium, deuterium and tritium.

1. Protium: It is the first isotope of hydrogen and has symbol ¹₁H. It has one proton and one electron. It is the most abundant isotopes of hydrogen among all the hydrogen found in nature. It is non reactive and most stable form too.
Uses of protium:

– It is used for synthesis of ammonia in Haber’s process.
– It is used for the manufacture of ethanol, HCl etc.
– It is used in hydrogenation of oil to produce artificial ghee.
– It is used as a reductant in metallurgical process.
– It is used as a fuel in rockets and missiles.
– It is used for filling balloons, study of atmosphere etc.

Note: The uses of ordinary hydrogen and protium are same.

2. Deuterium: It is the second isotope of hydrogen and has symbol ²₁H or D. It has one proton and one electron and one neutron. It is the second most abundant isotopes of hydrogen among all the hydrogen found in nature. It is non reactive and stable form too.
The main application of deuterium is: It is extensively used as moderator in nuclear reactors for the transformation of nuclear reaction.

Note: Since all the isotopes of hydrogen have same electronic configuration, they have similar chemical properties.

3. Tritium: It is the third isotope of hydrogen. It is not found in free state in nature. It is also represented by ³₁H or T. It has one proton, two neutrons and one electron. It is highly radioactive in nature.
Tritium is used in nuclear fusion reaction to liberate large amount of heat in nuclear bombs and weapons.

Pauli’s exclusion Principle

According to this principle, no two electrons in an atom have all the four quantum number same. Hence, in an orbital, more than two electrons are not accommodated. Take an example of first and second electron of helium,  i.e.,
(He)₂ = 1s²
The value of all quantum numbers is given below:

	n	l	m	s
Electron 1	1	0	0	+1/2
Electron 2	1	0	0	-1/2

In helium, two electrons e₁ and e₂ have same set of all quantum numbers except that of spin quantum number. This principle consider as the foundation of quantum mechanics for explaining the stability of an atom and other various phenomena.