position of hydrogen in the periodic table · the general chemical formula for covalent hydrides is...

Position of Hydrogen in the Periodic Table

We are all aware of our Modern Periodic table. But it took chemists

years and many attempts to arrive at our current periodic table. And

one main point of contention in the previous attempts was the position

of hydrogen in the periodic table. Let us take a look at the unique

position of hydrogen in the periodic table.

Position of Hydrogen in the Periodic Table

So if you glance at the periodic table, you will see hydrogen is the first

element in the table. It is the smallest element on the table. It has

atomic number one, which means it has only one electron orbiting it

its shell. In fact, Hydrogen has only one shell. It is also the lightest

element on the periodic table.

Now we know that the position of elements on the periodic table

largely depends on their electronic configuration. Hydrogen has the

electronic configuration of 1. It can get rid of one electron to attain

noble gas configuration. This characteristic of hydrogen matches those

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of alkali metals. But they hydrogen atoms can also gain one electron

similar to halogens. Let us see how this plays out.

Similarities to Metals

Hydrogen shares many similarities with alkali metals, i.e. elements in

group I-A. This is one of the factors that dictates the position of

hydrogen in the table. Let us take a look at the similarities

● Electronic Configuration: Like all the elements of the group,

Hydrogen also has one electron in its last shell, the valence

shell. Let us take a look at the composition of valence shells of

a few of these alkali metals.

H (z=1) : K1

Li (z=3) : K2, L1

Na (z=11) : K2, L8, M1

● Noble Gas Configuration: Like alkali metals, it needs to lose

one electron in its valence shell to achieve stable configuration

as that of the next noble gas, which in this case is helium. It

hence forms the H+ ion.

● Good Reducing Agent: Hydrogen is a strong reducing agent

like all the other alkali metals.

Fe2O3 + 4 H2 → 3Fe + 4H2O

B2O3 + 6 K → 2B + 3 K2O

● Forms Halides: Also just like alkali metals, hydrogen combines

with electronegative elements to form halides

2Na + Cl2 → 2NaCl

H2 + Cl2 → 2HCl

Browse more Topics under Hydrogen

● Hydrides

● Dihydrogen

● Preparation and Properties of Dihydrogen

● Water

● Heavy Water and Hydrogen Economy

● Hydrogen Peroxide

Video on Hydrogen

Differences with Metals

● Non-metal: Hydrogen is essentially not a metal like all alkali

metals, but a non-metal

● Loss of Electron: Although it has only one electron in its outer

shell, hydrogen cannot easily lose this electron to gain

electropositivity. All other alkali metals can do this with ease.

● State: At room temperatures where all alkali metals exist is the

solid state, hydrogen is a gas.

● Size of Atom: The H+ ion of hydrogen is much smaller than

ions of alkali metals.

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● Ionization Potential: The ionization potential of hydrogen is

over 300 Kcal per mole, The maximum ionization potential for

metals is 147 Kcal per mole.

Similarities to Halogens

● Noble Gas Configuration: Hydrogen can gain one electron to

complete its valence shells. Halogens also have seven electrons

in their last shell and can gain one electron to gain noble gas

configuration.

● Electronegativity: They also share the same electronegative

nature. Hydrogen also gains one electron (not looses) to

become stable and so do halogens.

H + e‾ → H‾

Cl + e‾ → Cl‾

● Diatomic Molecules: Both hydrogen and halogens form

diatomic molecules. Hydrogen forms H2 and, halogens are Cl2,

F2 etc

● Reaction with Metals: Hydrogen combines with metals to form

metallic hydrides. Similarly, halogens also combine with

metals to form metal halides.

2Na + H2 → 2NaH

Ca + H2 → CaH2

● Covalent Bonding: Halogens and hydrogen both also combine

with non-metals to form molecules with covalent bonding.

Differences with Halogens

● Structure of Atom: Hydrogen has only one electron in its outer

shell. All halogens have seven electrons in their last shell

● Size of Atom: The size of the H- ion is much larger than those

of the ions of Halogens. This is because hydrogen has only one

electron and proton and the pull of the nucleus is less.

● Reaction with Water: Also unlike halogens, the hydrogen ion

H- is unstable in water.

https://www.toppr.com/guides/chemistry/chemical-reactions-and-equations/chemical-reactions-and-equations/

Learn more about the Different type of Hydrides here.

Solved Question for You

Q: Hydrogen is mainly found in the combined state like in water and

not the free state. True or False?

Ans: This statement is True. The earth’s crust contains nearly 1% of

hydrogen by weight. In free state, hydrogen occurs only in traces in

the atmosphere. It is mainly found in combined state and not the free

state. Its main sources are water, acids, organic matter etc.

Hydrides

Hydrogen is a pretty reactive element. It reacts with every element on

the periodic table except non-metals of group VIIIA. These

compounds that hydrogen forms are known as hydrides. So every

element that hydrogen forms, like water, hydrochloric acid etc are all

actually hydrides! Let us learn more.

Hydrides

Very simply put a hydride is the product when hydrogen reacts with

any other element, except of course the noble gases. So hydrides are

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compounds where one atom is hydrogen bonded to another more

electropositive element. A hydride is an anion of hydrogen (H–).

The bonding between hydrogen and other elements is always covalent.

It just varies in its degrees of covalency. The general chemical

formula of a hydride is MHx. Here M is the other element with which

hydrogen makes a binary compound. And the x denotes the number of

hydrogen atoms. There are three possible types of hydrides based on

their structure and chemical properties. These are –


● Position of Hydrogen in the Periodic Table

● Dihydrogen


● Water



Saline Hydrides

These are hydrides that form when hydrogen reacts with s-Block

elements. These are elements found in Group 1 and Group 2. These

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alkali metals and alkaline earth metals are more electropositive than

hydrogen. The only two exceptions are beryllium hydride and

magnesium hydride. Due to the negatively charged ion (H–) they are

also known as Ionic Hydride.

Some examples are Lithium Hydride (LiH), Sodium Hydride (NaH)

etc. Let us look at some of their chemical properties

● These saline hydrides are in solid form. They are actually

crystals and usually have a whitish hue

● Ionic hydride has both high melting point and the boiling point

as all metal compounds usually do

● They have high density since these are stable molecules.

● They conduct electricity in molten state liberating a diatomic

molecule of hydrogen gas at the anode. This is also a way to

confirm the presence of hydrogen in the compound. Let us take

a look at the reaction of Calcium Hydride electrolysis

CaH2 (melt) → Ca2+ + 2H–

At anode: 2H– → H2 + 2e-

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At cathode: Ca2+ + 2e– → Ca

● One very unique chemical property of Saline hydrides is that

they react very vigorously with water and other solvents such

as ethanol and ammonia. In the process, they release pure

hydrogen gas. This is why they must be kept in moisture free

environments. Here is a reaction of Sodium Hydride reacting

with water and ammonia

NaH + H2O → NaOH + H2

NaH + NH3 → NaNH2 + H2

Covalent Hydrides

These are hydrides that form when hydrogen reacts with p-block

elements. These are highly electronegative elements, far more than

hydrogen. Hydrogen atoms form a covalent bond with these p-block

atoms. The general chemical formula for Covalent Hydrides is

XH(8-n), where “n” is the number of electrons the element has in its

outermost shell.

Some examples of covalent or molecular hydrides are HCl, HFl, H2O,

NH3 etc. Let us take a look at some chemical characteristics

● These hydrides consist of individual covalent molecules. These

covalent bonds are weak and have a weak interparticle force

● Molecular hydrides due to their weak covalent bonding have

very low melting and boiling points

● Like most non-metal compounds covalent hydrides are poor

conductors of electricity

● They can be in a liquid state if their properties are adapted due

to hydrogen bonding. Like in the case of water there is a

hydrogen bond, and it slightly alters the chemical properties of

these compounds.

● As we move across the periodic table the hydrogen compounds

with these non-metals become more acidic

● Hydrogen forms the most number of hydrides with Carbon.

These are all covalent hydrides, formed by covalent bonding

between hydrogen and carbon.

H2(g)+Cl2(g)→2HCl(g)

3H2(g)+N2(g)→2NH3(g)

Metallic Hydrides

These are compounds that Hydrogen forms by reacting with transition

elements. These transition elements are:

● Transitional Metals in group 3, 4 and 5 (which are in the

d-block)

● All the f-block elements

● One metal of group 6 – Chromium

There are three groups of the d-block Groups 7,8, and 9 that do not

form compounds with hydrogen. This phenomenon is known as the

Hydride Gap of the d-bock. Let us see some characteristics of metal

hydrides are:

● They are found in a solid state. They have the dark hues of

metals

● Again like all metal compounds, they are good conductors of

electricity since they have a high thermal capacity.

● They can decompose into metal and hydrogen gas. but this

reaction is easily reversible.


Q: Which of the following statements about a Saline Hydride is

correct?

a. They are dehydrating hydrides

b. Form by transition metals

c. Are very poor reducing agents

d. Are stable towards water and Alcohol

Sol: The correct answer is option “A”. Saline hydrides are formed by

alkali metals and alkaline earth metals (not transition metals). They

are good reducing agents and are readily decomposed by water and

alcohol to liberate hydrogen. They also act as powerful dehydrating

agents. Therefore the first statement is the correct answer. The saline

hydride is a good dehydrating hydride.

Dihydrogen

Everyone knows about Hydrogen. It is one of the most abundant

elements on earth. It is in the air we breathe, the water we drink, the

food we eat. But did you know that atomic hydrogen is not found on

Earth? It only exits in its diatomic molecular form as dihydrogen. Let

us study learn more about hydrogen.

Hydrogen

Hydrogen can be said to be the simplest element in existence. The first

element of our periodic table, hydrogen has one electron and one

proton. The interesting part is that it does not have any neutrons inside

its nucleus! This in turn also makes it the lightest element is known to

us.

The discovery of hydrogen was done by Henry Cavendish in 1766. He

discovered the inflammable air which was a product of reacting iron

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with sulphuric acid. The name Hydrogen comes from the Greek word

“Hydro” meaning provider since hydrogen produces water on

combustion.



● Hydrides


● Water



Dihydrogen

A hydrogen atom is extremely reactive in its original form. It has one

valence electron in its only shell, which makes it highly reactive and

unstable. So hydrogen combines with another atom of hydrogen to

give us Dihydrogen. We denote this as H2.

This molecular form of hydrogen is the most common form of

hydrogen on earth. Since the molecule is neutral (has no charge) and

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its orbit is now complete it is a stable gas. Actually, a dihydrogen

molecule (H2) is the smallest molecule on the planet.

Isotopes of Hydrogen

Hydrogen has three naturally occurring isotopes, namely Protium

(1H), Deuterium (2H) and Tritium(3H). Of these, tritium is the only

one not stable, it is in fact radioactive. The difference between the

three is the number of neutrons

● Protium: This is the most prevalent isotope of hydrogen in the

world. Over 99% of all hydrogen is in this form. Its name

comes from the fact that it has one proton in its nucleus. But it

has no neutrons at all. This is why it has a relatively low atomic

mass of 1.0078 u.

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● Deuterium: Having the atomic symbol 2H or sometimes D. It

has one proton and one neutron in its nucleus, giving it the

atomic mass of 2.014. It is also known as heavy hydrogen.

Most of the deuterium found on earth is found in the oceans or

sea water. Deuterium is also a stable isotope, It has the ability

to form a water molecule, and such water is known as heavy

water.

● Tritium: Tritium is 3H. It has one proton and two neutrons in its

structure. It is a radioactive isotope and has a half-life of about

12 years. Tritium also emits low energy β particles.

Solved Example for You

Q: Heavy hydrogen is used for which of the following?

a. Filling up balloons

b. Studying reaction mechanism

c. Oxidizing agent

d. All of the above

Sol: The correct option is “B”. We use heavy hydrogen to study

reaction mechanism as it is very easy to identify deuterium.

Q: The nuclei of tritium (H3) atom would contain neutrons equal to:

a. 1

b. 2

c. 3

d. 4

Sol: The correct option is “B”. The atomic number and mass number

of Tritium are 1 and 3 respectively. So it contains 1 electron, 1 proton,

and 2 neutrons.

Preparation and Properties of Dihydrogen

Although Hydrogen is abundantly present in our atmosphere, it is not

found in the free state. And there are various uses and applications of

Hydrogen for domestic and industrial purposes. So let us learn about

the properties and preparation of Dihydrogen.

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Preparation of Dihydrogen

Laboratory Preparation of Dihydrogen

● Reacting Zinc with Acid

One common way to prepare dihydrogen gas in the lab is by reacting

Zinc with a diluted acid. It results in Zinc forming Zinc Sulphate (or

zinc chloride) and releasing hydrogen in a gas form

Zn + H2SO4 → ZnSO4 + H2

Zn +HCl → ZnCl2 + H2

● Reacting Zinc with Alkaline

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Zinc can also react with an aqueous alkali such as Sodium Hydroxide,

The sodium and zinc will combine and give us dihydrogen as another

product. The reaction is as follows

Zn + 2NaOH → Na2ZnO2 + H2

Commercial Preparation of Dihydrogen

The commercial preparation of hydrogen is a huge industry because

there is a huge demand for hydrogen in the production of fertilizers

and oil-refining process.

● Electrolysis of Water

The method helps in the production of a very pure form of hydrogen

from water. Electricity passes through the water breaking up the water

molecule. The hydrogen collects at the cathode while oxygen collects

at the anode.

● Lane’s Process

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Here Water Gas (a mixture of carbon monoxide and hydrogen) and

steam are alternatively passed over iron at very high temperatures of

up to 800 C. The iron is first oxidized releasing hydrogen and has to

replaced with fresh metal. Then the iron reduces with water gas back

to a metal. The reactions are as follows

3Fe + 4H2O → Fe3O4 + 4H2

Fe3O4 + 4CO → 3Fe + 4CO2

Net reaction is

CO + H2O → CO2 + H2

● From Natural Gas

This remains the cheapest way to produce hydrogen on a commercial

scale. The gas is heated to high temperatures (up to 1100 C) with

steam and a Nickel catalyst. This results in the methane molecules

breaking into Carbon Monoxide and Hydrogen.

https://www.toppr.com/guides/science/heat/heat-and-measuring-temperature/

CH4 + H2O→ CO + 3H2

Properties of Dihydrogen

Physical Properties of Dihydrogen

● At room temperature, this diatomic molecule of hydrogen is

found in a gaseous state

● It is an absolutely colorless, odorless and tasteless gas which

makes it very hard to detect

● It is the lightest element of our periodic table. Has the

molecular weight f 1.00794

● It also has the lowest density of all elements

● It is highly combustible or inflammable. On combustion it

produces water. Also, it burns with a blue flame.

● It liquefies at very low temperatures of nearly -250 C

Chemical Properties of Dihydrogen

● Hydrogen is represented by its symbol ‘H’. However, in nature,

it occurs in its diatomic form as ‘H2‘ known as Dihydrogen

● Hydrogen has no effect on litmus paper, i.e. it is neutral

(neither acidic nor alkaline)

● Dihydrogen is a non-metal

● It is fairly non-reactive in nature. Since dihydrogen is a stable

molecule it is not very reactive

● Dihydrogen has high enthalpy due to its stable H-H bond

● It is an oxidizing agent when reacting with metals. It forms

metal hydrides.

● Reacts with unsaturated Hydrocarbon (for example ethene) to

form saturated hydrocarbons

● Reduces metal oxides of metals less reactive than iron (iron

included)


Question: When hydrogen burns in oxygen, water is formed and when

water is electrolyzed then hydrogen and oxygen are produced. What

type of a reaction takes place (i) in the first case, and (ii) in the second

case

a. In both the cases, i.e. (i) and (ii), a combination reaction takes

place.

b. In the first case, a combination reaction takes place and in the

second case, a decomposition reaction takes place.

c. The first case, a combination reaction takes place and in the

second case, a displacement reaction takes place.

d. Or in the first case, a displacement reaction takes place and in

the second case, a decomposition reaction takes place.

Ans: The correct answer is option “B”. In the first case, the two

elements, hydrogen, and oxygen combine to form a single compound

hence it is a combination reaction. In the second case, a decomposition

reaction takes place as a single compound, i.e. water, splits up to form

two simple substances-hydrogen and oxygen. This decomposition

reaction takes place by the action of electricity.

Water

The absolute necessity of Water in our lives and on our planet needs

no explanation as such. However, we need to understand the structure

and the properties of water to understand its chemical behaviour and

its importance. Let us take a look at these.

Physical Properties of Water

Water has some very important uses in our world. It is the universal

liquid and will find a use in 99% of life’s functions. This unique

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ability of water to be so versatile and ubiquitous is due to some of its

very impressive properties. These properties of water are often

overlooked, but are the main reason for what makes water such a

valuable compound,

● Chemical Formula: Chemical Formula of water as we are all

aware is H2O. The molecule of water has covalent bonding

between Hydrogen and Oxygen atoms. Two hydrogen atoms

form a bond with a single atom of oxygen.

● Appearance: Water is colorless, odorless and tasteless liquid in

its natural state.

● Boiling Point: As we know, water has a boiling point of 100 C.

But this relatively high boiling point of water defies the trend

in the periodic table. In comparison to Hydrogen Telluride and

Hydrogen Sulphide, both of which have low boiling points,

hydrogen (the next hydride) has a very high boiling point, This

is because the hydrogen bonds in the water molecule are very

strong, They require a huge amount of energy to break and start

boiling.

● Freezing Point: The same concept applies to the freezing point

of water as well. The freezing point of water is 0 C. The fact

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that water is very slow to freeze (or even boil for that matter) is

very important for our ecosystem and our survival.

● Density: One unique property of water is that in the sold state,

it is lense dense. Up to 4°C water’s density does increase on

cooling. But after that point water becomes less dense. This is

why ice floats in water,

● Viscosity: Water has high viscosity due to very strong

intermolecular interactions

● Solvency: Water is an excellent solvent. In fact, it is known as a

Universal Solvent. Due to a water molecule’s polarity, it can

dissolve almost any substance.

Chemical Properties of Water

● Amphoteric Nature: One of the unique qualities of Water is its

amphoteric nature. An amphoteric substance is one which can

act as an acid or a base. While Water is neither acidic or basic

it acts as both. This is because of its ability to both donate and

accept protons. For acids stronger than water it acts as a base.

And it acts like a acid to bases stronger than itself. These two

following reactions show this amphoteric nature.

H2O (l) + HCl (aq) ⇌ H3O++ Cl–

H2O (l) + NH3 (aq) ⇌ NH4+ + OH–

● Hydrolysis Reaction: Water has a very high dielectric constant.

This results in it having a strong hydrating tendency. Water has

strong reactions with ions of salts and creates hydrating shells

around them.

SiCl4 + 2H2O → SiO2 + 4HCl

● Redox Reactions: Water is a great source to obtain dihydrogen

since it can be reduced by reacting it with highly

electropositive metals such as Sodium.

H2O + Na → 2NaOH + H2

Structure of Water

Closely observe the structure of a water molecule. You will see one

atom of oxygen and two atoms of hydrogen. Each atom of hydrogen

bonds covalently with the atom of oxygen. So both atoms of hydrogen

share one pair of electrons with the oxygen atom.

Oxygen is a more electronegative element in comparison to water.

This results in an uneven distribution of electron density. This gives

the water molecule an angular bent structure. The H-O-H bond has a

slight angle of about 104.5°. So it can be said that water molecule is

polar. It has a slight negative polarity near the oxygen atom and slight

positive charge near both the hydrogen atoms.

Structure of Ice

The structure of the molecules of water in its frozen form i.e. ice is

very unique. It forms a Lattice Structure that does not generally occur

naturally in any other substance other than ice.

When water reaches its freezing point its atoms rearrange themselves

in a very specific three-dimensional pattern. The oxygen atom is

surrounded by four hydrogen atoms. Two of these form O-H bonds

normally seen in water molecules. The other two form a hydrogen

bond.

This very special hexagonal shape is what gives ice the unique

property of being less dense than water. Since in the structure of ice

there are empty spaces between the hexagonal structure, its density is

less than that of water in its liquid state. This is why ice floats on

water.


Q: Which of the following are properties of water?

I. It has a permanent dipole moment attributed to its molecular

structure.

II. It is a very good conductor of electricity.

III. It has its polar covalent bonds with hydrogen on opposite sides of

the oxygen atom so that the molecule is linear.

a. I only

b. II only

c. Both I and II

d. Both II and III

Ans: The correct answer is “A”. The property I only can be assigned

to water.

I. It has a permanent dipole moment attributed to its molecular

structure. Water is polar in nature due to electronegativity

difference between H atom and O atom. The individual bond

dipoles do not cancel each other as water has a bent shape.

II. It is a non-conductor of electricity as it lacks ions.

III. It has its polar covalent bond between hydrogen and oxygen

which forms V shape for the molecule so that the molecule is

non-linear or bent.

Heavy Water and Hydrogen Economy

The future of our world heavily relies on the fact whether we are able

to find alternative fuels to our fast diminishing conventional resources.

According to one such theory known as Hydrogen Economy, that fuel

could be hydrogen! Let us learn more about hydrogen as an alternative

fuel and heavy water.

Heavy Water

Hydrogen has three naturally occurring isotopes, namely Protium,

Deuterium, and Tritium. While 2 are stable one is radioactive. One of

the stable isotopes is Deuterium. It contains one proton and one

electron in its nucleus (which is know as a deuteron). It is the

Hydrogen 2 isotope and we denote it as 2H or D.

Heavy water is deuterium oxide i.e D2O. Here water contains a

deuterium atom instead of that of hydrogen. In fact, deuterium is often

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referred to as Heavy Hydrogen. And while it looks exactly like normal

water, heavy water has a few different chemical and physical

properties.

The name Heavy water comes from the fact that it is quite simply

heavier than water. Deuterium is almost twice as heavy as protium and

also less stable. D2O has the same qualities as water of being tasteless

and odorless. However, its boiling point is slightly heavier than that of

ordinary water.

Is Heavy Water Safe to Consume?

Heavy water is not radioactive since deuterium is not a radioactive

isotope. It is not toxic either. However, this does not mean D2O is safe

to consume. If you consume it in small quantities it will not affect you

and you may not even realize that you have consumed it.

However, in large quantities, D2O can be quite harmful and possibly

lethal. Since it is much heavier than normal water your cells will not

react with it normally. The first signs after a large consumption of

heavy water would probably be the dizziness and lack of balance. This

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could be followed by many medical complications. If 20% of the

water in your body was heavy water, it will be harmful to your health.

Uses of Heavy Water

● Heavy Water is used in nuclear power plants. In some plants,

we use it to cool down fuel rods, but this is now an outdated

method. We also use Heavy water as a neutron moderator as it

does not absorb neutrons easily and slows down neurons.

● It also finds a use in the production of tritium

● And also in organic chemistry to produce Deuterium

● Also finds some applications in the fields of biology and

medicine

Hydrogen Economy

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It is no secret that the world currently is facing an energy crisis.

Non-renewable energy sources such as oil, coal etc are depleting

globally. And renewable energy sources are still developing and not

able to meet our energy demands fast enough. So scientists from

around the world have come up with a vision, to use hydrogen as an

energy fuel! This concept is known as Hydrogen Economy.

The term was coined by John Bockrics who worked for General

Motors in 1970. It was then understood to be a revolutionary solution

to all our energy troubles. Since then a lot of development has taken

place in this field. Technological advancements in the last few decades

have made this vision very close to possible.

The most primary way to implement hydrogen economy is via

transport vehicles. This will be our first step into the future where

hydrogen can be used as an efficient energy source. Fuel cell electric

vehicles are the main focus of hydrogen economy currently. Now let

us look at a few advantages of hydrogen as a fuel over its conventional

counterparts.

● First and foremost advantage is that it is a non-polluting energy

source. The only by-product is Water.

● Hydrogen, in fact, has a higher efficiency than gasoline, oil,

petrol etc

● They operate silently, unlike those engines which work on

internal combustion

● Production of hydrogen energy can happen anywhere with

water and access to electricity. This helps with grid distribution

● They also have a low heat transmission making them an ideal

fuel source


Q: In a hydrogen economy, the energy required for automobiles or

electrical energy is obtained from the reaction between _____ and

oxygen.

a. hydrogen

b. oxygen

c. silver

d. none of the above

Sol: The correct answer is option “A”. In a hydrogen economy, the

energy required for automobiles or electrical energy is obtained from

the reaction between hydrogen and oxygen. This eliminates the use of

carbon-based fossil fuels. It reduces the emission of carbon dioxide

gas. Hydrogen acts as energy carrier as a substitute for petroleum

reserves that are in short supply.

Q: In nuclear reactors, heavy water is used as a:

a. Fuel

b. Coolant

c. Moderator

d. All of the above

Sol: The correct option is “C”. With the discovery of nuclear fission in

late 1938 and the need for a neutron moderator that captured few

neutrons, heavy water became a component of early nuclear energy

research. Since then, heavy water has been an essential component in

some types of reactors, both those that generate power and those

designed to produce isotopes for nuclear weapons. Most modern

reactors use enriched uranium with normal light water (H2O) as the

moderator.

Hydrogen Peroxide

As you know that hydrogen does not exist in the free state in our

environment. Instead due to its unique bonding nature, it forms

various compounds. One such important compound is Hydrogen

Peroxide. It has various applications and uses. Let us study.

Hydrogen Peroxide

A peroxide is any compound which has two oxygen atoms bonded

together. The O-O group is the peroxide group of the compound. And

Hydrogen Peroxide is the simplest peroxide. The chemical formula for

hydrogen peroxide is H2O2. It is a water molecule with one extra

atom of oxygen, It has various uses ranging from disinfectant to

propellant for rockets. Let us learn more about it

Browse more Topics Under Hydrogen


● Hydrides

● Dihydrogen


● Water


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Properties of Hydrogen Peroxide

● Hydrogen Peroxide is an almost clear, pale blue liquid in its

pure state

● It has an odour similar to that of nitric acid

● Hydrogen peroxide has a viscosity higher than water. It is

about 40% denser than water

● The pure concentrated form boils around 150°C. However, at

this temperature, it undergoes thermal decomposition and has

an explosive reaction. This is why hydrogen peroxide is

generally stored in an aqueous form. Also to avoid it reacting

with the light we store it in dark color containers. It must be

remembered that we have no practical knowledge of hydrogen

peroxide’s boiling point. It is only a theoretical assumption.

This is because it explodes before reaching its boiling point.

● It is miscible in water, i.e. it forms a homogeneous solution

when mixed with water

● It is acidic in nature.

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● H2O2 is a very strong oxidizing agent. It gives up one oxygen

atom and forms water as a byproduct

PbS + 4H2O2 → PbSO4 + 4H2O (in acidic medium)

2Fe + H2O2 → 2Fe + 2OH (in basic medium)

● It can also act as a reducing agent if one of the reactants is a

stronger oxidizing agent than hydrogen peroxide itself.

HOCl + H2O2 → H3O + Cl + O2 (in acidic medium)

I2 + H2O2 + 2OH → 2I + 2H2O + O2

Volume Strength of Hydrogen Peroxide

Preparation of Hydrogen Peroxide

1] From Barium Peroxide

This is a method for laboratory preparation of Hydrogen Peroxide.

Hydrated Barium Peroxide must be used, anhydrous barium peroxide

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will form a protective layer and not react with the sulphuric acid. The

resulting H2O2 is a 5% concentrate solution.

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

2] From Sodium Peroxide

Here too we will use a dilute solution of Sulphuric Acid and gradually

dissolve sodium peroxide in it. As you know H2O2 is explosive at high

temperatures, so we use cold sulphuric acid. On reacting these two we

get crystals of Sodium Sulphide and a 30% solution of hydrogen

peroxide. We can perform vacuum distillation on the said solution to

get pure hydrogen peroxide

Na2O2 + H2SO4 → Na2SO4 + H2O2 (30%)

Learn more about the Preparation of Dihydrogen here.

Structure of the Molecule

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If you look at the dot diagram of H2O2 you will see the O-O bond.

And you will see that both oxygen atoms have two pairs of unbound

electrons each. This brings into effect the valence shell electron

repulsion theory.

The hydrogen atoms will repel the unbonded electrons of oxygen. This

gives a bent molecular shape. And it has a bod angel of 109.5° In its

crystal form (solid state) this angel reduces due to hydrogen bonding

within the molecules being formed.

Have a look at the unique position of Hydrogen in the Periodic Table.

Uses of Hydrogen Peroxide

Antiseptic

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Hydrogen Peroxide has been used as an antiseptic for minor cuts,

bruises, laceration etc for years. It actually foams when it comes in

contact with blood. This helps in bringing up all the germs from

within the cut. The oxidizing property of hydrogen peroxide kills the

germs and also inhibits the growth of microorganisms such as

bacteria, virus etc. This prevents the spread of any infection.

Concentrate H2O2 is harmful and dangerous so we use only a 3%

dilute solution. Also, H2O2 is normally kept in dark brown bottles to

avoid any decomposition in the light. Since it is explosive at higher

temperatures, one must be careful in storing H2O2.

Bleaching

Hydrogen peroxide is extensively used for bleaching purposes. This

also comes down to the oxidizing abilities of H2O2 . When a large

organic molecule has a double bond this bond absorbs light.

During an oxidizing reaction when this bond breaks it destroys

pigments and removes colour. Hydrogen peroxide is a good bleaching

agent for hair since its byproducts (water and oxygen) are not harmful.

H2O2 is also an excellent bleaching agent for wood, tarnish and paper

and textiles.

Pollution Control Agent

Perhaps the most significant use of Hydrogen Peroxide is in

environmental protection. It finds a use in air pollution control where

it is used to treat effluents from industrial waste. It is also used in the

restoration of aerobic conditions to sewage and industrial waste

materials.

Read about Hydrides: Compounds that Hydrogen forms.

Solved Questions for You

Q: Last molecule of hydrogen is evolved from hydrogen peroxide by

which of the following?

a. Crystallization

b. Evaporation

c. Oxidation

d. Distillation

Sol: The correct answer is “A”. The last molecule of water separates

from H2O by crystallization. 90% solution of hydrogen peroxide is

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concentrated by cooling with solid carbon dioxide and ether bath. The

crystals of hydrogen peroxide are separated, melted and refrozen.

position of hydrogen in the periodic table · the general chemical formula for covalent hydrides is...

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