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Hydrogen is one of the most abundant elements found on earth and has the simplest atomic structure among all the known elements in nature. Hydrogen plays a significant role in sustaining all life forms. It co-exists with different compounds and enables different properties. The physical & chemical properties of hydrogen make it one of the most important and intriguing elements. The Class 11 Chemistry syllabus has a special chapter on this element, which provides in-depth information about hydrogen, its properties and different compounds. Looking for easy to understand study notes for the class 11 Hydrogen chapter? Here in this blog, we have summarised the chapter on Hydrogen along with the important exam questions you must practice.
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Also Read: Class 11 Environmental Chemistry
Hydrogen And Its Existence In Nature
Hydrogen is the most predominantly found element in the universe and the third most abundant on the globe’s surface, having the simplest atomic structure H2. In the elemental state, it exists as a diatomic molecule, hence H2. According to the Class 11 chapter on Hydrogen, this element exists in nature in many forms like water, the earth’s crust, and even the atmosphere. Moreover, isotopes that are elements with the same number of electrons but a different number of neutrons also exist. The three isotopes are Protium 1H1, Deuterium 1H2 or D, and Tritium 1H3 or T. The isotopes have one, two, and three neutrons.
Properties of Hydrogen
The class 11 Hydrogen chapter deals with elementary topics and does not delve into complex reactions and solutions. Hydrogen is so abundant that researchers have studied its properties for many years to make use of it for a sustainable future. Every element has physical and chemical properties. This particular section of the class 11 Hydrogen chapter elaborates on the physical and chemical properties of hydrogen.
Physical Properties
Here are the physical properties of Hydrogen:
| Property | Hydrogen |
| Relative atomic mass (g/mol) | 1.008 |
| Melting point (K) | 13.96 |
| Boiling point (K) | 20.39 |
| Density (g/L) | 0.09 |
| Covalent radius (pm) | 37 |
| Ionic radius (pm) | 208 |
| Colour | Colourless |
| Odour | Odourless |
| Taste | Tasteless |
Chemical Properties
The chemical properties of hydrogen depend entirely on the enthalpy (energy) of bond dissociation. The breaking of the H-H bond and the formation of new bonds with other atoms depend on the amount of energy required to do so. If the energy required to make a new bond is more, then forming a new molecule is less likely than if the energy required in forming new bonds is less. The prominent chemical properties of this element as detailed in the chapter on Hydrogen in Class 11 are:
| Property | Hydrogen |
| Enthalpy of fusion(KJ/mol) | 0.117 |
| Enthalpy of vaporization (KJ/mol) | 0.904 |
| Bond dissociation enthalpy(KJ/mol) | 435.98 |
| Ionisation enthalpy(KJ/mol) | 1312 |
| Electron gain enthalpy(KJ/mol) | -73 |
| Reaction with halogens | Forms halides |
| Reaction with nitrogen | Forms ammonia |
| Reaction with metals | Forms metal hydrides |
| Reaction with organic compounds | Forms hydrogenated products of commercial importance |
Also Read: Class 11 p Block Elements Notes
Compounds of Hydrogen and their Importance
Hydrogen forms a variety of paramount compounds on reaction with different elements. The true potential of hydrogen has still not been fully harnessed. The world has grown from making ammonia to hydrogen bombs, but still, there’s a lot left to discover. This section of the chapter on Hydrogen in Class 11 discussed the following prime compounds of hydrogen with their contribution to this world.
Water
First and foremost, water is a constituent of all living organisms on earth. The human body has up to 65% water and the plants have water as high as 95%. The importance of water is known to man for a long time as it is a necessity for survival and a solvent of great importance.
Physical Properties of Water
While going through the chapter on Hydrogen in Class 11, you will get to know about the physical properties of water. The properties of water in different states depend upon the extensive hydrogen bonding between the water molecules. To know more and learn about states of matter and how they are arranged to form different molecules, check out our blog on states of matter.
Water is a tasteless and odourless liquid. It can exist as a liquid at room temperature, as ice at low temperatures, and as a gas(water vapours) at high temperatures. Water has a high boiling point & freezing point due to its extensive hydrogen bonding. The energy required to cleave the bonds between hydrogen and oxygen atoms is high hence increasing the boiling point and freezing point. Water has a high specific heat, thermal conductivity, surface tension, dipole moment, and dielectric constant compared to other liquids.
Chemical Properties of Water
- Water can act both as acid as well as a base. This nature is called amphoteric.
- Due to the high dielectric constant, water can dissolve many ionic compounds. It has a strong hydrating tendency due to the large dielectric constant.
- Water can perform many redox reactions and also form hydrated salts.
Also Read: Class 11 Chapter 10 S Block
Hardness of Water
While naturally existing, water dissolves many compounds in it. It changes some properties of water, making it ‘hard’ and ‘soft.’ The dissolution of calcium and magnesium salts in water leads to hydrogen carbonate, chloride, and sulfate, which ultimately makes the water ‘Hard.’ Hard water doesn’t give lather with soap. Water-free from calcium and magnesium salt is called ‘Soft’ water, which gives lather easily with soaps. As per the Class 11 chapter on Hydrogen, the hardness of the water could be completely removed easily by various techniques, either temporarily or permanently.
Also Read: Class 11 Redox Reactions
Hydrides
Hydrogen combines with all elements, except the noble gases under different conditions to form hydrides. If M is the element, then its hydride can be represented as MxHy(Eg. MgH2, B2H5). Hydrides are crucial compounds in the chemical industry. These are of the following three types:
- Ionic or saline hydrides
- Covalent or molecular hydrides
- Metallic or non-stoichiometric hydrides
Hydrides have different properties like crystalline structure, non-volatility, electron deficiency, electron efficiency, etc. The relevance of hydrides is not elaborated in detail in the class 11 Hydrogen chapter. However, in the field of industrial chemistry and Chemical engineering, it plays a significant role.
Also Read: Class 11 Hydrocarbons
Hydrogen Peroxide/ Peroxide Hydrogen (H2O2)
Another prime compound of Hydrogen as mentioned in this chapter in Class 11 is Hydrogen peroxide which is also named chemically as dihydrogen dioxide and is a powerful reducing agent. Since the advent of modern times, it is used to control pollution from the industries. Peroxide hydrogen is a synthetically produced compound that involves high thermal reactions.
Physical Properties of Peroxide Hydrogen
The physical properties of Hydrogen Peroxide as mentioned in the class 11 Hydrogen chapter are:
| Property | H2O2 |
| Colour | Colourless |
| Odour | Odourless |
| Melting Point (K) | 272.4 |
| Boiling Point (K) | 423 |
| Density (g/cm3) | 1.44 |
| Vapour pressure (mmHg) | 1.9 |
| Dielectric constant | 70.7 |
Chemical Properties Of Hydrogen Peroxide
Hydrogen peroxide acts as oxidizing and reducing agents in both acidic and alkaline mediums. These properties make it viable for using it as bleach and mild disinfectant. Moreover, under certain temperature conditions, peroxide hydrogen can manufacture chemicals like sodium perborate and percarbonate, which are used in making detergents.
Heavy Water
Studying the Class 11 chapter on Hydrogen, you will also get to know about Heavy Water or D2O which is widely utilised as a moderator in nuclear reactors as well as to explore reaction mechanisms. It is mainly created with the process of exhaustive electrolysis of water or can be formed as a by-product in fertilizer industries. Many deuterium compounds are also created with the help of Heavy water.
Use of Dihydrogen as a Fuel
The concluding section in the Class 11 chapter on Hydrogen elaborates on the applications of Dihydrogen as a fuel as it releases immense heat on combustion. This energy released comes around three times more than petrol when it comes to mass for mass basis. It is also observed that pollutants released with the combustion of petrol are less than petrol thus the only ones left will be oxides of dinitrogen and they can be decreased with the addition of a little water into the cylinder. This will lessen the temperature which will lead to the absence of reaction between dinitrogen and dioxygen. But, we need to consider the mass of the dihydrogen container in such a case. Dihydrogen gas can also be converted to liquid form through cooling it to 2K but this would need costly insulated tanks which have posed as a limitation for the researcher to resort to alternative techniques for utilising dihydrogen. Thus, the Hydrogen Economy can work as a great alternative in such a case as it transports and stores energy in a liquid or gaseous form of dihydrogen.
The first time hydrogen economy was implemented in India was through a pilot project which used dihydrogen as fuel was introduced in October 2005 for automobiles. In the beginning, 5% dihydrogen was also combined with CNG in four-wheeler vehicles and then this percentage was boosted to an optimum level. It is also worth mentioning that dihydrogen is also widely used in fuel cells for electricity generation and researchers constantly working on bringing better viable and safe energy sources of Dihydrogen.
Production of Dihydrogen
Dihydrogen is formed through coal and the process is called coal gasification. It is also known as syngas. The mixture of carbon monoxide and hydrogen that is used in the formation of dihydrogen is known as water gas. The water gas is also known as syngas or synthesis gas. To increase the production of dihydrogen it is meant to react with carbon monoxide of singers mixture along with the scheme in presence of iron chromate as a catalyst. This reaction is known as the water-gas shift reaction. The H-H bond has a dissociation enthalpy of dihydrogen which is the highest for a single bond between two connected atoms of an element. This property is used in the atomic hydrogen torch that generates a temperature of around 4000K that is ideal for the welding of high melting metals.
Isotopes of Hydrogen
Hydrogen mainly has three isotopes such as protium, deuterium and tritium. Out of all three, tritium is radioactive in nature. Even after having a slight resemblance with both halogens and alkali metals, it has a separate space in the periodic table because of its distinct features and properties. Let us now study the properties of hydrogen with respect to its resemblance with halogens and alkali metals-
Alkali Metals
As hydrogen is a tasteless, colourless and odourless gas, it is diatomic in nature. Lets us study the properties of hydrogen with respect to alkali metals-
| Resemblance with Alkali Metals | Difference from Alkali Metals |
| It shows an oxidation state of +1 | Ionization enthalpy of hydrogen is very high whereas that of alkali metals is less |
| Shoes electropositive characters such as H+, Na+ K+, etc | Alkali metals are monoatomic whereas hydrogen diatomic |
| It reacts with halogens to form halides such as Hcl, NaCl, KCl, etc | Hydrogen is a non-metal whereas alkali metals are the common type of metals |
| It operates binary compounds with electronegative elements like alkali metals | The compounds of hydrogen are covalent in nature. On the other hand, the compounds of alkali metals are ionic. |
| – | Oxides of alkali metals are basic. While the oxides of hydrogen are neutral |
Halogens
Below are the similarities as well as the similarities of hydrogen and its isotopes with respect to hydrogens-
| Similarities with Halogens | Dissimilarities with Halogens |
| In the electronic configuration of hydrogen and halogens, both contained an electron less than the nearest noble gas configuration. | -An unshared pairs of electrons that is absent-Water has less tendency for hydride formation -The oxides of hydrogen are neutral but the oxides of halogens are acidic in nature |
Important Questions
Here are some of the important questions for the chapter on hydrogen:
- How can the hardness of water be removed?
- How does hydrogen peroxide act as both reducing and oxidizing agents?
- What explains the different states in which water exists?
Also Read: Class 11 Thermodynamics
Important Questions
The first element in the periodic table is hydrogen. Hydrogen exhibits dual behaviour because it has only 1 electron on its one ‘S’ shell.(i.e.,) hydrogen resembles both halogens and alkali metals.
Electronic configuration of hydrogen = [1s1]Hydrogens resemblance with alkali metals:
Hydrogen has 1 valence electron on its valency shell-like alkali metals.
[He] 2s 1 – Li
1s1 – H
[Ne] 3s1 – Na
Therefore, to form a uni positive ion, it can lose one of its electrons.
To form halides, oxides and sulphides, it combines with electro –ve elements which are the same as alkali metals.
Hydrogens resemblance with halogens:
Only 1 electron is required to complete their respective octets for both the halogen and hydrogen.
H : 1s 1
F : 1s 2 2s 2 2p 5
Cl : 1s 2 2s 3 2p 6 3s 2 3p 5
It forms several covalent compounds and diatomic molecules like halogens. Even though hydrogen has certain similarities among halogen and alkali metals, it differs from them. Hydrogen won’t possess metallic characteristics; it possesses higher ionization enthalpy and reacts less than halogens.
Due to these reasons, hydrogen cant is replaced with alkali metal of the 1st group or with the halogens of the 2nd group. Therefore, it is best to place hydrogen separate in the periodic table.
There are three isotopes in total.
(i) tritium ^{3}_{1}H13H or T
(ii) protium ^{3}_{1}H13H
(iii) deuterium ^{2}_{1}H12H or D
Mass Ratio:
Tritium : Protium : deuterium = 1 : 2 : 3
The ionization enthalpy of a hydrogen atom is higher. Therefore, it is harder to remove its electron. This results in its tendency to exist in the low monoatomic form. Instead of that, a covalent bond is formed by hydrogen with another hydrogen atom and exists as a diatomic molecule.
The ionization enthalpy of the H–H bond is higher (1312 kJ mol–1 ) which shows that hydrogen has a low tendency to form H+ ions. Its ionization enthalpy value is comparable to that of halogens. Hence, it forms
→ a large number of covalent bonds
→ diatomic molecules (H2)
→ hydrides with element
Hydrogen does not possess metallic characteristics (lustre, ductility, etc.) like metals because ionization enthalpy is very high.
Molecular hydride is classified on the basis of the presence of the bonds and the total number of electrons in their Lewis structures as:
Electron-deficient hydrides
Electron-precise hydrides
Electron-rich hydrides
An electron-deficient hydride has very less electrons, less than that required for representing its conventional Lewis structure.
Justify the position of hydrogen in the periodic table on the basis of its electronic configuration. The first element in the periodic table is hydrogen. Hydrogen exhibits dual behaviour because it has only 1 electron on its one ‘S’ shell.(i.e.,) hydrogen resembles both halogens and alkali metals.
Electronic configuration of hydrogen = [1s1]Hydrogens resemblance with alkali metals:
Hydrogen has 1 valence electron on its valency shell-like alkali metals.
[He] 2s 1 – Li
1s1 – H
[Ne] 3s1 – Na
Therefore, to form a uni positive ion, it can lose one of its electrons.
To form halides, oxides and sulphides, it combines with electro –ve elements which are the same as alkali metals.
Hydrogens resemblance with halogens:
Only 1 electron is required to complete their respective octets for both the halogen and hydrogen.
H : 1s 1
F : 1s 2 2s 2 2p 5
Cl : 1s 2 2s 3 2p 6 3s 2 3p 5
It forms several covalent compounds and diatomic molecules like halogens. Even though hydrogen has certain similarities among halogen and alkali metals, it differs from them. Hydrogen won’t possess metallic characteristics; it possesses higher ionization enthalpy and reacts less than halogens.
Due to these reasons, hydrogen cant is replaced with alkali metal of the 1st group or with the halogens of the 2nd group. Therefore, it is best to place hydrogen separate in the periodic table.
Write the names of isotopes of hydrogen. What is the mass ratio of these isotopes? There are three isotopes in total.
(i) tritium ^{3}_{1}H13H or T
(ii) protium ^{3}_{1}H13H
(iii) deuterium ^{2}_{1}H12H or D
Mass Ratio:
Tritium : Protium : deuterium = 1 : 2 : 3
Why does hydrogen occur in a diatomic form rather than in a monoatomic form under normal conditions? The ionization enthalpy of a hydrogen atom is higher. Therefore, it is harder to remove its electron. This results in its tendency to exist in the low monoatomic form. Instead of that, a covalent bond is formed by hydrogen with another hydrogen atom and exists as a diatomic molecule.
Discuss the consequences of high enthalpy of H–H bond in terms of chemical reactivity of dihydrogen The ionization enthalpy of the H–H bond is higher (1312 kJ mol–1 ) which shows that hydrogen has a low tendency to form H+ ions. Its ionization enthalpy value is comparable to that of halogens. Hence, it forms
→ a large number of covalent bonds
→ diatomic molecules (H2)
→ hydrides with element
Hydrogen does not possess metallic characteristics (lustre, ductility, etc.) like metals because ionization enthalpy is very high.
What do you understand by (i) electron-rich – compounds of hydrogen, (ii) electron-precise, and (iii) electron-deficient? Provide justification with suitable examples.
Molecular hydride is classified on the basis of the presence of the bonds and the total number of electrons in their Lewis structures as:
Electron-deficient hydrides
Electron-precise hydrides
Electron-rich hydrides
An electron-deficient hydride has very less electrons, less than that required for representing its conventional Lewis structure.
– Diborane (B2 H6)
In B2 H6, there are 6 bonds in all, out of which only 4 bonds are regular 2 centred-2 electron bonds.
The remaining 2 bonds are 3 centred-2 electron bonds i.e., 2 electrons are shared by 3 atoms. Hence, its conventional Lewis structure cannot be drawn.
By conventional Lewis structure, an electron-precise hydride has a sufficient number of electrons to be represented.
e.g. CH4
The Lewis structure can be written as:
4 regular bonds are formed where 2 electrons are shared by 2 atoms. An electron-rich hydride contains excess electrons as lone pairs.
e.g. NH3
There are 3 regular bonds in all with a lone pair of electrons on the nitrogen atom. – Diborane (B2 H6)
In B2 H6, there are 6 bonds in all, out of which only 4 bonds are regular 2 centred-2 electron bonds.
The remaining 2 bonds are 3 centred-2 electron bonds i.e., 2 electrons are shared by 3 atoms. Hence, its conventional Lewis structure cannot be drawn.
By conventional Lewis structure, an electron-precise hydride has a sufficient number of electrons to be represented.
e.g. CH4
The Lewis structure can be written as:
4 regular bonds are formed where 2 electrons are shared by 2 atoms. An electron-rich hydride contains excess electrons as lone pairs.
e.g. NH3
There are 3 regular bonds in all with a lone pair of electrons on the nitrogen atom.
Metallic hydrides are hydrogen deficient. They don’t follow the law of constant composition.
It has been established that in the hydrides of Pd, Ac, Ni, and Ce, hydrogen occupies the interstitial position in lattices which allow further absorption of hydrogen on these metals.
Metals like Pt and Pd have the capacity to accommodate a large volume of hydrogen. Hence, metallic hydrides serve as a source of energy and are used for the storage of hydrogen.
The atomic hydrogen torch is also known as an oxyhydrogen torch. These atoms are produced through dihydrogen dissociation with the help of an electric arc which results in a huge amount of energy.
The energy released = 435.88 kJ mol-1
This energy is used in the generation of 4000 K temperature which is used in cutting and welding of metals.
Therefore, atomic hydrogen torches are used for this purpose i.e., it allows to recombine on the particular surface to be welded for the generation of a particular temperature.
Water is very essential for our life. It consists of many dissolved nutrients that are required for us and also for plants and animals. Demineralised water is free from all soluble minerals and so it cannot be used for drinking purposes.
After adding desired minerals in specific amounts that are required for growth, this water can be made useful.
We hope that this blog helped you understand the major concepts covered in the class 11 chapter on Hydrogen. Confused about finding the right course after 12? Reach out to our experts at Leverage Edu and we will assist you in exploring the best course and university combinations as per your interests and aspirations! Sign up for a free session with us today!
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