CBSE Class 11 Chemistry – Chapter 9 Hydrogen – Study Materials

Subtopics of Class 11 Chemistry Chapter 9 – Hydrogen

  1. Position Of Hydrogen In The Periodic Table
  2. Dihydrogen, H2
    1. Occurrence
    2. Isotopes Of Hydrogen
  3. Preparation Of Dihydrogen, H2
    1. Laboratory Preparation Of Dihydrogen
    2. Commercial Production Of Dihydrogen
  4. Properties Of Dihydrogen
    1. Physical Properties
    2. Chemical Properties
    3. Uses Of Dihydrogen
  5. Hydrides
    1. Ionic Or Saline Hydrides
    2. Covalent Or Molecular Hydride
    3. Metallic Or Non-stoichiometric (Or Interstitial ) Hydrides
  6. Water Ex
    1. Physical Properties Of Water Ex
    2. Structure Of Water
    3. Structure Of Ice
    4. Chemical Properties Of Water
    5. Hard And Soft Water
    6. Temporary Hardness
    7. Permanent Hardness
  7. Hydrogen Peroxide (H2O2)
    1. Preparation
    2. Physical Properties
    3. Structure
    4. Chemical Properties
    5. Storage
    6. Uses
  8. Heavy Water, D2O
  9. Dihydrogen as a Fuel

Hydrogen Class 11 Notes Chemistry Chapter 9

• Electronic Configuration of Hydrogen 1s1
Position of hydrogen in the periodic table: Position of hydrogen in periodic table is not justified because it resembles both alkali metals as well as halogens.
• Resemblance of Hydrogen with Alkali Metals
(i) Electronic Configuration: Hydrogen has one electron in its valence shell like alkali metals.
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(ii) Both hydrogen and alkali metals form unipositive ions.
For example,
Na ———–> Na+ + e
H ———-> H+ + e
(iii) Hydrogen and alkali metals both shows +1 oxidation state.
(iv) Hydrogen as well as other alkali metals acts as reducing agents.
(v) Both have affinity for electronegative element For example, Na2O, NaCl, H20, HCl.
• Resemblance with Hologens
(i) Electronic configuration: Hydrogen and halogen family both require one electron to fulfil the inert gas configuration
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(ii) Ionisation energy of hydrogen is almost similar to halogens.
(iii) Hydrogen as well as halogens are Diatomic in nature.
(iv) Many compounds of hydrogen as well as of halogens are of covalent nature.
For example, CH4, SiH4CCl4, SiCl4
• Occurrence of Hydrogen
Hydrogen is the most abundant element in the universe. It is present in combined state as water, coal, animal and vegetable matter. All organic compounds contain hydrogen as an essential constituent.
• Isotopes of Hydrogen
Hydrogen has three isotopes.
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• Preparation of Dihydrogen, H2
Laboratory Preparation of Dihydrogen
(i) It is prepared by the reaction of granulated zinc with dil HCl.
Zn + 2HCl ——–> ZnCl2 + H2
(ii) It is prepared by the action of zinc with aqueous alkali.
hydrogen-cbse-notes-for-class-11-chemistry-4
• Properties of Dihydrogen
Physical properties
(i) Dihydrogen is a colourless, odourless and tasteless gas.
(ii) It is a combustible gas.
(iii) It is insoluble in water.
(iv) It is lighter than air.
Chemical properties
Reaction with halogens: It reacts with halogens, X2 to give hydrogen halides. HX.
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• Hydrides
The hydrides are classified into three types:
(i) Ionic or saline or salt like hydrides
(ii) Covalent or molecular hydrides (iii) Metallic or non-stoichiometric hydrides.
• Ionic or Saline Hydrides
Hydrides formed between hydrogen and electropositive element of group I and II belonging to s-block. These are known as stoichiometric compounds.
Properties of saline or ionic hydrides:
(i) The hydrides of lighter elements like Li, Be, Mg etc. have significant covalent character.
(ii) Ionic hydrides are crystalline, non-volatile and non-conducting in solid state.
(iii) They conduct electricity in molten state and liberate hydrogen at anode.
• Covalent or Molecular Hydrides
These are binary compounds of hydrogen with non-metals belonging to p-block.
For example, NH3, CH4, H20, HF They are mostly volatile compounds with low boiling points. They are classified as:
(i) Electron-Deficient Molecular Hydride: Molecular hydrides in which central atom does not have octet are called electron deficient hydrides e.g., BH3, MgH2, BeH2.
(ii) Electron precise hydrides: Those hydrides in which the central atom has its octet complete e.g., group 14 hydrides. They are tetrahedral in geometry.
(iii) Electron rich hydrides: Those metal hydrides which contain lone pair of electrons are called electron rich hydrides, e.g., NH3, PH3, H20 and H2S.
NH3 and PH3 has 1 lone pair and H20 and H2S have 2 lone pairs of electrons.
• Metallic or Non-Stoichiometric Hydrides
These hydrides are also known as interstitial hydrides. Transition metals group 3, 4 and 5 form metallic hydrides. In group 6, chromium alone has a tendency to form CrH. Metals of 7, 8 and 9 do not form hydrides. This is called as hydride gap.
Latest study shows that only Ni, Pd, Ce and Ac are interstitial in nature, that means they can occupy hydrogen atom in the interstitial sides. The hydrides are generally non-stoichiometric and their composition varies with temperature and pressure, for example, Ti H1.73, CeH2.7′ , LaH2.8  etc.
These hydrides have metallic lock and their properties are closely related to those of the parent metal. They are strong reducing agents in most of the cases due to the presence of free hydrogen atom in the metal lattice.
• Water
Human body has about 65% and some plants have nearly 95% water.
Physical properties of water:
(i) Freezing point of water is 273.15 K and boiling point 373.15 K.
(ii) Maximum density of water at 4°C is 1 gm cm-3
(iii) It is a colourless and tasteless liquid.
(iv) Due to hydrogen bonding with polar molecules, even covalent compounds like alcohol and carbohydrates dissolve in water.
Structure of Water:
In gas phase, it is a bent molecule with HOH bond angle 104.5° and O—H bond length of 95.7 pm. It is highly polar in nature. Its orbital overlap picture is also shown below.
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Water in Crystalline Form:
Ice is the crystalline form of water. At atmospheric pressure ice crystallise in the hexagonal form. At low temperature it condenses to cubic form. Density of ice is less than that of water. Therefore, ice cubes can float on water.
Structure of ice:
hydrogen-cbse-notes-for-class-11-chemistry-8
Chemical Properties of Water:
(i) Amphoteric nature: It behaves like an amphoteric substance because it can act as an acid as well as base.
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Autoprotolysis of water also accounts for its amphoteric nature according to Bronsted-Lowry concept.
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(ii) Oxidising and Reducing Nature: Water can act as an oxidising as well as reducing agent.
hydrogen-cbse-notes-for-class-11-chemistry-11
(iii) Hydrolysis Reaction: It has a very strong hydrating tendency. It can hydrolyse a large number of compounds such as oxides, halides, carbides etc.
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• Hydrates Formation
From aqueous solutions many salts can be crystallised as hydrated salts. Hydrates are of three types:
(i) Coordinated water
For example: [Ni(H20)6]2+ (N03)2 and [Cr(H20)6]3+ 3CP
(ii) Interstitial water
For example: BaCl2. 2H20
(iii) Hydrogen bonded water
For example: [Cu(H20)4]2+ S042- H20 in CuS04.5H20
• Hard and Soft Water
Hard water: Water which does not produce lather with soap easily is called hard water. Presence of calcium and magnesium salts in the form of hydrogen carbonate, chloride and sulphate in water makes the water hard.
Types of Hardness of Water:
(i) Temporary hardness: It is due to the presence of bicarbonates of calcium and magnesium in water. It is known as temporary because it can be easily removed by simple boiling of hard water.
(ii) Permanent hardness: It is due to the presence of chlorides and sulphates of calcium and magnesium. It cannot be removed on boiling water. Permanent hardness of water can be removed by chemical methods.
Soft water: Water which readily forms lather with soap is called soft water.
For example: rain water, distilled water.
• Hydrogen Peroxide (H202)
Preparation:
hydrogen-cbse-notes-for-class-11-chemistry-13
Uses of H202:
(i) It is used as a mild disinfectant. It is marketed as perhydrol (an Antiseptic).
(ii) It is used in the manufacture of high quality detergents.
(iii) It is used in the synthesis of hydroquinone tartaric acid and certain food products and pharmaceuticals.
(iv) It is used as bleaching agent for textilies, paper pulp etc.
(v) It is used for pollution control treatment of domestic and industrial effluents.
(vi) 93% H202 is used as an oxidant for rocket fuel.
• Heavy Water (D20)
It is used in the preparation of other deuterium compounds.
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Uses of D2O:
(i) It is used as moderator in nuclear reactors.
(ii) It is used in the exchange reaction study of reaction mechanisms.
• Hydrogen as a Fuel
Hydrogen Economy: The basic principle of hydrogen economy is the transportation and storage of energy in the form of liquid or gaseous dihydrogen. Advantage is that energy is transmitted in the form of dihydrogen and not as electric power.
Advantage as a fuel:
– It is used as fuel cells for the generation of electric power.
– One major advantage of combustion of hydrogen is that it produces very little pollution and there is not any emission of unbumt carbon particles in the form of smoke.
– It is evident from the study that dihydrogen in the gaseous state as well as in liquefied form releases more energy on combustion as compared to the other fuel commonly used.
– 5% of dihydrogen is mixed in CNG for use in four wheeler vehicles.

CBSE Class 11 Chemistry Chapter-9 Important Questions


1 Marks Questions

1.Which isotope of hydrogen

(i) does not contain neutron?

(ii) is radioactive?

Ans. (i) Protium

(ii) Tritium


2.Give the electronic configuration of hydrogen

Ans. 1s’


3.Name the isotopes of hydrogen.

Ans. Hydrogen has three isotopes:

Protium, 

deuterium, 

tritium, 


4.What is syn-gas?

Ans. Mixture of CO and H2 is used for the synthesis of methanol and a number of hydrocarbons it is also called synthesis gas or ‘syngas’


5.What is coal gasification?

Ans. The process of producing syn gas from coal is called ‘coal gasification.


6.Give the laboratory method of preparation of hydrogen.

Ans. Hydrogen is usually prepared by the reaction of granulated zinc with dilute hydrochloric acid


7.Give the commercial method of preparation of dihydrogen.

Ans. Electrolysis of acidified water using platinum electrodes give hydrogen.


8.What is water – gas shift reaction?

Ans.The production of dihyrogen can be increased by reacting carbon monoxide of syn gas mixtures with steam in the presence of iron chromate as catalyst.

This is called water gas – shift reaction.


9.Why is dihydrogen gas not preferred in balloons?

Ans. Dihydrogen is the lightest gas and should have been used in balloons. But it is not preferred due to its highly combustible nature.


10.What is the pH of water?

Ans.The pH value of water is 7.


11.How is methrol prepared using dihydrogen?

Ans.CO on reacting with dihydrogen yields bulk amount of methanol.


12.How is ammonia prepared using dihydrogen?

Ans.With dinitrogen it form ammonia.

This is the method for the manufacture of ammonia by the Haber process.


13.Name the categories into which hydrides are categorized.

Ans.The hydrides are classified into three categories –

(i)Ionic or saline or salt like hydrides.

(ii) Covalent or molecular hydrides

(iii) Metallic or non-stoichiometric hydrides.


14.What are hydrides?

Ans. Dihydrogen under certain reaction conditions combines with almost all elements, except noble gases, to form binary compounds, called hydrides.


15.Give an example of each of an ionic hydride and a covalent hydride.

Ans.Ionic hydride: LiH, NaH Covalent hydride CH4, NH3 and H2O


16.What happens when water is added to calcium hydride?

Ans.Calcium hydroxide is formed 


17.Give an example of electron – deficient hydride.

Ans. Diborane.


18.What is the behavioral similarity between NH3, H2O HF compounds?

Ans. They behave as Lewis is bases i.e. electron donors. The presence of lone pairs on highly electronegative atoms like N, O and F in hydrides results in hydrogen bond formation between the molecules.


19.Give a reaction in which water acts as an oxidizing agent.

Ans.


20.Write the Name of a zeolite used in softening of hard water.

Ans. Sodium aluminum silicate Na2Al2Si2O8. X H2O.


21.Define hard water.

Ans. Water which does not produce lather with soap solution readily is called hard water. eg. hand pump water, river water, sea water etc.


22.What is calgon?

Ans. Sodium hexameta phosphate (Na6P6O18) is commercially called calgon.


23.Why is H2O2 a better oxidant than water?

Ans. H2O2 is easily reduced to form O and H2O.


24.What happens when H2O2 reacts with ethylene?

Ans.


25.What do you mean by 100 volume of hydrogen peroxide?

Ans. It means that one milliliter of 30% H2Osolution will give 100v of oxygen at STP


26.What happens when BaO2 is treated with phosphoric acid?

Ans. H2O2 is obtained



2 Marks Questions

1.Why does hydrogen occupy unique position in the periodic table?

Ans. Inspite of the fact that hydrogen, to a certain extent resembles both with alkali metals (ns’) and halogens (ns2 np5), it differs from them as well. Hydrogen has very small size as a consequence H+ does not exist freely and is always associated with other atoms or molecules. Thus, it is unique in behaviors and is therefore, best placed separately in the periodic table.


2.Give the main characteristics of isotopes.

Ans. Since, the isotopes have the same electronic configuration, they have almost the same chemical properties. The only difference is in their rates of reactions, mainly due to their different enthalpy of bond dissociation. However, in physical properly of these isotopes differ considerably due to their large mass differences.


3.How can the production of dlhydrogen obtained from ‘coal gasification be increased’?

Ans. By reacting carbon monoxide of syngas mixtures with steam in the presence of iron chromate as catalyst


4.Why is dihydrogen used an fuel cells for generating electrical energy?

Ans. Because it does not produce any pollution and releases greater energy per unit mass of fuel in comparison to gasoline or any other fuel.


5.What is understood by hydrogenation?

Ans. Hydrogenation is used for the conversion of polyunsaturated oils into edible fats.


6.Which fuel is used as a rocket fuel?

Ans. Dihydrogen is used as a rocket fuel in space research.


7.What happens when sodium hydride reacts with water?

Ans. Saline hydride (sodium hydride) react violently with water producing dihydrogen gas 


8.What is the geometry of the compound formed by group 14 to form molecular hydride?

Ans. Tetrahedral in structure.


9.What are the characteristic features of ionic or saline hydrides?

Ans. The ionic hydrides are crystalline, non – volatile non – conducting in solid state. However their melts conduct electricity.


10.Which gas is produced on electrolysis of ionic hydride?

Ans. Dihydrogen gas is produced at the anode on electrolysis of ionic hydride.


11.How does H+ ion forms hydronium ion (OH3+) in water?

Ans. In water H+ ion forms a covalent bond with H2O and forms hydronium ion, (H3O+).


12.Show with reaction the amphoteric nature of water.

Ans. Water acts as an acid with NH3 and base with H2S


13.Why is ice less dense then water and what kind of attractive forces must be overcome to melt ice?

Ans. The structure of ice is an open structure having a number of vacant spaces. Therefore, the density of ice is less than water. When ice melts the hydrogen bonds are broken and the water molecules go in between the vacant spaces. As a result, the structure of liquid water is less open than structure of ice. Thus ice is less dense than water


14.Why does hard water not form lather with soap?

Ans. Hard water does not produce lather with soap readily because the cations (Ca2+ and Mg2+) present in hard water react with soap to precipitate of tcalcium and magnesium salts of fatly acids.

From hard water sodium stearate form Ca/Mg stearate


15.Why is water an excellent solvent for ionic or polar substances?

Ans. Water is a polar solvent with a high dielectric constant. Due to high dielectric constant of water the force of attraction between cation and anion gets weakened. Thus water molecules are able to remove ions from the lattice site using in dipole forces easily.


16.How many hydrogen – bonded water molecule are associated in CuSO4. 5H2O? 

Ans. Only one water molecule, which is outside the brackets (coordinator spheres), is hydrogen bonded. The other four molecules of water are co-ordinated.


17.What happens when H2O2 reacts with acidified KMnO4?

Ans. Reducing property of H2O2 is observed.


18.Hydrogen peroxide acts as oxidizing agent as well as a reducing agent. Why?

Ans. Hydrogen peroxide can act as an oxidizing agent because it readily decomposes to evolve oxygen and also take up oxygen from water.


19.Why is hydrogen peroxide stored in wax-lined glass or plastic vessels in dark?              

Ans. H2O2 decomposes slowly on exposure to light

In the presence of metal surfaces or traces of alkali (present in glass containers), the above reaction is catalyzed.


20.What is the volume strength of 2M-H2O2

Ans. Since 1M – H2O2 solution contains 17g H2O2

∴ 2 M – H2O2 solution contains 34g of H2O2

But 68g of H2O2 contains = 

= 11200ml of O2 at NTP

Thus 1000ml of H2Osoln. gives off O= 11200ml at NTP

Hence 1 ml of H2O2 soln gives off

Thus volume strength of H2O


21.Calculate the strength in volumes of a solution containing 30.36 g/l of H2O2.                    

Ans.      

22.4l at NTP

68g of H2O2 produce 22.4 l O2 at NTP

30.36g of H2Oproduce =  

= 10l O2 at NTP

∴ volume strength = 10 volumes.


22.What happens when hydrogen peroxide reacts with acidified K2Cr2O7?

Ans. Acidified K2Cr2O7 is oxidized to blue peroxide of chromium (Cr2O3) which is unstable.

However, it is soluble in ether and produces blue colored solution.

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