Hydrogen - Solutions

 CBSE Class 11 Chemistry

NCERT Solutions
Chapter 9

1. Justify the position of hydrogen in the periodic table on the basis of its electronic configuration.

Ans. Hydrogen is the first element of the periodic table. Its electronic configuration is []. Due to the presence of only one electron in its 1s shell, hydrogen exhibits a dual behaviour, i.e., it resembles both alkali metals and halogens.

Resemblance with alkali metals:

1. Like alkali metals, hydrogen contains one valence electron in its valency shell.

H: 1s1

Li: [He] 2s1

Hence, it can lose one electron to form a unipositive ion.

2. Like alkali metals, hydrogen combines with electronegative elements to form oxides, halides, and sulphides.

Resemblance with halogens:

1. Both hydrogen and halogens require one electron to complete their octets.


F: 1s2s2 2p5

Hence, hydrogen can gain one electron to form a uninegative ion.

2. Like halogens, it forms a diatomic molecule and several covalent compounds.

Though hydrogen shows some similarity with both alkali metals and halogens, it differs from them on some grounds. Unlike alkali metals, hydrogen does not possess metallic characteristics. On the other hand, it possesses a high ionization enthalpy. Also, it is less reactive than halogens.

Owing to these reasons, hydrogen cannot be placed with alkali metals (group I) or with halogens (group VII). In addition, it was also established that H+ ions cannot exist freely as they are extremely small. H+ ions are always associated with other atoms or molecules. Hence, hydrogen is best placed separately in the periodic table.

2. Write the names of isotopes of hydrogen. What is the mass ratio of these isotopes?

Ans. Hydrogen has three isotopes. They are:

1. Protium,  ,

2. Deuterium, or D, and

3. Tritium, or T

The mass ratio of protium, deuterium and tritium is 1:2:3.

3. Why does hydrogen occur in a diatomic form rather than in a monoatomic form under normal conditions?

Ans. The ionization enthalpy of hydrogen atom is very high (1312 kJ mol-1 ). Hence, it is very hard to remove its only electron. As a result, it show the tendency to exist in the monoatomic form, for which ionization enthalpy is rather low. So. Hydrogen forms a covalent bond with another hydrogen atom. in this form, the K shell of hydrogen is complete (1s2), so it is more stabe in this form.

4. How can the production of dihydrogen, obtained from 'coal gasification', be increased?

Ans. Dihydrogen is produced by coal gasification method as:

The yield of dihydrogen (obtained from coal gasification) can be increased by reacting carbon monoxide (formed during the reaction) with steam in the presence of iron chromate as a catalyst.

This reaction is called the water-gas shift reaction. Carbon dioxide is removed by scrubbing it with a solution of sodium arsenite.

5. Describe the bulk preparation of dihydrogen by electrolytic method. What is the role of an electrolyte in this process?

Ans. Dihydrogen is prepared by the electrolysis of acidified or alkaline water using platinum electrodes. Generally, 15 -20% of an acid ( ) or a base (NaOH) is used.

Reduction of water occurs at the cathode as:

At the anode, oxidation of  ions takes place as:

 Net reaction can be represented as:

Electrical conductivity of pure water is very low owing to the absence of ions in it. Therefore, electrolysis of pure water also takes place at a low rate. If an electrolyte such as an acid or a base is added to the process, the rate of electrolysis increases. The addition of the electrolyte makes the ions available in the process for the conduction of electricity and for electrolysis to take place.

6. Complete the following reactions:





Ans. (i) 

7. Discuss the consequences of high enthalpy of H-H bond in terms of chemical reactivity of dihydrogen.

Ans. The ionization enthalpy of H-H bond is very high (1312 kJ ). This indicates that hydrogen has a low tendency to form  ions. Its ionization enthalpy value is comparable to that of halogens. Hence, it forms diatomic molecules ( ), hydrides with elements, and a large number of covalent bonds.

Since ionization enthalpy is very high, hydrogen does not possess metallic characteristics (lustre, ductility, etc.) like metals.

8. What do you understand by (i) electron-deficient, (ii) electron-precise, and (iii)electron-rich compounds of hydrogen? Provide justification with suitable examples.

Ans. Molecular hydrides are classified on the basis of the presence of the total number of electrons and bonds in their Lewis structures as:

1. Electron-deficient hydrides

2. Electron-precise hydrides

3. Electron-rich hydrides

An electron-deficient hydride has very few electrons, less than that required for representing its conventional Lewis structure e.g. diborane ( ). In B2H6, there are six bonds in all, out of which only four bonds are regular two centered-two electron bonds. The remaining two bonds are three centered-two electron bonds i.e., two electrons are shared by three atoms. Hence, its conventional Lewis structure cannot be drawn.

An electron-precise hydride has a sufficient number of electrons to be represented by its conventional Lewis structure e.g. . The Lewis structure can be written as:


Four regular bonds are formed where two electrons are shared by two atoms.

An electron-rich hydride contains excess electrons as lone pairs e.g. .


There are three regular bonds in all with a lone pair of electrons on the nitrogen atom.

9. What characteristics do you expect from an electron-deficient hydride with respect to its structure and chemical reactions?

Ans. An electron-deficient hydride does not have sufficient electrons to form a regular bond in which two electrons are shared by two atoms e.g.,  etc.

These hydrides cannot be represented by conventional Lewis structures. , for example, contains four regular bonds and two three centered-two electron bond. Its structure can be represented as:


Since these hydrides are electron-deficient, they have a tendency to accept electrons. Hence, they act as Lewis acids.

10. Do you expect the carbon hydrides of the type ( ) to act as 'Lewis' acid or base? Justify your answer.


Ans. For carbon hydrides of type , the following hydrides are possible for

For a hydride to act as a Lewis acid i.e., electron accepting, it should be electron-deficient. Also, for it to act as a Lewis base i.e., electron donating, it should be electron-rich.

Taking  as an example, the total number of electrons are 14 and the total covalent bonds are seven. Hence, the bonds are regular  -2 centered bonds.


Hence, hydride has sufficient electrons to be represented by a conventional Lewis structure. Therefore, it is an electron-precise hydride, having all atoms with complete octets. Thus, it can neither donate nor accept electrons to act as a Lewis acid or Lewis base.

11. What do you understand by the term “non-stoichiometric hydrides”? Do you expect this type of the hydrides to be formed by alkali metals? Justify your answer.

Ans. Non-Stoichiometric hydrides are hydrogen-deficient compounds formed by the reaction of dihydrogen with d-block and f-block elements. These hydrides do not follow the law of constant composition. For example:  etc.

Alkali metals form stoichiometric hydrides. These hydrides are ionic in nature. Hydride ions have comparable sizes (208 pm) with alkali metal ions. Hence, strong binding forces exist between the constituting metal and hydride ion. As a result, stoichiometric hydrides are formed.

Alkali metals will not form non-stoichiometric hydrides.

12. How do you expect the metallic hydrides to be useful for hydrogen storage? Explain.

Ans. Metallic hydrides are hydrogen deficient, i.e., they do not hold the law of constant composition. It has been established that in the hydrides of Ni, Pd, Ce, and Ac, hydrogen occupies the interstitial position in lattices allowing further absorption of hydrogen on these metals. Metals like Pd, Pt, etc. have the capacity to accommodate a large volume of hydrogen. Therefore, they are used for the storage of hydrogen and serve as a source of energy.

13. How does the atomic hydrogen or oxy-hydrogen torch function for cutting and welding purposes? Explain.

Ans. Atomic hydrogen atoms are produced by the dissociation of dihydrogen with the help of an electric arc. This releases a huge amount of energy (435.88 kJ ). This energy can be used to generate a temperature of 4000 K, which is ideal for welding and cutting metals. Hence, atomic hydrogen or oxy-hydrogen torches are used for these purposes. For this reason, atomic hydrogen is allowed to recombine on the surface to be welded to generate the desired temperature.

14. Among   and HF, which would you expect to have highest magnitude of hydrogen bonding and why?

Ans. The extent of hydrogen bonding depends upon electronegativity and the number of hydrogen atoms available for bonding. Among nitrogen, fluorine, and oxygen, the increasing order of their electro negativities are N < O < F.

Hence, the expected order of the extent of hydrogen bonding is .

But, the actual order is .

Although fluorine is more electronegative than oxygen, the extent of hydrogen bonding is higher in water. There is a shortage of hydrogens in HF, whereas there are exactly the right numbers of hydrogens in water. As a result, only straight chain bonding takes place. On the other hand, oxygen forms a huge ring-like structure through its high ability of hydrogen bonding.

In case of ammonia, the extent of hydrogen bonding is limited because nitrogen has only one lone pair. Therefore, it cannot satisfy all hydrogens.


15. Saline hydrides are known to react with water violently producing fire. Can , a well known fire extinguisher, be used in this case? Explain.

Ans. Saline hydrides (i.e., NaH, LiH, etc.) react with water to form a base and hydrogen gas. The chemical equation used to represent the reaction can be written as:

The reaction is violent and produces fire.

COis heavier than dioxygen. It is used as a fire extinguisher because it covers the fire as a blanket and inhibits the supply of dioxygen, thereby dousing the fire.

CO2 can be used in the present case as well. It is heavier than dihydrogen and will be effective in isolating the burning surface from dihydrogen and dioxygen.

16. Arrange the following

(i)  in order of increasing electrical conductance.

(ii) LiH, NaH and CsH in order of increasing ionic character.

(iii) H-H, D-D and F-F in order of increasing bond dissociation enthalpy.

(iv) NaH,  in order of increasing reducing property.

Ans. (i) The electrical conductance of a molecule depends upon its ionic or covalent nature. Ionic compounds conduct, whereas covalent compounds do not.

 is a covalent hydride. Hence, it does not conduct.  is an ionic hydride, which conducts electricity in the molten state. Titanium hydride,  is metallic in nature and conducts electricity at room temperature. Hence, the increasing order of electrical conductance is as follows:

(ii) The ionic character of a bond is dependent on the electronegativities of the atoms involved. The higher the difference between the electronegativities of atoms, the smaller is the ionic character.

Electronegativity decreases down the group from Lithium to Caesium. Hence, the ionic character of their hydrides will increase (as shown below).

LiH < NaH < CsH

(iii) Bond dissociation energy depends upon the bond strength of a molecule, which in turn depends upon the attractive and repulsive forces present in a molecule.

The bond pair in D-D bond is more strongly attracted by the nucleus than the bond pair in H-H bond. This is because of the higher nuclear mass of . The stronger the attraction, the greater will be the bond strength and the higher is the bond dissociation enthalpy. Hence, the bond dissociation enthalpy of D-D is higher than H-H.

However, bond dissociation enthalpy is the minimum in the case of F-F. The bond pair experiences strong repulsion from the lone pairs present on each F-centre.

Therefore, the increasing order of bond dissociation enthalpy is as follows:

F-F < H-H < D-D

(iv) Ionic hydrides are strong reducing agents. NaH can easily donate its electrons. Hence, it is most reducing in nature.

Both,and  are covalent hydrides.  is less reducing than  since the bond dissociation energy of  is higher than .

Hence, the increasing order of the reducing property is  <  < NaH.

17. Compare the structures of .

Ans. In gaseous phase, water molecule has a bent form with a bond angle of 104.5°. The O-H bond length is 95.7 pm. The structure can be shown as:


Hydrogen peroxide has a non-planar structure both in gas and solid phase. The dihedral angle in gas and solid phase is 111.5° and 90.2° respectively.


18. What do you understand by the term 'auto-protolysis' of water? What is its significance?

Ans. Auto-protolysis (self-ionization) of water is a chemical reaction in which two water molecules react to produce a hydroxide ion ( ) and a hydronium ion ( ).

The reaction involved can be represented as:

Auto-protolysis of water indicates its amphoteric nature i.e., its ability to act as an acid as well as a base.

The acid-base reaction can be written as:

19. Consider the reaction of water with  and suggest, in terms of oxidation and reduction, which species are oxidized/reduced.

Ans. The reaction between fluorine and water can be represented as:


This is an example of a redox reaction as water is getting oxidized to oxygen, while fluorine is being reduced to fluoride ion.

The oxidation numbers of various species can be represented as:


Fluorine is reduced from zero to (--1) oxidation state. A decrease in oxidation state indicates the reduction of fluorine.

Water is oxidized from (- 2) to zero oxidation state. An increase in oxidation state indicates oxidation of water.

20. Complete the following chemical reactions.






Classify the above into (a) hydrolysis, (b) redox and (c) hydration reactions.

Ans. (i) PbS(s)+4H2O2(aq)PbSO4(s)+4H2O(l)

 is acting as an oxidizing agent in the reaction. Hence, it is a redox reaction.


 is acting as a reducing agent in the acidic medium, thereby oxidizing  . Hence, the given reaction is a redox reaction.


The reactions in which a compound reacts with water to produce other compounds are called hydrolysis reactions. The given reaction is hydrolysis.


The reactions in which a compound reacts with water to produce other compounds are called hydrolysis reactions. The given reaction represents hydrolysis of .


The reactions in which a compound reacts with water to produce other compounds are called hydrolysis reactions. The given reaction represents hydrolysis of .

21. Describe the structure of the common form of ice.

Ans. Ice is the crystalline form of water. It takes a hexagonal form if crystallized at atmospheric pressure, but condenses to cubic form if the temperature is very low.

The three-dimensional structure of ice is represented as:


The structure is highly ordered and has hydrogen bonding. Each oxygen atom is surrounded tetrahedrally by four other oxygen atoms at a distance of 276 pm. The structure also contains wide holes that can hold molecules of appropriate sizes interstitially.

22. What causes the temporary and permanent hardness of water?

Ans. Temporary hardness of water is due to the presence of soluble salts of magnesium and calcium in the form of hydrogen carbonates ( , where M = Mg, Ca) in water.

Permanent hardness of water is because of the presence of soluble salts of calcium and magnesium in the form of chlorides in water.

23. Discuss the principle and method of softening of hard water by synthetic ion-exchange resins.

Ans. The process of treating permanent hardness of water using synthetic resins is based on the exchange of cations (e.g.,  etc) and anions (e.g.,  etc) present in water by  ions respectively.

Synthetic resins are of two types:

1) Cation exchange resins

2) Anion exchange resins

Cation exchange resins are large organic molecules that contain the  H group. The resin is firstly changed to RNa (from ) by treating it with NaCl. This resin then exchanges  ions withand  ions, thereby making the water soft.

There are cation exchange resins in H+ form. The resins exchange  ions for ,  , andions.

Anion exchange resins exchange  ions for anions like , , and  present in water.

During the complete process, water first passes through the cation exchange process. The water obtained after this process is free from mineral cations and is acidic in nature.

This acidic water is then passed through the anion exchange process where OH ions neutralize the H ions and de-ionize the water obtained.

24. Write chemical reactions to show the amphoteric nature of water.

1) Reaction with H2S

2) Reaction with NH3

3) Self-ionization of water

Ans. The amphoteric nature of water can be described on the basis of the following reactions:

1) Reaction with H2S

The reaction takes place as:

In the forward reaction,  accepts a proton from . Hence, it acts as a Lewis base.

2) Reaction with NH3

The reaction takes place as:


In the forward reaction, denotes its proton to . Hence, it acts as a Lewis acid.

3) Self-ionization of water

In the reaction, two water molecules react as: