Chemistry Class 11 NCERT Solutions: Chapter 9 Hydrogen Part 4

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Q: 16. Arrange the following

(i) , and in order of increasing electrical conductance.

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

(iii) and in order of increasing bond dissociation enthalpy.

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

Answer:

(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).

(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:

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

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

Q: 17. Compare the structures of .

Answer:

In gaseous phase, water molecule has a bent form with a bond angle of The

bond length is . 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 and respectively.

Q: 18. What do you understand by the term ‘auto-protolysis’ of water? What is its significance?

Answer:

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:

Q: 19. Consider the reaction of water with F2 and suggest, in terms of oxidation and reduction, which species are oxidized/reduced.

Answer:

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.