P Block Elements

7.31

In interhalogen compound ICl and I₂ the atoms are bonded by covalent bonds

Reactivity refers to the rate at which a chemical species will undergo reaction in time. For reaction to take place the compound needs to be broken into separate elements first, then the individual elements react with other elements to form new compounds. So any compound which can easily break into their individual elements can react faster.

The covalent bonds between dissimilar atoms I and Cl atoms in ICl are weaker than between similar atoms in I₂. Therefore the bond between ICL will break easily so the I and Cl atom will be easily available to form another compound as compared to the strong bond between I

Therefore bond dissociation enthalpy of all bond is less than that of I₂ Therefore ICl is more reactive than I₂.

7.29

Moist chlorine is a good reducing agent because it can accept electrons from other species as it is very electronegative.

(iii) The bleaching action of chlorine is due to oxidation by nascent oxygen produced. This nascent oxygen can be produced by

Chlorine dissolves in water in absence of sunlight and forms hydrochloric acid and hypochlorous acid (HOCl)

Since hypochlorous acid is not much stable it decomposes giving nascent oxygen. Cl₂ + H₂O  HCl + HOCl

HOCl HCl + [O]

This [O] is called as nascent oxygen. This nascent oxygen is responsible for bleaching action.

NOTE: Since chlorine has the ability to kill harmful micro-organism it also acts as a good disinfecting agent.

7.27

The reasons for the anomalous behaviour of fluorine are as follows:

The smallest size of fluorine

The highest electronegativity

Low bond dissociation enthalpy of F-F bond

Non-availability of d-orbitals in its valence shell The anomalous properties are as follows;

Fluorine shows only one oxidation state-1 while all other halogens show variable oxidation states like

-1, +1, +3, +5, +7 (due to small size, high electronegativity)

Fluorine forms strong hydrogen bonding in its hydrides unlike other halogens. (due to high hydration energy than other elements)

The compounds of fluorine have higher ionic character than other halogens. (due to high electronegativity forming polar interactions)

Fluorine has no tendency to form polyhalide ion whereas other halogen form polyhalideion (due to non-availability of d orbitals in valence shell)

7.26

The oxidizing power of an element depends on three factors.

1. Bond dissociation energy

2. Electron gain enthalpy

3. Hydration enthalpy

Fluorine is considered the strongest oxidizing agent compared to all known elements due to its small size (being the first member of 17 group and ionic radii decrease downwards), low bond enthalpy of F-F (due to high electron-electron repulsion).

High negative value of electron gain enthalpy (the highest electronegativity ) and very high hydration energy of F^- ions (hydration energy is the energy released when a compound is dissolved in water .

 In case of fluorine it is more due to the strong interaction between F^- ions and water, as it is electro-negative). The comparison of both the elements makes this clear that F₂ is a strong oxidizing power as compared to Cl_2.

The electron gain enthalpy of chlorine (-349 KJ mol^-1) is more negative than that of fluorine (-333 KJ mol^{- 1}). However, the bond dissociation energy of fluorine (158.8 KJ mol^-1) is much lesser than that of chlorine (242.6 KJ mol^-1).

Also, because of its small size, the hydration energy of fluorine (515 KJ mol^-1) is much higher than that of chlorine (381 KJ mol^-1). Therefore, the latter two factors more than compensate for the less negative electron gain enthalpy of fluorine. Thus, fluorine is a much stronger oxidizing agent than chlorine.