Question

Give reasons:
(i) Alkali metals are good reducing agents.
(ii) Alkali metals can be obtained only by electrolysis of their fused salts.
(iii) Hydrogen is obtained on electrolysis of aqueous solution of salts of alkali metals.
(iv) Alkali metals dissolve in liquid ammonia to give blue solutions.
(v) The blue colour of metal–ammonia solution disappears on standing.

Answer 1

(i) A substance is known to be a reducing agent, if  

a. It causes reduction in the "oxidation number" of the other substance involved in the chemical reaction.

b. Its oxidation number is increased.

"Alkali metals" have only one electron in their "valence shells" which they would lose easily to attain stability and become oxidized. Hence, their oxidation numbers would increase whereas on giving that electron, the substance that gets this electron will have its oxidation number reduced.  

Hence, alkali metals are termed as strong reducing agents.

(ii) There are three main reasons:

a. As alkali metals are strong reducing agents, they cannot be isolated by the chemical reaction of their oxides

b. As alkali metals are highly electropositive, they cannot be prepared  by reactions involving displacement of one element over another in their salt solutions

c. During electrolysis of  aqueous solutions of their salts, the liberated metals react with water and eventually hydrogen gets liberated at the cathode.  

Thus, in order to extract alkali metals, electrolysis of their fusion chloride is implemented as shown below:

$NaCl \quad \underrightarrow { Fusion } \quad { Na }^{ + }+{ Cl }^{ - }$

During electrolysis

At anode

${ 2Cl }^{ - }\longrightarrow { Cl }_{ 2 }+2{ e }^{ - }$

At cathode

${ 2Na }^{ + }+2{ e }^{ - }\longrightarrow 2Na$

(iii) During the process of electrolysis of the aqueous solution of any alkali metal like for example sodium, it would produce sodium hydroxide by immediately reacting with water. Thus, hydrogen gets liberated at the cathode. This happens because the standard electrode potential of water is much higher than that of alkali metals.

(iv) When an alkali metal gets dissolved in liquid ${ NH }_{ 3 }$, the atom of the alkali metal loses its only electron in its valence shell very easily. Now, the cation so formed will merge with ammonia to form ammoniated cation.  

$M+(x+y){ NH }_{ 3 }\longrightarrow { M }^{ + }{ ({ NH }_{ 3 }) }_{ x }+{ e }^{ -1 }{ ({ NH }_{ 3 } })_{ y }$

The solvated electron so formed, absorbs energy from the red region of the visible light in order to get excited to higher energy levels. Eventually, the blue colour is imparted to the solution due to the transmitted light.

(v) The blue colour gets disappeared on standing for some time because the solvated electrons reacts with ammonia which will thus liberate hydrogen

${ NH }_{ 3 }+{ e }^{ - }\longrightarrow { NH }_{ 2 }^{ - }+\frac { 1 }{ 2 } { H }_{ 2 }$