Question

The correct order of reducing abilities of hydrides of V group element is:
A.$N{H_3} < {\text{ }}P{H_3} < {\text{ }}As{H_3} < {\text{ }}Sb{H_3} < {\text{ }}Bi{H_3}$
B.$N{H_3} > {\text{ }}P{H_3} > {\text{ }}As{H_3} > {\text{ }}Sb{H_3} > {\text{ }}Bi{H_3}$
C.$N{H_3} < {\text{ }}P{H_3} > {\text{ }}As{H_3} > {\text{ }}Sb{H_3} > {\text{ }}Bi{H_3}$
D.$\;Sb{H_3} > {\text{ }}Bi{H_3} > {\text{ }}As{H_3} > {\text{ }}N{H_3} > {\text{ }}P{H_3}$

Answer 1

These are the compounds of Nitrogen family hydrides and reducing ability over here corresponds to the ease of losing hydrogen very easily. Hence the compound which will lose hydrogen fast will be a good reducing agent and this ease of losing hydrogen will depend on Metal-Hydrogen bond strength.

Complete step-by-step answer: In the given question, the compounds are from Nitrogen group i.e. group 15 elements and we have to compare reducing abilities of these elements as we move down the group. Since all the compounds are hydrides so the reducing ability directly relates to ease of losing Hydrogen from themselves.
But in general case, the oxidation process is defined as loss of electrons from the species and reducing agent is the one which itself undergoes oxidation i.e. which loses electrons from itself.
Here the reducing abilities correspond to the ease of losing hydrogen and the one which loses hydrogen very fast will be the highest reducing agent.
This reducing character of hydrides depends on the Metal – Hydrogen bond strength and the one which will be strong will not fast loose hydrogen and the one with weak metal- hydrogen bond will be losing hydrogen very easily.
So, we see that as we move from Nitrogen to Bismuth, the size of the atom goes on increasing and therefore the Metal-Hydrogen strength will go on decreasing because of this increasing size.
This ultimately makes more reducing in nature and therefore here $Bi{H_3}$will have the highest reducing ability as compared to $N{H_3}$
That’s also why $N{H_3}$is a mild reducing agent whereas BiH3 is a stronger one.
So, the correct order is $N{H_3} < {\text{ }}P{H_3} < {\text{ }}As{H_3} < {\text{ }}Sb{H_3} < {\text{ }}Bi{H_3}$

Hence, the correct answer is A.

Note: Redox reaction is the one in which oxidation and reduction takes place in a reaction simultaneously and there are reducing agents as well as oxidizing agents in that reaction. Those could be identified by looking at the change in the oxidation state of the particular species. There are few basic properties of elements which correspond to trends in other properties as well like atomic size, mass, electronegativity, electron affinity, ionization energy.