Question

Why does the reducing character of tri hydrides of group 15 elements increase down the group?

Answer 1

You can start by defining what elements are present in group 15 and their corresponding trihydrates. Then define what reducing the character of a compound means. Then describe how the size of atoms increases, bond length increases and bond strength decreases when we move down the group. Then relate it to the reducing character to reach the solution.

Complete step by step answer:
Group 15 elements include nitrogen ( $N$ ), phosphorus ( $P$ ), arsenic ( $As$ ), antimony ( $Sb$ ), and bismuth ( $Bi$ ).
The general chemical formula of tri hydrides of group 15 elements is $X{H_3}$ , with $X$ representing an element of group 15.
Group 15 elements make the following tri hydrides with hydrogen: $N{H_3}$ , $P{H_3}$ , $As{H_3}$ , $Sb{H_3}$ and $Bi{H_3}$ .
Arranging the trip hydrides of group according to the reducing character of the reducing character, we get
$N{H_3} < P{H_3} < As{H_3} < Sb{H_3} < Bi{H_3}$

As you can clearly see that the reducing character of group 15 elements increase as we move down the group. To understand this we have to understand what reducing character is.
Reducing character refers to the tendency of a compound to donate an electron to another chemical species in order to reduce (being reduced means to accept one electron) it.
As we move down in group 15 the size of the group 15 element increases which causes the length of the bond between the hydrogen atom and the group 15 element to increase. This in turn decreases the strength of the bond between hydrogen atom and the group 15 element, hence making it easier to donate a hydrogen atom.
This is why the reducing character of the tri hydrides of group 15 elements decrease as we move down the group.

Note: We have used Henry’s law here to solve this problem but this law does not hold true under certain conditions. It is not applicable at very high pressure and very low temperature. It is also not applicable if the gas reacts chemically with the solvent or dissociates in it.