Question

In the following reaction
4P + 3KOH + 3H₂O → 3KH₂PO₂ + PH₃
(a) phosphorus is both oxidised and reduced.
(b) only phosphorus is reduced.
(c) phosphorus is not oxidised
(d) None of these

Answer 1

In the following reaction

4P + 3KOH + 3H₂O → 3KH₂PO₂ + PH₃

(a) phosphorus is both oxidised and reduced.

We have to find the oxidation states of P in KH₂PO₂ and PH₃.

In P,the oxidation state is 0.

In KH₂PO₂, the oxidation state of K is +1, H is +1, O is -2.

Let us consider the oxidation state of P is 'x'.

1 + 2(1) + x +2(-2) = 0

⇒  x - 1 = 0

⇒  x = 1

In PH₃, the oxidation state of H is +1.

Let us consider the oxidation state of P is 'y'.

y + 3(1) = 0

⇒  x + 3 = 0

⇒  x = -3

So, the oxidation number of P increases to +1 in KH₂PO₂ and reduces to -3 in PH₃.

Thus, it is both oxidised and reduced.

Option (a) is correct.