Question

why is phospheric acid is strong reducing agent but H2PO4 is not​

Answer 1

Answer:

in H3PO3, check the oxidation state of P…… it is +3, which can be further oxidised to higher oxidation state. Thus it can act as reducing agent

here we can say that h3po2 will act as stronger reducing agent for the same reason.(oxidation stat +1)

But now lets come to the phosphoric acid or H3PO4, the oxidation state of phosphorous here is +5 that is the highest oxidation state for the P. Hence it cannot be oxidised to higher state and so it cannot act as a reducing agent

Phosphoric acid, in fact, can act as an oxidising agent at very high temperatures. It is worth mentioning here that even concentrated sulfuric acid requires elevated temperatures to show its oxidising power.

Answer 2

Answer:

The oxidation and reduction processes always occur in pairs. Phosphoric acid, H3PO4, is unable to be oxidized because P is in its highest oxidation state. That is why phosphorous acid is a better reducing agent. ... When H3PO3 reduces another compound it is oxidized from P^+3 to P^+5.