Question

For ${H_3}P{O_4}$ and${H_3}P{O_3}$, the correct choice is?
A) ${H_3}P{O_3}$is dibasic and reducing agent
B) ${\text{ }}{H_3}P{O_3}$ is a dibasic and non-reducing agent.
C) ${H_3}P{O_4}$is tribasic and reducing agent
D) ${H_3}P{O_3}$is a tribasic and non-reducing agent.

Answer 1

The dibasic and tribasic nature of an acid is indicated by no. of H atoms it can furnish. And the reducing and non-reducing behavior is explained on the basis of its oxidation state.

Complete answer:
${H_3}P{O_3}$ is known as phosphorus acid and ${H_3}P{O_4}$ is known as phosphoric acid. As phosphorus has a tendency to make one double bond so both the acid consists of double bonds each with oxygen atoms. If we talk about the nature of the acids so ${H_3}P{O_3}$ is dibasic in nature as phosphorus is bonded to two –OH bonds (hydroxyl atoms) which shows that it can furnish two hydrogen atoms. On the other side ${H_3}P{O_4}$ is tribasic in nature as it consists of three –OH bonds which make it furnish three hydrogen atoms. Hence option D is eliminated.

Reducing agents are those which themselves undergo oxidation by losing electrons in a redox reaction. In this process, there is an increase in the oxidation state of the reducing agent. These reducing agents reduce other molecules. In case of acids if a central atom is present in its maximum oxidation state then it cannot further increase its oxidation state. So, it would not act as a reducing agent. In ${H_3}P{O_4}$ , phosphorus is present in its maximum oxidation state $( + 5)$. Hence it will not act as a reducing agent. Hence option C is eliminated.

On the other hand, the phosphorus atom in ${H_3}P{O_3}$ , has$\left( { + 3} \right)$oxidation state. So, it can be further oxidized. Hence ${H_3}P{O_3}$ will act as a reducing agent. So, option B is also wrong.

As a rule of thumb, if a hydrogen atom is directly attached to a phosphorus atom in the oxyacid of phosphorus, then that oxyacid is a reducing agent. In case of phosphorous acid, the hydrogen atom is attached directly to phosphorus acid. Hence, phosphorus acid is a reducing agent. However, for phosphoric acid, the hydrogen atom is not directly attached to the phosphorus atom. Hence, phosphoric acid is not a reducing agent.

Hence option A is the correct answer.

Note: In the structure of the oxyacid of phosphorus, count the number of hydrogen atoms that are a part of the hydroxyl group. This number will tell you whether the acid is dibasic or tribasic. Also find out if any hydrogen atom is directly attached to phosphorus atom. This will tell you if the oxyacid is reducing in nature or non-reducing in nature.