Identify the substance oxidized, the substance reduced, the oxidizing agent, and the reducing agent? 2HNO3 + 3H3AsO3 -----> 2NO + 3H3AsO4 + H2O

2 Answers
Mar 27, 2015

the substance oxidized is H3AsO3, since the oxidation number of As goes from +3 to +5, the substance reduced is HNO3, with the oxidation number of N going from +5 to +2, the oxidizing agent is the substance reduced ( HNO3) and the reducing agent is the substance oxidized (H3AsO3)

Nitric acid, #HNO_3#, is the substance reduced to NO, while arsenous acid, #H_3AsO_3#, is oxidized to arsenic acid, #H_3AsO_4#.

There are three ways to determine this.

a) The easiest one is to observe the oxygen/non-metal ratio: 3:1 in nitric acid and 1:1 in NO.

So nitrogen undergoes an oxygen loss, that is, the same as a reduction, while arsenic gains an extra oxygen, so it is oxidized.

b) The second way is to balance half-reactions to deduce which is the substance that "absorbs" electrons (the oxidant that is getting reduced) and which one donates electrons (while getting oxidized).

The nitric acid half-reaction is:

#HNO_3 + 3H^+ + 3e^-##NO + 2H_2O#

The three electrons were needed to balance electrical charge on the left side, so this is a reduction half-reaction.

The arsenous acid half-reaction is:

#H_3AsO_3 + H_2O -> H_3AsO_4 + 2H^+ + 2e^-#

The electrons were donated, so the half-reaction is an oxidation.

c) The third way consists in "monitoring" the oxidation number (o. n.) of all the elements before and after the reaction.

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Once again we have an increase of oxidation number of arsenic, the oxidized element, and a decrease in the o. n. of nitrogen, the reduced element.