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Question: In the equation: \[NO{}_2^ - + {H_2}O \to NO_3^ - + 2{H^ + } + n{e^ - }\]; \[n\] stands for:...

In the equation: NO2−+H2O→NO3−+2H++ne−NO{}_2^ - + {H_2}O \to NO_3^ - + 2{H^ + } + n{e^ - }; nn stands for:

Explanation

Solution

In the given question as we can see the reaction represents the oxidation reaction as there is removal of electrons in the reaction.

Complete step by step solution
The number of electrons is represented by “nn”, in the redox reaction. In a redox reaction the value of “nn” will be the same as the change in the oxidation state of the oxidizing species.
Rules for the assigning of oxidation numbers:
1. All species in their elemental form are given the oxidation number of zero.
2. All monatomic ions have the same oxidation number as the charge on the ion. e.g. Fe2+F{e^{2 + }}has the oxidation number of +2 + 2.
3. All combined hydrogen has an oxidation number of+1 + 1 (except metal hydrides where its oxidation number is −1 - 1).
4. All combined oxygen has an oxidation number of −2 - 2 (except peroxides where the oxidation number is −1 - 1).
5. In polyatomic species, the sum of the oxidation numbers of the element in the ion equals the charge on that species (we can use this to find the oxidation number of elements in polyatomic species).
Redox reaction represents the oxidation of one species and reduction of another species, if one oxidizes then other needs to reduce, the number of electrons transferred is represented by “n”. The number of electrons should be the same in both reduction and oxidation half reaction, then we can get the balance redox reaction. In the given reaction, in the oxidation of nitrite ion to nitrate ion, the change in oxidation state of nitrogen atom can be calculated as follows:

NO2−+H2O  →   NO3−+2H++2e−   +3                                        +5N{O^{2 - }} + {H_2}O\; \to \,\;N{O^{3 - }} + 2{H^ + } + 2{e^ - }\\\ \; + 3{\rm{ }}\;{\rm{ }}\;{\rm{ }}\;{\rm{ }}\;{\rm{ }}\;{\rm{ }}\;{\rm{ }}\;{\rm{ }}\;{\rm{ }}\;{\rm{ }}\;{\rm{ }}\;{\rm{ }}\;{\rm{ }}\; \; \; \; \; \; \; \; + 5

The change in oxidation state is +2 + 2, hence, the value of “n” =2 = 2.

Note:
As we know if the direction of the arrow is reversed the reaction becomes reduction half reaction and nitrite reduction to nitrate is the reaction taking place in soil if the amount oxygen available is less than expected.