Question

The following steps are unbalanced half-reactions involved in the nitrogen cycle. Balance each half-reaction to show the number of electrons lost or gained, and state whether it is an oxidation or a reduction (all occur in acidic conditions):

${\mathbf{\text{(a)N}}}_{\mathbf{\text{2}}}\mathbf{\text{(g)}}\mathbf{\to }\mathbf{\text{NO(g)}}$

${\mathbf{\text{(b)N}}}_{\mathbf{\text{2}}}\mathbf{\text{O(g)}}\mathbf{\to }{\mathbf{\text{NO}}}_{\mathbf{\text{2}}}\mathbf{\text{(g)}}$

${\mathbf{\text{(c)NH}}}_{\mathbf{\text{3}}}\mathbf{\text{(aq)}}\mathbf{\to }{\mathbf{\text{NO}}}_{\mathbf{\text{2}}}^{\mathbf{\text{-}}}\mathbf{\text{(aq)}}$

${{\mathbf{\text{(d)NO}}}_{\mathbf{\text{3}}}}^{\mathbf{\text{-}}}\mathbf{\text{(aq)}}\mathbf{\to }{{\mathbf{\text{NO}}}_{\mathbf{\text{2}}}}^{\mathbf{\text{-}}}\mathbf{\text{(aq)}}$

${\mathbf{\text{(e)N}}}_{\mathbf{\text{2}}}\mathbf{\text{(g)}}\mathbf{\to }{\mathbf{\text{NO}}}_{\mathbf{\text{3}}}^{\mathbf{\text{-}}}\mathbf{\text{(aq)}}$

Short answer

a. Oxidation, ${\text{N}}_2{\text{(g)+O}}_2\left(\mathrm{g}\right)\to \text{2NO}\left(\mathrm{g}\right){\text{+4e}}^{\text{-}}$.

Gaining oxygen is oxidation.

b. Oxidation, ${\text{2N}}_2{\text{O+3O}}_2\to {\text{4NO}}_2{\text{+12e}}^{\text{-}}$.

Gaining oxygen is oxidation.

c.Oxidation, ${\text{2NH}}_{\text{4}}^{\text{+}}{\text{(aq)+3O}}_{\text{2}}\to {\text{2NO}}_{\text{2}}^{\text{-}}{\text{(aq)+4H}}^{\text{+}}{\text{(aq)+2H}}_{\text{2}}\text{O(l)}$

Losing hydrogen is oxidation.

d. Reduction, ${\text{2NO}}_{\text{3}}^{\text{-}}\text{(aq)}\to {\text{2NO}}_{\text{2}}^{\text{-}}{\text{(aq)+O}}_{\text{2}}\text{(g)}$

Losing oxygen is oxidation.

e. Oxidation: ${\text{2N}}_{\text{2}}{\text{(g)+5O}}_{\text{2}}{\text{(g)+2H}}_{\text{2}}\text{O(l)}\to {\text{4NO}}_{\text{3}}^{\text{-}}{\text{(aq)+4H}}^{\text{+}}\text{(aq)}$

Losing hydrogen is oxidation.

Step-by-step solution

Oxidation

Oxidation is a chemical reaction that results in the loss of one or more electrons in an atom or molecule. Learn the difference between oxidation and reduction by diving further into the definition, process, and real-world examples of oxidation reactions.

Subpart (a)

Reduction is the process of obtaining electrons, whereas oxidation is the process of losing electrons.

The complete equation is:

${\text{N}}_{\text{2}}{\text{(g)+O}}_{\text{2}}\text{(g)}\to \text{2NO(g)}$

The nitrogen half-reaction is:

${\text{N}}_{\text{2}}^{\text{o}}\text{(g)}\to {\text{2N}}^{\text{II}}{\text{+4e}}^{\text{-}}$

The process is called oxidation.

Subpart (b)

The complete equation is:

${\text{2N}}_{\text{2}}{\text{O(g)+3O}}_{\text{2}}\text{(g)}\to {\text{4NO}}_{\text{2}}\text{(g)}$

The nitrogen half-reaction is:

${\text{4N}}^{\text{I}}\text{(g)}\to {\text{4N}}^{\text{IV}}{\text{(g)+12e}}^{\text{-}}$

The process is called oxidation.

To obtain oxygen:

${\text{3O}}_{\text{2}}^{\text{o}}{\text{(g)+12e}}^{\text{-}}\to {\text{6O}}^{\text{2-}}\text{(g)}$

The process is called reduction.

Therefore, oxidation process.

Subpart (c)

The complete equation is:

${\text{2NH}}_{\text{4}}^{\text{+}}{\text{(aq)+3O}}_{\text{2}}\to {\text{2NO}}_{\text{2}}^{\text{-}}{\text{(aq)+4H}}^{\text{+}}{\text{(aq)+2H}}_{\text{2}}\text{O(l)}$

So, because reaction takes place in an acidic environment, the ammonia will be in the form of ${\text{NH}}_{\text{4}}^{\text{+}}$rather than ${\text{NH}}_{\text{3}}$.

The nitrogen half-reaction is:

${\text{2N}}^{\text{III-}}\text{(aq)}\to {\text{2N}}^{\text{III}}{\text{(aq)+12e}}^{\text{-}}$

The process is called oxidation.

Subpart (d)

The complete equation is:

${\text{2NO}}_{\text{3}}^{\text{-}}\text{(aq)}\to {\text{2NO}}_{\text{2}}^{\text{-}}{\text{(aq)+O}}_{\text{2}}\text{(g)}$

The nitrogen half-reaction is:

${\text{2N}}^{\text{V}}{\text{(g)+4e}}^{\text{-}}\to {\text{2N}}^{\text{III}}$

The process is called reduction.

Subpart (e)

The whole equation is:

${\text{2N}}_{\text{2}}{\text{(g)+5O}}_{\text{2}}{\text{(g)+2H}}_{\text{2}}\text{O(l)}\to {\text{4NO}}_{\text{3}}^{\text{-}}{\text{(aq)+4H}}^{\text{+}}\text{(aq)}$

The nitrogen half-reaction is:

${\text{2N}}_{\text{2}}^{\text{o}}\text{(g)}\to {\text{4N}}^{\text{V}}{\text{+20e}}^{\text{-}}$

The process is called oxidation.