Question

How would you balance the following equation: Ca+HNO3 --> Ca(NO3)2+ N2O3+ H2O?

Answer 1

By the method of half-equations: calcium metal is oxidized, and nitrogen is reduced.

Explanation:

Oxidation:

`Ca(s) rarr Ca^(2+) + 2e^-` `(i)`

Reduction:

`2HNO_3(aq) +4e^(-) +4H^(+) rarr N_2O_3 +3H_2O` `(ii)`

Note that dinitrogen trioxide is a metastable species, and is a mixed valence oxide of `N(II+)` and `N(IV+)`, hence an average oxidation state of `III+`.

Both `(i)` and `(ii)` are balanced with respect to mass and charge (as they must be!). We add `2xx(i) +(ii)` to give:

`2Ca(s) + 2HNO_3(aq) +4H^(+) rarr 2Ca^(2+) + N_2O_3(aq) +3H_2O(aq)`

Since the source of `H^+` is undoubtedly nitric acid, we could write:

`2Ca(s) + 6HNO_3(aq) rarr 2Ca(NO_3)_2 + N_2O_3(aq) +3H_2O(aq)`

Is this last equation balanced with respect to mass and charge? Do not trust my arithmetic!