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

Feb 11, 2016

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

#### Explanation:

Oxidation:

$C a \left(s\right) \rightarrow C {a}^{2 +} + 2 {e}^{-}$ $\left(i\right)$

Reduction:

$2 H N {O}_{3} \left(a q\right) + 4 {e}^{-} + 4 {H}^{+} \rightarrow {N}_{2} {O}_{3} + 3 {H}_{2} O$ $\left(i i\right)$

Note that dinitrogen trioxide is a metastable species, and is a mixed valence oxide of $N \left(I I +\right)$ and $N \left(I V +\right)$, hence an average oxidation state of $I I I +$.

Both $\left(i\right)$ and $\left(i i\right)$ are balanced with respect to mass and charge (as they must be!). We add $2 \times \left(i\right) + \left(i i\right)$ to give:

$2 C a \left(s\right) + 2 H N {O}_{3} \left(a q\right) + 4 {H}^{+} \rightarrow 2 C {a}^{2 +} + {N}_{2} {O}_{3} \left(a q\right) + 3 {H}_{2} O \left(a q\right)$

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

$2 C a \left(s\right) + 6 H N {O}_{3} \left(a q\right) \rightarrow 2 C a {\left(N {O}_{3}\right)}_{2} + {N}_{2} {O}_{3} \left(a q\right) + 3 {H}_{2} O \left(a q\right)$

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