# What are the spectator ions in Mg(s) + Zn^(2+) + 2NO_3^(-) -> Mg^(2+) + 2NO_3^(-) + Zn(s)?

Jun 18, 2016

The nitrate anions, ${\text{NO}}_{3}^{-}$.

#### Explanation:

A spectator ion is an ion that is present on both sides of a chemical equation.

In other words, in order for an ion to be a spectator ion, it must exist as such as a reactant and as a product in the balanced chemical equation.

You're dealing with a single replacement reaction in which magnesium metal, $\text{Mg}$, replaces the zinc cations in zinc nitrate, "Zn"("NO"_3)_2.

The products of this reaction are zinc metal, $\text{Zn}$, and aqueous magnesium nitrate*, "Mg"("NO"_3)_2.

The problem provides you with the complete ionic equation for this reaction

${\text{Mg"_ ((s)) + "Zn"_ ((aq))^(2+) + 2"NO"_ (3(aq))^(-) -> "Mg"_ ((aq))^(2+) + 2"NO"_ (3(aq))^(-) + "Zn}}_{\left(s\right)}$

As you can see, the reactants' side contains two ions

• the zinc cation, ${\text{Zn}}^{2 +}$
• the nitrate anion, ${\text{NO}}_{3}^{2 -}$

Likewise, the product's side also contains two ions

• the magnesium cation, ${\text{Mg}}^{2 +}$
• the nitrate anion, ${\text{NO}}_{3}^{-}$

Because the nitrate anions are present on both sides of the equation, they are spectator ions.

This means that you can eliminate them from the complete ionic equation

${\text{Mg"_ ((s)) + "Zn"_ ((aq))^(2+) + color(red)(cancel(color(black)(2"NO"_ (3(aq))^(-)))) -> "Mg"_ ((aq))^(2+) + color(red)(cancel(color(black)(2"NO"_ (3(aq))^(-)))) + "Zn}}_{\left(s\right)}$

to get the net ionic equation

${\text{Mg"_ ((s)) + "Zn"_ ((aq))^(2+) -> "Mg"_ ((aq))^(2+) + "Zn}}_{\left(s\right)}$