Step 1: Identify the species present in water
Magnesium is a metal in elemental form: \( \mathrm{Mg(s)} \). Magnesium nitrate is a soluble ionic compound and behaves as a strong electrolyte in water:
\[ \mathrm{Mg(NO_3)_2(aq)} \;\rightarrow\; \mathrm{Mg^{2+}(aq)} + 2\,\mathrm{NO_3^{-}(aq)} \]
Step 2: Test common driving forces for reaction
In aqueous solution, a reaction is expected when at least one of these occurs:
- Formation of a precipitate (an insoluble solid).
- Formation of a gas.
- Formation of a weak electrolyte (such as water in neutralization).
- A redox (single-replacement) process where one metal displaces another metal ion.
Nitrates are soluble, so precipitate formation from \( \mathrm{NO_3^-} \) is not expected. No acid or carbonate is present to generate a gas, and no neutralization can occur. The only remaining possibility is a single-replacement/redox idea.
Step 3: Check the single-replacement (redox) possibility
A single-replacement reaction has the form \[ \mathrm{M(s)} + \mathrm{M'^{n+}(aq)} \rightarrow \mathrm{M^{n+}(aq)} + \mathrm{M'(s)} \] and requires \( \mathrm{M} \) to be a stronger reducing agent than \( \mathrm{M'} \).
In magnesium + magnesium nitrate, the ion present is \( \mathrm{Mg^{2+}} \), which corresponds to the same metal as the solid \( \mathrm{Mg(s)} \). Trying to “displace” \( \mathrm{Mg^{2+}} \) with \( \mathrm{Mg(s)} \) produces no change:
\[ \mathrm{Mg(s)} + \mathrm{Mg^{2+}(aq)} \rightarrow \mathrm{Mg^{2+}(aq)} + \mathrm{Mg(s)} \]
The reactants and products are identical, so there is no thermodynamic or chemical driving force for a net reaction.
Step 4: Write molecular, complete ionic, and net ionic forms
| Form | Result for magnesium + magnesium nitrate (aqueous) | Interpretation |
|---|---|---|
| Molecular equation | NR (no reaction) | No new products form under the stated conditions. |
| Complete ionic equation | \[ \mathrm{Mg(s)} + \mathrm{Mg^{2+}(aq)} + 2\,\mathrm{NO_3^{-}(aq)} \rightarrow \mathrm{Mg(s)} + \mathrm{Mg^{2+}(aq)} + 2\,\mathrm{NO_3^{-}(aq)} \] | Everything present on the left appears unchanged on the right. |
| Net ionic equation | No net ionic equation (all species cancel) | Cancellation leaves no chemical change to represent. |
Common confusion: magnesium does react readily with many other metal-ion solutions (for example, \( \mathrm{Mg(s)} \) can displace \( \mathrm{Cu^{2+}} \) from copper(II) salts), but it cannot “displace” \( \mathrm{Mg^{2+}} \) from magnesium nitrate because that would not change the identity of any species.
Optional note: when a reaction could be observed in a nitrate solution
If an acid is present (providing \( \mathrm{H^+(aq)} \)), magnesium metal reacts with \( \mathrm{H^+} \) to produce hydrogen gas:
\[ \mathrm{Mg(s)} + 2\,\mathrm{H^+(aq)} \rightarrow \mathrm{Mg^{2+}(aq)} + \mathrm{H_2(g)} \]
In that situation, nitrate may still be present as a spectator ion, but the reacting species is \( \mathrm{H^+} \), not \( \mathrm{Mg^{2+}} \).