Question

Mercury(I) ions \(\left(\mathrm{Hg}_{2}^{2+}\right)\) can be removed from solution by precipitation with \(\mathrm{Cl}^{-}\). Suppose that a solution contains aqueous \(\mathrm{Hg}_{2}\left(\mathrm{NO}_{3}\right)_{2} .\) Write complete ionic and net ionic equations for the reaction of aqueous \(\mathrm{Hg}_{2}\left(\mathrm{NO}_{3}\right)_{2}\) with aqueous sodium chloride to form solid \(\mathrm{Hg}_{2} \mathrm{Cl}_{2}\) and aqueous sodium nitrate.

Short answer

Complete ionic equation: \(\mathrm{Hg}_{2}^{2+}(aq) + 2\mathrm{NO}_{3}^{-}(aq) + 2\mathrm{Na}^{+}(aq) + 2\mathrm{Cl}^{-}(aq) \rightarrow \mathrm{Hg}_{2}\mathrm{Cl}_{2}(s) + 2\mathrm{Na}^{+}(aq) + 2\mathrm{NO}_{3}^{-}(aq)\). Net ionic equation: \(\mathrm{Hg}_{2}^{2+}(aq) + 2\mathrm{Cl}^{-}(aq) \rightarrow \mathrm{Hg}_{2}\mathrm{Cl}_{2}(s)\).

Step-by-step solution

Write Down All Reactants and Products

Identify all reactants and products. Reactants are aqueous Mercury(I) nitrate \(\mathrm{Hg}_{2}(\mathrm{NO}_{3})_{2}\) and aqueous sodium chloride (NaCl). Products are solid Mercury(I) chloride \(\mathrm{Hg}_{2}\mathrm{Cl}_{2}\) and aqueous sodium nitrate (NaNO_3). Write down the balanced molecular equation for the reaction: \[\mathrm{Hg}_{2}(\mathrm{NO}_{3})_{2}(aq) + 2\mathrm{NaCl}(aq) \rightarrow \mathrm{Hg}_{2}\mathrm{Cl}_{2}(s) + 2\mathrm{NaNO}_{3}(aq)\]

Write the Complete Ionic Equation

Break all the soluble ionic compounds into their constituent ions. The complete ionic equation is expressed as: \[\mathrm{Hg}_{2}^{2+}(aq) + 2\mathrm{NO}_{3}^{-}(aq) + 2\mathrm{Na}^{+}(aq) + 2\mathrm{Cl}^{-}(aq) \rightarrow \mathrm{Hg}_{2}\mathrm{Cl}_{2}(s) + 2\mathrm{Na}^{+}(aq) + 2\mathrm{NO}_{3}^{-}(aq)\]

Cancel Out the Spectator Ions

Spectator ions are ions that remain unchanged on both sides of a chemical equation and do not participate in the reaction. Cancel them out to form the net ionic equation. In this case, the spectator ions are \(\mathrm{Na}^{+}\) and \(\mathrm{NO}_{3}^{-}\). After canceling, the net ionic equation is: \[\mathrm{Hg}_{2}^{2+}(aq) + 2\mathrm{Cl}^{-}(aq) \rightarrow \mathrm{Hg}_{2}\mathrm{Cl}_{2}(s)\]

Key concepts

Complete Ionic Equation

When exploring chemical reactions in an aqueous solution, we often use the complete ionic equation to fully represent the system's behavior. The complete ionic equation shows every ion present in the reaction as it actually exists in the solution. This differs from a molecular equation where compounds are written as molecules or complete units.

For example, when aqueous Mercury(I) nitrate reacts with aqueous sodium chloride, we represent their ionic forms separately in the complete ionic equation. Each soluble compound is split into its respective ions. The equation becomes more descriptive as it shows Mercury(I) as \(\mathrm{Hg}_{2}^{2+}\) and chloride ions as \(\mathrm{Cl}^{-}\), along with the nitrate \(\mathrm{NO}_{3}^{-}\) and sodium ions \(\mathrm{Na}^{+}\). This allows us to view the actual participants of the precipitation reaction and aids in identifying the spectator ions which do not change throughout the reaction.

Writing the complete ionic equation is an essential step in understanding the nuances of chemical interactions in a solution, and sets the stage for moving on to the net ionic equation, which provides an even more focused view on the reaction.

Spectator Ions

In any aqueous reaction like the one we examined earlier, not all ions undergo a chemical change. Those that remain unchanged in both the reactants and products are called spectator ions. They are present in the solution but do not participate in the actual chemical reaction, similar to spectators watching a game without affecting the outcome.

In the reaction between Mercury(I) nitrate and sodium chloride, the \(\mathrm{Na}^{+}\) and \(\mathrm{NO}_{3}^{-}\) ions are found on both the reactant and product sides of the complete ionic equation, without undergoing any change. Thus, these are spectator ions. They just 'spectate' the key event, which here is the formation of the solid Mercury(I) chloride precipitate. Understanding spectator ions is crucial as it helps us identify the ions that drive the chemical reaction and simplify the equations we use to describe these reactions.

Chemical Precipitation Reactions

Precipitation is a process where soluble ions in a solution form an insoluble solid. In our example, the reaction between aqueous Mercury(I) nitrate and sodium chloride results in the formation of solid Mercury(I) chloride - this is a precipitation reaction. Such reactions typically occur when two aqueous solutions that contain ions are mixed and form a product that is not soluble in water.

The product, a precipitate, is the solid that forms and falls out of solution. The identification of the precipitate is an essential aspect of writing net ionic equations. In our case, \(\mathrm{Hg}_{2}\mathrm{Cl}_{2}\) is the precipitate. To predict whether a chemical precipitation reaction will occur, we can use solubility rules. Knowing these rules enables us to determine which combinations of ions will result in the formation of a precipitate. This knowledge is not only fundamental in chemistry labs but also has practical applications in various industries such as pharmaceuticals, environmental science, and materials engineering.