Question

\(\mathrm{Cd}(\mathrm{OH})_{2}\) is an insoluble compound. It dissolves in a \(\mathrm{NaOH}\) solution. Write a balanced ionic equation for this reaction. What type of reaction is this?

Short answer

The net ionic equation for the reaction is: Cd(OH)2 (s) ⟶ Cd^2+ (aq) + 2OH^- (aq). This reaction is a dissolution or dissociation reaction.

Step-by-step solution

Write the Balanced Molecular Equation

Cd(OH)2 (s) + 2NaOH (aq) ⟶ Na2Cd(OH)4 (aq)

Dissociate All Strong Electrolytes

Cd^2+ (aq) + 2OH^- (aq) + 2Na^+ (aq) + 2OH^- (aq) ⟶ 2Na^+ (aq) + Cd^2+ (aq) + 4OH^- (aq)

Cancel the Spectator Ions

From the equation in step 2, we can see the spectator ions are Na^+ and OH^- ions. So, after removing these we can get the net ionic equation: Cd(OH)2 (s) ⟶ Cd^2+ (aq) + 2OH^- (aq)

Identify the Type of Reaction

The reaction is a type of dissolution or dissociation reaction, where a compound breaks into its component ions in a solution.

Key concepts

Insoluble Compounds

An insoluble compound is a substance that does not readily dissolve in a solvent to form a solution. Insolubility is a relative term since some amount of the substance may still dissolve, but not enough to go unnoticed. In this context, we can think of a compound as insoluble if it remains largely undissolved when mixed with a solvent.
For example, \(\mathrm{Cd}(\mathrm{OH})_{2}\) is considered an insoluble compound in water. Once introduced to a slightly different environment, like \(\mathrm{NaOH}\) solution, it may react differently.
Understanding the behavior of insoluble compounds is fundamental in predicting the outcomes of chemical reactions. This includes knowing which substances form precipitates, which are essentially solid particles formed when a solute becomes insoluble. Additionally, recognizing insoluble compounds helps in crafting chemical reactions, whether in the laboratory or in industrial processes.

Net Ionic Equation

A net ionic equation is a streamlined version of a chemical reaction that only showcases the substances that undergo a change. This form of equation removes spectator ions, which are ions present in the same form on both the reactant and product side of the equation. Spectator ions do not participate in the actual chemical change and remain dissolved throughout the process.
For the reaction involving \(\mathrm{Cd}(\mathrm{OH})_{2}\), writing the balanced molecular equation is the first step. The reaction is as follows:
\[\mathrm{Cd}(\mathrm{OH})_{2} \,(\mathrm{s}) + 2\mathrm{NaOH} \,(\mathrm{aq}) \rightarrow \mathrm{Na}_{2}\mathrm{Cd}(\mathrm{OH})_{4} \,(\mathrm{aq})\]
After the affiliation of ions in the aqueous phase, some, like \(2\mathrm{Na}^{+}\) and \(\mathrm{OH}^{-}\), do not change through the reaction. These ions are spectators.
Once these are removed, we get the net ionic equation:
\[\mathrm{Cd}(\mathrm{OH})_{2} \,(\mathrm{s}) \rightarrow \mathrm{Cd}^{2+} \,(\mathrm{aq}) + 2\mathrm{OH}^{-} \,(\mathrm{aq})\]
This equation offers a clearer view of the dissolution process, focusing solely on the active dissolution of the cadmium hydroxide.

Dissolution Reaction

A dissolution reaction is where a solid substance dissolves in a solvent to form a solution. During this reaction, the compound dissociates into its constituent ions. Dissolution is an essential process in chemistry because it is the basis for various reactions.
In the given example, \(\mathrm{Cd}(\mathrm{OH})_{2}\) undergoes a dissolution reaction in the presence of \(\mathrm{NaOH}\) solution. The solid compound \(\mathrm{Cd}(\mathrm{OH})_{2}\) separates into \(\mathrm{Cd}^{2+}\) and \(\mathrm{OH}^{-}\) ions in the solution:
\[\mathrm{Cd}(\mathrm{OH})_{2} \,(\mathrm{s}) \rightarrow \mathrm{Cd}^{2+} \,(\mathrm{aq}) + 2\mathrm{OH}^{-} \,(\mathrm{aq})\]
Understanding how and why certain reactions occur through dissolution helps in numerous scientific and practical applications. Predicting outcomes of dissolution reactions allows chemists to manipulate reaction conditions to favor desired results, whether creating solutions, removing contaminants, or synthesizing new compounds. In industrial settings, dissolution reactions are critical for processes like ore extraction and pharmaceuticals.