Problem 9 - Chemistry and Chemical Reactivity

Question

Balance the equation for the following precipitation reaction, and then write the net ionic equation. Indicate the state of each species \((s, \ell, \text { aq, or } g\) ). $$\mathrm{CdCl}_{2}+\mathrm{NaOH} \longrightarrow \mathrm{Cd}(\mathrm{OH})_{2}+\mathrm{NaCl}$$

Step-by-Step Solution

Answer

The balanced equation is \( \mathrm{CdCl}_2 (aq) + 2\mathrm{NaOH} (aq) \rightarrow \mathrm{Cd(OH)}_2 (s) + 2\mathrm{NaCl} (aq) \). The net ionic equation is \( \mathrm{Cd}^{2+} (aq) + 2\mathrm{OH}^- (aq) \rightarrow \mathrm{Cd(OH)}_2 (s) \).

Step 1: Write the unbalanced chemical equation

Write the chemical equation for the given reaction: \( \text{CdCl}_2 (aq) + \text{NaOH} (aq) \rightarrow \text{Cd(OH)}_2 (s) + \text{NaCl} (aq) \). This equation includes the states of each species.

Step 2: Balance the chemical equation

First, let's balance the Cd atoms and then balance the Cl and Na atoms together. The equation \( \text{CdCl}_2 (aq) + 2\text{NaOH} (aq) \rightarrow \text{Cd(OH)}_2 (s) + 2\text{NaCl} (aq) \) is balanced, as there are two Na and two Cl atoms on each side and one Cd and two OH groups on each side.

Step 3: Write the complete ionic equation

To write the ionic equation, split the soluble ionic compounds into their ions: \[ \text{Cd}^{2+} (aq) + 2\text{Cl}^- (aq) + 2\text{Na}^+ (aq) + 2\text{OH}^- (aq) \rightarrow \text{Cd(OH)}_2 (s) + 2\text{Na}^+ (aq) + 2\text{Cl}^- (aq) \].

Step 4: Identify the spectator ions

Spectator ions are ions that appear unchanged on both sides of the equation. Here, \(\text{Na}^+\) and \(\text{Cl}^-\) are spectator ions since they do not participate in the formation of the solid.

Step 5: Write the net ionic equation

Remove the spectator ions from the ionic equation to write the net ionic equation: \[ \text{Cd}^{2+} (aq) + 2\text{OH}^- (aq) \rightarrow \text{Cd(OH)}_2 (s) \]. This equation shows only the species that participate in the reaction.

Key Concepts

precipitation reaction

A precipitation reaction is a type of chemical reaction where two solutions combine to form an insoluble solid, known as a precipitate. This precipitate emerges from the solution and can be observed as a solid particle falling out of the liquid. Precipitation reactions are common in chemistry and are often used in laboratories to separate components of mixtures, purify compounds, and produce new materials.

In the given exercise, the precipitation reaction occurs when solutions of \( \text{CdCl}_2 (aq) \) and \( \text{NaOH} (aq) \) are mixed, resulting in the formation of \( \text{Cd(OH)}_2 (s) \), a solid precipitate. The presence of this solid indicates that a chemical change has occurred.

Key points to remember about precipitation reactions include:

• The formation of a solid precipitate, which is usually not soluble in water.
• They often involve exchange or double displacement reactions.
• Balancing the chemical equation is crucial to accurately represent the reaction.

Recognizing precipitation reactions helps in predicting the products of chemical reactions and understanding solubility behavior of different compounds in a solution.

net ionic equation

The net ionic equation is a simplified version of a chemical equation that shows only the species actively participating in the chemical reaction. It provides a clear picture of the actual chemical change occurring by removing spectator ions that do not undergo any change during the reaction.

Writing the net ionic equation involves a few essential steps:

• Complete ionic equation: Begin by splitting all aqueous compounds into their constituent ions.
• Identify spectator ions: These ions are present on both sides of the equation and remain unchanged.
• Simplify to the net ionic equation: Remove the spectator ions to reveal the net reaction.

In the exercise, the net ionic equation for the precipitation reaction between \( \text{CdCl}_2 \) and \( \text{NaOH} \) is \[\text{Cd}^{2+} (aq) + 2\text{OH}^- (aq) \rightarrow \text{Cd(OH)}_2 (s)\].

This representation highlights the formation of the solid precipitate and emphasizes the ions involved in this key transformation. Using net ionic equations helps to focus on the essential parts of the reaction, aiding in a deeper understanding of the chemical processes involved.

spectator ions

Spectator ions are ions in a chemical reaction that do not participate in the actual chemical change. They appear on both sides of the complete ionic equation, remaining in solution even after the reaction has occurred. These ions play a supporting role, balancing the charge in the solution without being directly involved in forming the precipitate or any other products.

In the original exercise, the spectator ions identified are \( \text{Na}^+ \) and \( \text{Cl}^- \). These ions remain in the aqueous solution during the reaction and are not part of the formation of the \( \text{Cd(OH)}_2 \) precipitate.

Key points about spectator ions:

• They do not affect the overall stoichiometry of the reaction.
• They help maintain charge neutrality in the reaction mixture.
• Removing them leads to a cleaner, clearer net ionic equation.

Understanding spectator ions is essential when simplifying and analyzing chemical reactions, as they allow chemists to focus on the real action of the reacting species.