Question

Write net ionic equations for the following qualitative cation analysis procedures. (a) precipitation of \(\mathrm{PbCl}_{2}(\mathrm{s})\) from a solution containing \(\mathrm{Pb}^{2+}\) (b) dissolution of \(\mathrm{Zn}(\mathrm{OH})_{2}(\mathrm{s})\) in a solution of \(\mathrm{NaOH}(\mathrm{aq})\) (c) dissolution of \(\mathrm{Fe}(\mathrm{OH})_{3}(\mathrm{s})\) in \(\mathrm{HCl}(\mathrm{aq})\) (d) precipitation of \(\mathrm{CuS}(\mathrm{s})\) from an acidic solution of \(\mathrm{Cu}^{2+}\) and \(\mathrm{H}_{2} \mathrm{S}\)

Short answer

The net ionic equations for these reactions are: (a) Pb2+ (aq) + 2Cl- (aq) -> PbCl2 (s), (b) Zn(OH)2 (s) + 2OH- (aq) -> Zn(OH)4^2- (aq), (c) Fe(OH)3 (s) + 3H+ (aq) -> Fe3+ (aq) + 3H2O (l) and (d) Cu2+ (aq) + S2- (aq) -> CuS (s).

Step-by-step solution

Write the chemical equation for precipitation of PbCl2

Pb2+ (aq) + 2Cl- (aq) -> PbCl2 (s). This reaction describes the formation of PbCl2 precipitate from the Pb2+ ion. The net ionic equation is thus the same as the full ionic equation in this case, because there are no spectator ions.

Write the chemical equation for dissolution of Zn(OH)2

Zn(OH)2 (s) + 2OH- (aq) -> Zn(OH)4^2- (aq). This reaction describes the solubility of Zn(OH)2. In this case, the net ionic equation and the full ionic equation are also the same, as there are no spectator ions.

Write the chemical equation for dissolution of Fe(OH)3 in HCl

Fe(OH)3 (s) + 3H+ (aq) -> Fe3+ (aq) + 3H2O (l). This reaction is the acid-base reaction between Fe(OH)3 and HCl. The net ionic equation in this case is the same as the full ionic equation, because there are no spectator ions.

Write the chemical equation for precipitation of CuS from an acidic solution

Cu2+ (aq) + S2- (aq) -> CuS (s). This reaction is the formation of a precipitate of CuS from Cu2+ and S2-. The net ionic equation is thus the same as the full ionic equation in this case, because there are no spectator ions.

Key concepts

Understanding Cation Analysis

Cation analysis is a technique used to identify and distinguish between different cations present in a solution. This process is essential in qualitative chemical analysis, often used in educational chemistry labs and industrial settings. During this process, various tests help determine the presence of specific ions based on their physical and chemical properties.

In a qualitative cation analysis:

• You can identify particular cations in a mixture through characteristic reactions.
• Reagents are added and observable changes, like color change or precipitate formation, indicate the presence of certain ions.
• It's crucial to perform the reactions in a controlled sequence to avoid interference between ions.

A well-understood example of cation analysis is the precipitation and dissolution of ionic compounds, integral for identifying the presence of specific cations. Analyzing cations precisely is important for determining their roles in broader chemical reactions.

Understanding Precipitation Reactions

Precipitation reactions occur when two aqueous solutions react and form an insoluble solid called a precipitate. This reaction is useful for separating ions from a solution and is commonly used in cation analysis to identify specific ions again.

Key points about precipitation reactions include:

• They can occur when a solution containing a specific cation is combined with another solution that has a suitable anion, resulting in the formation of a solid.
• In the equation Pb\(^{2+}\) (aq) + 2Cl\(^-\) (aq) \(\rightarrow\) PbCl\(_2\) (s), lead (II) chloride precipitates from the solution, confirming the presence of Pb\(^{2+}\) ions.
• Net ionic equations focus on the components that actually undergo the chemical change, excluding spectator ions.

Understanding how different ions come together to form solids helps chemists predict and manipulate reactions in laboratory or industrial processes.

The Role of Dissolution Reactions

Dissolution reactions are essentially the opposite of precipitation reactions. During dissolution, a solid solute dissolves in a solvent, forming a homogeneous mixture; this makes it easier to analyze ions in a solution, especially during cation analysis.

Key aspects of dissolution reactions are:

• Dissolution reactions occur when a solid compound, like Zn(OH)\(_2\), reacts with a solvent or reagent to form an aqueous solution, such as Zn(OH)\(_4^{2-}\) in the presence of OH\(^-\).
• In Fe(OH)\(_3\) (s) + 3H\(^+\) (aq) \(\rightarrow\) Fe\(^{3+}\) (aq) + 3H\(_2\)O (l), the solid iron hydroxide dissolves in the acidic solution, indicating solubility and ion presence.
• The products of dissolution reactions can further participate in additional reactions, showcasing their significance in broader chemical processes.

Dissolution reactions are crucial for analyzing complex mixtures, facilitating separation, and understanding compound solubility dynamics.