Question

When \(\mathrm{H}_{2} \mathrm{S}\) is passed into a solution of copper(II) sulfate acidified with \(\mathrm{H}_{2} \mathrm{SO}_{4},\) copper(II) sulfide precipitates. When concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) is heated with metallic \(\mathrm{Cu}\), the principal sulfur-containing product is \(\mathrm{SO}_{2}\), but a residue of copper(II) sulfide is also formed. Account for these reactions.

Short answer

When \(\mathrm{H}_2\mathrm{S}\) reacts with \(\mathrm{CuSO}_4\) solution, \(\mathrm{CuS}\) precipitates. Heating \(\mathrm{Cu}\) with \(\mathrm{H}_2\mathrm{SO}_4\) forms \(\mathrm{SO}_2\) and \(\mathrm{CuS}\) residue.

Step-by-step solution

Understanding the Reaction Conditions

Identify the initial conditions. In the first scenario, we have hydrogen sulfide gas (\(\mathrm{H}_2\mathrm{S}\)) being passed through a solution of copper(II) sulfate (\(\mathrm{CuSO}_4\)) that has been acidified with sulfuric acid (\(\mathrm{H}_2\mathrm{SO}_4\)). In the second scenario, concentrated sulfuric acid is heated with metallic copper (\(\mathrm{Cu}\)).

Analyzing the Formation of Copper(II) Sulfide

When \(\mathrm{H}_2\mathrm{S}\) is passed through \(\mathrm{CuSO}_4\) solution, a precipitation reaction occurs. \(\mathrm{H}_2\mathrm{S}\) provides sulfide ions (\(\mathrm{S}^{2-}\)) which react with \(\mathrm{Cu}^{2+}\) ions from \(\mathrm{CuSO}_4\), forming insoluble copper(II) sulfide (\(\mathrm{CuS}\)), which precipitates out. The reaction is: \[ \mathrm{Cu}^{2+} + \mathrm{S}^{2-} \rightarrow \mathrm{CuS} \downarrow \]

Explaining the Reaction with Metallic Copper

When metallic copper is heated with concentrated \(\mathrm{H}_2\mathrm{SO}_4\), copper is oxidized to \(\mathrm{Cu}^{2+}\), while \(\mathrm{H}_2\mathrm{SO}_4\) acts as an oxidizing agent and is reduced to sulfur dioxide (\(\mathrm{SO}_2\)). The reaction releases \(\mathrm{SO}_2\) gas and can also form \(\mathrm{CuS}\) as a residue under limited oxygen conditions: \[ \mathrm{Cu} + \mathrm{H}_2\mathrm{SO}_4 \rightarrow \mathrm{CuSO}_4 + \mathrm{SO}_2 + \mathrm{H}_2\mathrm{O} \] and leads to \(\mathrm{CuS}\) formation under these conditions.

Key concepts

Copper(II) Sulfide

Copper(II) sulfide, commonly represented as CuS, is an insoluble dark compound that forms as a result of a precipitation reaction. In a laboratory setting, this often occurs when a solution of copper(II) sulfate encounters hydrogen sulfide gas. The copper ions, denoted as Cu\(^{2+}\), in the solution bind with sulfide ions, S\(^{2-}\), provided by the H\(_2\)S gas. This leads to the formation of a solid compound that does not dissolve in water, thus it precipitates out of the solution.

• Reacts with: Hydrogen sulfide.
• Appearance: Dark, insoluble solid.
• Formation Reaction: Cu\(^{2+}\) + S\(^{2-}\) → CuS.

Precipitation reactions like these are fundamental in chemical analysis for detecting the presence of metal cations by the formation of a visible solid.

Acidified Solution

An acidified solution is typically a chemical solution containing an added acid, often to adjust the pH or facilitate certain reactions. In our scenario involving copper(II) sulfate (CuSO\(_4\)), the solution is acidified with sulfuric acid (H\(_2\)SO\(_4\)). Adding acid can enhance the overall reactivity in various reactions by increasing the concentration of hydrogen ions in the solution.

• Common Acids Used: Sulfuric acid, hydrochloric acid.
• Purpose: Increase reaction rates/effectiveness.
• Effect: Often leads to faster and more complete reactions.

Understanding how and why solutions are acidified is essential in performing successful chemical reactions in both laboratory and industrial contexts.

Oxidizing Agent

An oxidizing agent is a substance that has the ability to accept electrons, thereby getting reduced while oxidizing another substance in the process. Concentrated sulfuric acid in the reaction with metallic copper acts as an oxidizing agent. It facilitates the transfer of electrons by accepting them from copper, which is why we see copper transition from a metallic state to copper(II) ions as it forms copper sulfate.

• Role: Electron acceptor in reactions.
• Outcome: Oxidizes substances by gaining electrons.
• Example: Sulfuric acid in the discussed reaction.

Recognizing oxidizing agents is crucial in predicting the outcomes of chemical reactions and understanding energy flow in chemical processes.

Sulfur Dioxide

Sulfur dioxide (SO\(_2\)) is a colorless gas with a pungent odor, serving as a critical product in some oxidation reactions involving sulfuric acid. During the oxidation of metallic copper with H\(_2\)SO\(_4\), sulfur dioxide gas is released as a byproduct. This shows the dual role of sulfuric acid, acting both as an oxidizing agent and a source of SO\(_2\).

• Appearance: Colorless gas, pungent smell.
• Produced in: Oxidation of metals with H\(_2\)SO\(_4\).
• Significance: Indicator of oxidation reactions involving sulfur compounds.

Comprehending the creation and implications of sulfur dioxide in this context aids in grasping broader concepts in chemical transformations and industrial applications.