Problem 140 - General Chemistry

Question

Mercury(II) nitrate is treated with hydrogen sulfide, \(\mathrm{H}_{2} \mathrm{~S}\), forming a precipitate and a solution. Write the molecular equation and the net ionic equation for the reaction. An acid is formed; is it strong or weak? Name each of the products. If \(65.65 \mathrm{~g}\) of mercury(II) nitrate and \(4.26 \mathrm{~g}\) of hydrogen sulfide are mixed in \(395.0 \mathrm{~g}\) of water to form \(54.16 \mathrm{~g}\) of precipitate, what is the mass of the solution after the reaction?

Step-by-Step Solution

Answer

Molecular equation: \( \text{Hg(NO}_3\text{)}_2 + \text{H}_2\text{S} \rightarrow \text{HgS} + 2\text{HNO}_3 \). Net ionic: \( \text{Hg}^{2+} + \text{S}^{2-} \rightarrow \text{HgS} \). \( \text{HNO}_3 \) is a strong acid. Solution mass: 410.75g.

Step 1: Write the Chemical Reaction

The reaction of mercury(II) nitrate with hydrogen sulfide can be represented as \( \text{Hg(NO}_3\text{)}_2 + \text{H}_2\text{S} \rightarrow \text{HgS} + 2\text{HNO}_3 \). Mercury(II) sulfide \( \text{HgS} \) is the precipitate, and nitric acid \( \text{HNO}_3 \) is the solution formed.

Step 2: Identify Acid Strength

Nitric acid \( \text{HNO}_3 \) is a product resulting from the reaction. \( \text{HNO}_3 \) is a well-known strong acid.

Step 3: Write the Net Ionic Equation

The net ionic equation includes ions that participate in forming the precipitate: \( \text{Hg}^{2+} + \text{S}^{2-} \rightarrow \text{HgS(s)} \). This shows the formation of the mercury(II) sulfide precipitate.

Step 4: Calculate Mass of Solution After Reaction

The mass of the solution after the reaction is the initial total mass minus the mass of the precipitate. Total initial mass = mass of mercury(II) nitrate + mass of hydrogen sulfide + mass of water: \( 65.65 \text{ g} + 4.26 \text{ g} + 395.0 \text{ g} = 464.91 \text{ g} \). Subtract the mass of the precipitate: \( 464.91 \text{ g} - 54.16 \text{ g} = 410.75 \text{ g} \).

Key Concepts

Net Ionic Equation

Chemical reactions often involve a transition where ions in a solution form new substances. In a net ionic equation, only the ions and compounds that directly participate in the formation of the products are shown. For this reaction, the molecular equation is:
\(\mathrm{Hg(NO_3)_2} + \mathrm{H_2S} \rightarrow \mathrm{HgS} + 2\mathrm{HNO_3} \)
The net ionic equation highlights the formation of a precipitate. In this scenario, only mercury(II) ions \(\mathrm{Hg}^{2+} \)and sulfide ions \(\mathrm{S}^{2-} \)participate directly in forming mercury(II) sulfide:

• \( \mathrm{Hg}^{2+} + \mathrm{S}^{2-} \rightarrow \mathrm{HgS(s)} \)

This equation informs us that when these ions encounter each other in the solution, they create a solid through a chemical change. This precise focus on the active components simplifies the scenario and allows for easier understanding of chemical interactions.

Precipitation Reactions

Precipitation reactions are a type of chemical reaction where two soluble ions mix in solution to form an insoluble solid. This solid is known as a precipitate. In our reaction, when mercury(II) nitrate interacts with hydrogen sulfide, the mercury(II) ions and sulfide ions come into contact.
When they combine, mercury(II) sulfide \(\mathrm{HgS}\)forms as a black precipitate. Several features define precipitation reactions:

• Two solubles create an insoluble product: The reactants are typically soluble ionic compounds in water.
• Formation of a solid precipitate: The molecules interact to form a solid not soluble in the present medium.
• Observed as a solid settling or cloudiness: This visual sign indicates a chemical reaction has occurred.

Precipitation reactions are crucial in areas like waste treatment, where specific poisons are removed from solutions.

Strong Acids

Acids are chemicals that release hydrogen ions in a solution, and nitric acid \(\mathrm{HNO}_3\)is one of the products of the reaction between mercury(II) nitrate and hydrogen sulfide. Strong acids are those that dissociate completely in water. When \(\mathrm{HNO}_3\) is dissolved, it fully ionizes to produce hydrogen ions \(\mathrm{H}^+\)and nitrate ions \(\mathrm{NO}_3^-\).Strong acids have several characteristics:

• Completely dissociate in water, fully separating into ions.
• Conduct electrical current efficiently, making them good conductors.
• Have a low pH, typically less than 3, indicating the high concentration of \(\mathrm{H}^+\) ions.

In the laboratory, understanding acid strength is essential for safely handling and neutralizing chemical solutions.

Solution Mass Calculation

Calculating the mass of a solution after a chemical reaction is key in stoichiometry, as it helps understand conservation of mass in a reaction. For the given reaction, we need to account for all components. Initially, we have:

• Mercury(II) nitrate: 65.65 g
• Hydrogen sulfide: 4.26 g
• Water: 395.0 g

The total initial mass amounts to 464.91 g.
After the reaction, mercury(II) sulfide precipitate forms, which weighs 54.16 g. The new mass of the solution can be calculated by subtracting the precipitate's mass from the total initial mass:
\(464.91 \, \text{g} - 54.16 \, \text{g} = 410.75 \, \text{g}\)This mistakeless calculation ensures complete comprehension of how mass is conserved across a chemical process, affirming that mass is neither created nor destroyed in chemical reactions.