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 \(81.15 \mathrm{~g}\) of mercury(II) nitrate and \(8.52 \mathrm{~g}\) of hydrogen sulfide are mixed in \(550.0 \mathrm{~g}\) of water to form \(58.16 \mathrm{~g}\) of precipitate, what is the mass of the solution after the reaction?

Short answer

The solution mass is 581.51 g; \(\mathrm{HNO_3}\) is a strong acid.

Step-by-step solution

Identify Reactants and Products

The reactants are mercury(II) nitrate, \(\mathrm{Hg(NO_3)_2}\), and hydrogen sulfide, \(\mathrm{H_2S}\). The product of their reaction is mercury(II) sulfide, \(\mathrm{HgS}\), a precipitate, and nitric acid, \(\mathrm{HNO_3}\), which remains in solution.

Write the Molecular Equation

The balanced molecular equation for the reaction is:\[\mathrm{Hg(NO_3)_2 (aq) + H_2S (aq) \rightarrow HgS (s) + 2HNO_3 (aq)}.\]

Write the Net Ionic Equation

Since \(\mathrm{HgS}\) is the precipitate, the net ionic equation includes only the ions that participate in forming \(\mathrm{HgS}\):\[\mathrm{Hg^{2+} (aq) + S^{2-} (aq) \rightarrow HgS (s)}.\]

Determine the Acid's Strength

The acid formed is nitric acid, \(\mathrm{HNO_3}\), which is a strong acid because it completely dissociates in water.

Calculate Mass of Products and Solution

The total mass before the reaction is the sum of the masses of the reactants and the solvent: \[81.15 \, \mathrm{g} + 8.52 \, \mathrm{g} + 550.0 \, \mathrm{g} = 639.67 \, \mathrm{g}.\]The mass of the precipitate formed is given as \(58.16 \, \mathrm{g}\). Subtract this from the total mass to find the mass of the solution:\[639.67 \, \mathrm{g} - 58.16 \, \mathrm{g} = 581.51 \, \mathrm{g}.\] Therefore, the mass of the solution after the reaction is \(581.51 \, \mathrm{g}\).

Key concepts

Molecular Equation

A molecular equation represents a chemical reaction by writing out all the reactants and products in their molecular forms. It provides a clear view of the substances involved before and after a chemical change. For instance, when mercury(II) nitrate reacts with hydrogen sulfide, the molecular equation is:

• \( \mathrm{Hg(NO_3)_2 \,(aq) + H_2S \,(aq) \rightarrow HgS \,(s) + 2HNO_3 \,(aq)} \)

This equation shows:

• The reactants: \( \mathrm{Hg(NO_3)_2} \) and \( \mathrm{H_2S} \).
• The products: a precipitate of \( \mathrm{HgS} \) and \( \mathrm{HNO_3} \), which stays dissolved in water.

Each compound is labeled to indicate its state (aqueous or solid), which helps in visualizing the process and outcomes of the reaction.

Net Ionic Equation

A net ionic equation focuses on the chemical species that actually change during the reaction. It eliminates spectator ions, which are ions that do not participate in forming the precipitate or the main chemical change. This simplifies the equation and highlights essential reactive components:

• For our reaction: \( \mathrm{Hg^{2+} \,(aq) + S^{2-} \,(aq) \rightarrow HgS \,(s)} \)

Here, the mercury ions \( \mathrm{Hg^{2+}} \) and sulfide ions \( \mathrm{S^{2-}} \) combine to form mercury(II) sulfide \( \mathrm{HgS} \), the precipitate. Spectator ions like \( \mathrm{NO_3^-} \) are not included, as they remain unchanged in the solution.

Strong Acid

In chemistry, strong acids are those that completely dissociate in water, releasing all their hydrogen ions. Nitric acid \( \mathrm{HNO_3} \) is a prime example of a strong acid, which means when it is in water, it releases its hydrogen ion \( \mathrm{H^+} \) completely:

• \( \mathrm{HNO_3 \rightarrow H^+ + NO_3^-} \)

This characteristic of complete ionization is essential for reactions where the strength of an acid impacts equilibrium and reaction rates. Knowing an acid's strength helps predict its behavior in chemical processes and reactions.

Precipitate Formation

Precipitation is a process where solid particles form in a solution when certain ionic compounds become insoluble. This occurs during reactions where two aqueous solutions are mixed and an insoluble compound forms. This solid is called a precipitate.

• In our reaction, mercury(II) sulfide \( \mathrm{HgS} \) forms as a precipitate when mercury ions \( \mathrm{Hg^{2+}} \) react with sulfide ions \( \mathrm{S^{2-}} \).

Precipitates are often less soluble at standard conditions and can be filtered out from the solution, revealing visible evidence of a chemical change. Understanding precipitate formation is crucial in predicting the outcomes of reactions, especially in fields like analytical chemistry and environmental science.