Problem 139 - 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 \(81.15 \mathrm{~g}\) 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?

Step-by-Step Solution

Answer

The solution mass after the reaction is 581.51 g; nitric acid (strong acid) is formed.

Step 1: Identify Reactants and Products

Mercury(II) nitrate is written as \(\text{Hg(NO}_3\text{)}_2\) and hydrogen sulfide is \(\text{H}_2\text{S}\). When they react, mercury(II) sulfide \(\text{HgS}\) is formed as a precipitate, along with nitric acid \(\text{HNO}_3\).

Step 2: Write the Molecular Equation

The molecular equation for the reaction is:\[\text{Hg(NO}_3\text{)}_2 + \text{H}_2\text{S} \rightarrow \text{HgS} \downarrow + 2\text{HNO}_3\]The downward arrow indicates that \(\text{HgS}\) forms as a precipitate.

Step 3: Write the Net Ionic Equation

The net ionic equation involves the ions directly participating in the formation of the precipitate:\[\text{Hg}^{2+} + \text{S}^{2-} \rightarrow \text{HgS} \downarrow\]Only the ions that form the precipitate are included.

Step 4: Identify the Acid Formed and Its Strength

The acid formed in this reaction is nitric acid, \(\text{HNO}_3\). Nitric acid is considered a strong acid because it completely dissociates in water.

Step 5: Determine the Mass of the Solution After Reaction

The initial mass of all components (reactants and water) is:\[81.15\,\text{g} + 8.52\,\text{g} + 550.0\,\text{g} = 639.67\,\text{g}\]The mass of the precipitate formed is 58.16 g. Thus, the mass of the solution after the reaction is:\[639.67\,\text{g} - 58.16\,\text{g} = 581.51\,\text{g}\]

Key Concepts

Mercury(II) nitrate

Mercury(II) nitrate is a chemical compound with the formula \( \text{Hg(NO}_3\text{)}_2 \), where mercury is in the +2 oxidation state. It is an important compound in chemistry due to its role in certain types of reactions, such as precipitation reactions. Mercury(II) nitrate is used primarily for its ability to produce other mercury compounds and is often employed in laboratory experiments.
When it reacts with other substances, mercury(II) nitrate can form precipitates, which are solid compounds that come out of solution. Its reaction with hydrogen sulfide, for example, results in the formation of mercury(II) sulfide (\( \text{HgS} \)), which is a red or black precipitate depending on its crystalline form. This property makes mercury(II) nitrate a valuable reagent in analytical chemistry.

Hydrogen sulfide

Hydrogen sulfide (\( \text{H}_2\text{S} \)) is a colorless, toxic gas known for its characteristic odor of rotten eggs. It is produced naturally by the decomposition of organic matter and in some geothermal and volcanic emissions.
In the context of our chemical reaction, hydrogen sulfide acts as a sulfide ion donor. When it reacts with mercury(II) nitrate, the sulfide ions (\( \text{S}^{2-} \)) combine with mercury ions (\( \text{Hg}^{2+} \)) to form mercury(II) sulfide (\( \text{HgS} \)), resulting in a solid precipitate. This reaction is a typical example of a precipitation reaction, which is used to separate substances and purify solutions in chemical processes.

Net ionic equation

A net ionic equation simplifies chemical equations by showing only the species that undergo a chemical change. This means that only the ions or molecules that participate actively in the formation of a precipitate or a gas are included.
For the reaction between mercury(II) nitrate and hydrogen sulfide, the net ionic equation is:\[\text{Hg}^{2+} + \text{S}^{2-} \rightarrow \text{HgS} \downarrow\]This equation highlights the essential ions that react to form the mercury(II) sulfide precipitate. Spectator ions, like nitrate and hydrogen ions from the original compounds that do not participate in the reaction, are omitted. Understanding net ionic equations helps simplify chemical reactions and highlights the actual chemical processes at work. It also makes it easier to predict the products of different reactions.

Strong acids

Strong acids are acids that completely dissociate into their ions in water. This means that when dissolved, a strong acid releases all of its hydrogen ions (\( \text{H}^+ \)) into the solution. This complete dissociation increases the acidity of the solution significantly.
In the given reaction, the strong acid formed is nitric acid (\( \text{HNO}_3 \)). Nitric acid is well-known for its strong acidic properties, and it plays an important role in various chemical processes, including nitration reactions and the manufacture of fertilizers and explosives.
Understanding the properties of strong acids like nitric acid is crucial in predicting their behavior in reactions. Their ability to fully dissociate makes them highly reactive and effective in catalyzing or driving chemical processes.