Problem 93 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is formed when mercurous nitrate reacts with \(\mathrm{NaCl}\) ? (a) \(\mathrm{Hg}_{2} \mathrm{Cl}_{2} \cdot 2 \mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{Hg}\left(\mathrm{Na}_{2}\right) \mathrm{Cl}\) (c) \(\mathrm{HgCl}_{2}\) (d) \(\mathrm{Hg}_{2} \mathrm{Cl}_{2}\)

Step-by-Step Solution

Answer

The reaction forms \( \text{Hg}_2\text{Cl}_2 \), option (d).

Step 1: Identify the Reactants and Their Ions

The reactants are mercurous nitrate, which is \( \text{Hg}_2(\text{NO}_3)_2 \), and sodium chloride, \( \text{NaCl} \). In solution, these dissociate into their constituent ions, \( \text{Hg}_2^{2+} \), \( \text{NO}_3^- \), \( \text{Na}^+ \), and \( \text{Cl}^- \).

Step 2: Determine Possible Reaction Products

When the ions from mercurous nitrate and sodium chloride interact, the possible precipitate could be formed with \( \text{Hg}_2^{2+} \) and \( \text{Cl}^- \), leading to \( \text{Hg}_2\text{Cl}_2 \). The other ions, \( \text{Na}^+ \) and \( \text{NO}_3^- \), remain in solution as they form a soluble salt, \( \text{NaNO}_3 \).

Step 3: Write the Chemical Equation

The chemical equation for the reaction is: \[ \text{Hg}_2(\text{NO}_3)_2 (aq) + 2 \text{NaCl} (aq) \rightarrow \text{Hg}_2\text{Cl}_2 (s) + 2 \text{NaNO}_3 (aq) \]\( \text{Hg}_2\text{Cl}_2 \) is the solid precipitate formed in this reaction.

Step 4: Confirm the Product

Based on the equation, the precipitate formed, \( \text{Hg}_2\text{Cl}_2 \), matches option (d). The others are either incorrect compounds or involve incorrect stoichiometry of the reaction.

Key Concepts

Chemical Reactions

Chemical reactions are the processes where substances, called reactants, interact to form new substances known as products. In every chemical reaction, the atoms of the reactants are rearranged to create something different.
These reactions are foundational to understanding chemistry and how substances interact at a molecular level. In our specific example, mercurous nitrate reacts with sodium chloride. During this interaction, the ions from these compounds rearrange to form new products.

Key points about chemical reactions include:

• Conservation of Matter: The total mass of reactants equals the total mass of products.
• Energy Changes: Reactions may release or absorb energy.
• Reaction Rate: Different reactions occur at varying speeds, influenced by factors like temperature and concentration.

Recognizing the nature of these reactions, such as whether they result in a precipitate or gas, is crucial. This understanding can predict the products formed as seen in the reaction between mercurous nitrate and sodium chloride.

Precipitation Reactions

Precipitation reactions occur when two aqueous solutions combine, and one of the products is an insoluble solid called a precipitate. This solid falls out of the solution as it is not soluble in water.
An understanding of solubility rules helps to predict whether a precipitate will form in a reaction.
In the reaction between mercurous nitrate and NaCl, mercurous chloride \(\text{Hg}_2\text{Cl}_2\) precipitates out as it is insoluble in water, forming the solid observed. This reaction exemplifies the classic precipitation reaction:

• Two compounds in aqueous solution, mercurous nitrate and sodium chloride, dissociate into ions.
• When mixed, mercurous ions \(\text{Hg}_2^{2+}\) and chloride ions \(\text{Cl}^-\) combine to form mercurous chloride, a solid precipitate.

These reactions are common in aqueous chemistry and important in processes like water treatment and the formation of minerals.

Chemical Equations

Chemical equations are symbolic representations of chemical reactions. They show how reactants convert to products using chemical symbols and formulas. Writing a balanced chemical equation ensures that the number of atoms for each element is the same on both sides, complying with the law of conservation of mass.
In our reaction, the balanced chemical equation is:\[ \text{Hg}_2(\text{NO}_3)_2 (aq) + 2 \text{NaCl} (aq) \rightarrow \text{Hg}_2\text{Cl}_2 (s) + 2 \text{NaNO}_3 (aq) \]

• Aqueous (aq) denotes that the substances are dissolved in water.
• Solid (s) indicates the formation of a precipitate, as with \(\text{Hg}_2\text{Cl}_2\).

Interpreting chemical equations allows chemists to visualize the reactants and products and understand the stoichiometry—the relative quantities of reactants and products in a reaction. This helps predict how much product can be formed from the given reactants.