Problem 74 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is obtained when \(\mathrm{NH}_{4} \mathrm{OH}\) is added to \(\mathrm{AlCl}_{3}\) solution? (a) \(\mathrm{NH}_{3}\) (b) \(\mathrm{NH}_{4} \mathrm{Cl}\) (c) \(\mathrm{NH}_{4}\) (d) \(\mathrm{Al}(\mathrm{OH})_{3}\)

Step-by-Step Solution

Answer

(d) \( \mathrm{Al}(\mathrm{OH})_{3} \).

Step 1: Write the Reactants

Identify and write down the chemical formulas for the given reactants in the problem: \( \mathrm{NH}_{4} \mathrm{OH} \) and \( \mathrm{AlCl}_{3} \).

Step 2: Predict the Reaction

When ammonium hydroxide \( \mathrm{NH}_{4} \mathrm{OH} \) is added to aluminum chloride \( \mathrm{AlCl}_{3} \), a double displacement reaction takes place. The products formed are aluminum hydroxide \( \mathrm{Al(OH)}_{3} \) and ammonium chloride \( \mathrm{NH}_{4} \mathrm{Cl} \).

Step 3: Write the Balanced Chemical Equation

The balanced chemical equation for the reaction is: \[ \mathrm{AlCl}_{3} + 3 \mathrm{NH}_{4} \mathrm{OH} \rightarrow \mathrm{Al(OH)}_{3} + 3 \mathrm{NH}_{4} \mathrm{Cl} \] Here, the aluminum hydroxide \( \mathrm{Al(OH)}_{3} \) is produced as a precipitate.

Step 4: Identify the Main Product

The problem asks what is obtained when \( \mathrm{NH}_{4} \mathrm{OH} \) is added to \( \mathrm{AlCl}_{3} \). The main compound that forms as a precipitate in this reaction is \( \mathrm{Al(OH)}_{3} \), aluminum hydroxide.

Key Concepts

Double Displacement Reactions

Double displacement reactions are an exciting type of chemical reactions where two compounds exchange ions to form two new compounds. This swapping of ions can result in different products, including precipitates, gases, or water. In these reactions, the general form can be seen as:

• AB + CD → AD + CB

Here, AB and CD are the reactant compounds, and the cations and anions switch places. This swapping can often result in a precipitate, a solid that forms from the reaction and drops out of the solution.

In the given problem, when ammonium hydroxide (\(\mathrm{NH}_4 \mathrm{OH}\)) reacts with aluminum chloride (\(\mathrm{AlCl}_3\)), a double displacement reaction occurs. The ions involved in this reaction are the aluminum ions (\(\mathrm{Al}^{3+}\)), hydroxide ions (\(\mathrm{OH}^-\)), ammonium ions (\(\mathrm{NH}_4^+\)), and chloride ions (\(\mathrm{Cl}^-\)). By exchanging ions, aluminum hydroxide (\(\mathrm{Al(OH)}_3\)) and ammonium chloride (\(\mathrm{NH}_4 \mathrm{Cl}\)) are formed. The aluminum hydroxide emerges as a precipitate, representing a classic outcome of double displacement reactions.

Chemical Equations

Chemical equations are symbolic representations of chemical reactions. They show the reactants transforming into products. On one side, we have the reactants, and on the other, the resulting products. The arrangement is concise, illustrating the transformation through chemical notation.

In our problem scenario, the chemical equation involving (\(\mathrm{NH}_4 \mathrm{OH}\)) and (\(\mathrm{AlCl}_3\)) can be written as:

• \[ \mathrm{AlCl}_3 + 3 \mathrm{NH}_4 \mathrm{OH} \rightarrow \mathrm{Al(OH)}_3 + 3 \mathrm{NH}_4 \mathrm{Cl} \]

This equation expresses the combination and transformation of chemicals from initial reactants to their respective outcome products.

Key symbols used in equations often denote the physical states such as solid (s), liquid (l), gas (g), and aqueous solutions (aq). The plus sign indicates multiple reactants or products, while the arrow signifies the direction of the reaction—reactants becoming products. Chemical equations offer a simplified yet detailed view of reactions, providing insights into what components are involved, their proportions, and the resulting products.

Balancing Equations

Balancing chemical equations ensures that there are equal numbers of each type of atom on both sides of the equation, maintaining the law of conservation of mass. This is crucial because, in a closed system, matter is neither created nor destroyed in chemical reactions.

To balance a chemical equation, follow these steps:

• Identify each type of atom or ion involved in the reaction.
• Count the number of each type of atom in the reactants and the products.
• Adjust coefficients—these are numbers placed before compounds—to balance each type of atom across both sides of the equation.

In the case of our reaction with (\(\mathrm{AlCl}_3\)) and (\(\mathrm{NH}_4 \mathrm{OH}\)), the following balanced equation is achieved:

• \[ \mathrm{AlCl}_3 + 3 \mathrm{NH}_4 \mathrm{OH} \rightarrow \mathrm{Al(OH)}_3 + 3 \mathrm{NH}_4 \mathrm{Cl} \]

The equation shows that to form one molecule of aluminum hydroxide, three ammonium hydroxide molecules are needed. Similarly, three ammonium chloride molecules are produced. Coefficients ensure that the number of atoms for elements like aluminum, chlorine, nitrogen, hydrogen, and oxygen are balanced across both the reactants and products. Balancing equations is crucial to accurately represent what happens in chemical reactions, adhering to fundamental scientific laws.