Problem 149 - Chemistry

Question

When chlorine reacts with water, the resulting solution is weakly acidic and reacts with \(\mathrm{AgNO}_{3}\) to give a white precipitate. Write balanced equations to represent these reactions. Explain why manufacturers of household bleaches add bases such as \(\mathrm{NaOH}\) to their products to increase their effectiveness.

Step-by-Step Solution

Answer

Chlorine reacts with water to form HCl and HOCl. HOCl reacts with AgNO3 to form a white AgCl precipitate. Bases like NaOH enhance bleach effectiveness by forming stable NaOCl.

Step 1: Reaction of Chlorine with Water

When chlorine gas \( \text{Cl}_2 \) is added to water, it reacts to form hydrochloric acid (HCl) and hypochlorous acid (HOCl). The balanced chemical equation representing this reaction is:\[ \text{Cl}_2 + \text{H}_2\text{O} \rightarrow \text{HCl} + \text{HOCl} \] Both HCl and HOCl contribute to the weakly acidic nature of the solution.

Step 2: Reaction of Hypochlorous Acid with Silver Nitrate

Hypochlorous acid (HOCl) reacts with silver nitrate (\( \text{AgNO}_3 \)) to form a white precipitate of silver chloride (\( \text{AgCl} \)) and nitric acid (HNO3). The balanced equation for this reaction is:\[ \text{HOCl} + \text{AgNO}_3 \rightarrow \text{AgCl} + \text{HNO}_3 \]The precipitate of \( \text{AgCl} \) is white, confirming the presence of chloride ions in the solution.

Step 3: Importance of Adding Bases to Bleaches

Manufacturers add bases like \( \text{NaOH} \) to household bleaches to increase their effectiveness. The base reacts with hypochlorous acid to form sodium hypochlorite (\( \text{NaOCl} \)), which is a more stable bleaching agent. The reaction is:\[ \text{HOCl} + \text{NaOH} \rightarrow \text{NaOCl} + \text{H}_2\text{O} \]Sodium hypochlorite is a powerful oxidizing agent, thus enhancing the bleaching and disinfecting properties of the solution.

Key Concepts

Acid-Base Reactions

Acid-base reactions are central to many chemical processes, including the reactions of chlorine with water. When chlorine gas \( \text{Cl}_2 \) is introduced to water, it forms hydrochloric acid (HCl) and hypochlorous acid (HOCl). This transformation exemplifies an acid-base reaction, where water acts as a base, accepting chlorine to form these acids.
The acids formed (HCl and HOCl) are responsible for the weakly acidic characteristic of the resultant solution.

• HCl is a strong acid that fully dissociates in water, contributing hydrogen ions \( \text{H}^+ \).
• HOCl, however, is a weak acid, partly dissociating to provide some hydrogen ions.
  This partial dissociation characterizes it as a weak acid in the solution.

When manufacturers add sodium hydroxide \( \text{NaOH} \) to bleach, the interaction illustrates another acid-base reaction. Here, \( \text{NaOH} \), a strong base, neutralizes HOCl, enhancing bleach stability and effectiveness.

Chemical Equations

Chemical equations succinctly describe chemical reactions using symbols and formulas. In the reaction of chlorine with water, the chemical equation is \( \text{Cl}_2 + \text{H}_2\text{O} \rightarrow \text{HCl} + \text{HOCl} \).
This balanced equation illustrates the reactants (chlorine and water) transforming into products (hydrochloric acid and hypochlorous acid). The equation respects the law of conservation of mass, showing that the number of each type of atom is the same on both sides of the equation.
Balancing chemical equations:

• Involves adjusting coefficients to have equal numbers of each atom on both sides.
• Ensures mass and charge conservation during reactions.

Another example is the reaction of hypochlorous acid with silver nitrate, balanced as \( \text{HOCl} + \text{AgNO}_3 \rightarrow \text{AgCl} + \text{HNO}_3 \).
These equations provide a clear and concise description of the chemical changes occurring during the reactions.

Hypochlorous Acid Reactions

Hypochlorous acid (HOCl) plays a crucial role in several chemical reactions. One key reaction involves its interaction with silver nitrate (\( \text{AgNO}_3 \)).
In this reaction, HOCl reacts with silver nitrate to produce a white precipitate of silver chloride (\( \text{AgCl} \)) and nitric acid (HNO3). The balanced chemical equation for this process is:\[ \text{HOCl} + \text{AgNO}_3 \rightarrow \text{AgCl} + \text{HNO}_3 \]This reaction is vital as it confirms the presence of chloride ions through the formation of a solid precipitate. The white silver chloride precipitate is easily observable, making it a handy test for chloride ions.
Moreover, hypochlorous acid can react with bases like sodium hydroxide to form sodium hypochlorite (\( \text{NaOCl} \)), a more stable and effective bleaching agent.

• This reaction is represented by \( \text{HOCl} + \text{NaOH} \rightarrow \text{NaOCl} + \text{H}_2\text{O} \).
• It highlights how chemical transformations can enhance chemical properties for practical applications, such as in cleaning products.

Silver Nitrate Reactions

Silver nitrate (\( \text{AgNO}_3 \)) is a reactive compound, often used to test for halides like chloride ions. When silver nitrate comes into contact with hypochlorous acid (HOCl) in this context, it results in a reaction that produces a distinct white precipitate of silver chloride (\( \text{AgCl} \)).
This precipitate formation is described by the balanced chemical equation:\[ \text{HOCl} + \text{AgNO}_3 \rightarrow \text{AgCl} + \text{HNO}_3 \]The ability of silver nitrate to form a precipitate with chloride ions is frequently exploited in qualitative analysis to detect the presence of such ions.
Besides its role in confirming chloride ions, \( \text{AgNO}_3 \) participates in other reactions:

• It can combine with other halides to form respective silver halide precipitates, which are distinctly colored.
• This behavior makes it a versatile compound in analytical chemistry, particularly in titrations and gravimetric analysis.

Understanding these reactions helps in recognizing how they are applied in real-world scenarios, especially in laboratory settings.