Problem 1 - Chemistry and Chemical Reactivity

Question

Does the \(\mathrm{pH}\) of the solution increase, decrease, or stay the same when you (a) add solid ammonium chloride to a dilute aqueous solution of \(\mathrm{NH}_{3} ?\) (b) add solid sodium acetate to a dilute aqueous solution of acetic acid? (c) add solid NaCl to a dilute aqueous solution of NaOH?

Step-by-Step Solution

Answer

(a) Decrease, (b) Increase, (c) Stay the same.

Step 1: Understand the Concept of pH Change

The pH of a solution changes when substances that affect the concentration of hydrogen ions are added. Adding acids increases the hydrogen ion concentration, decreasing pH, while bases decrease the hydrogen ion concentration, increasing pH. Salts can either increase or decrease pH depending on the ions they produce in solution.

Step 2: Analyze Scenario (a): Adding Ammonium Chloride to Ammonia Solution

Ammonium chloride \((NH_4Cl)\) is a salt that dissolves in water to form ammonium ions \((NH_4^+)\) and chloride ions \((Cl^-)\). The ammonium ion is a weak acid and will release hydrogen ions \((H^+)\) in solution, decreasing pH. The addition of \(NH_4Cl\) to \(NH_3\), which is a weak base, will establish an equilibrium, thereby decreasing the pH.

Step 3: Analyze Scenario (b): Adding Sodium Acetate to Acetic Acid Solution

Sodium acetate \((CH_3COONa)\) is a salt that dissolves in water to produce acetate ions \((CH_3COO^-)\) and sodium ions \((Na^+)\). The acetate ion acts as a weak base that reacts with hydrogen ions \((H^+)\), reducing their concentration in solution. This reaction will cause an increase in pH when \(CH_3COONa\) is added to acetic acid, as the acetate ions will partially neutralize the acid.

Step 4: Analyze Scenario (c): Adding NaCl to NaOH Solution

Sodium chloride \((NaCl)\) dissociates in water into sodium ions \((Na^+)\) and chloride ions \((Cl^-)\). Neither of these ions has an effect on the pH because \(NaCl\) is a neutral salt. Thus, adding \(NaCl\) to a sodium hydroxide \((NaOH)\) solution does not change the concentration of hydroxide ions \((OH^-)\) and hence does not change pH.

Key Concepts

Ammonium Chloride

Ammonium chloride, or \(NH_4Cl\), is a salt formed from the reaction of ammonia and hydrochloric acid. When dissolved in water, it separates into ammonium ions \((NH_4^+)\) and chloride ions \((Cl^-)\). The ammonium ion is an interesting player because it behaves as a weak acid in aqueous solutions.

When an ammonium ion comes into play, it donates protons \((H^+)\) to the solution, effectively lowering the solution's pH. This makes the solution more acidic than it originally was. In the context of a weak base like ammonia \((NH_3)\), adding ammonium chloride establishes a buffer system. This system strives to keep changes in pH minimal by balancing added acids or bases.

• Behavior in solution: Acts as a source of hydrogen ions, decreasing pH.
• Impact on weak base ammonia: Leads to a decrease in pH when ammonium chloride is added.

Understanding this interaction helps explain why adding ammonium chloride to ammonia doesn't make it more basic, but rather slightly acidic.

Sodium Acetate

Sodium acetate \((CH_3COONa)\) is widely acknowledged for its role as a "conjugate base" in chemistry. In water, it dissociates into sodium ions \((Na^+)\) and acetate ions \((CH_3COO^-\)). Here, the acetate ion acts in an alkaline manner, neutralizing excess hydrogen ions in solutions.

When you add sodium acetate to a solution containing acetic acid, you have an intriguing reaction. Acetic acid, a weak acid, partially dissociates in water. The presence of acetate ions prompts the reaction towards forming more undissociated acetic acid, thereby reducing the \(H^+\) concentration.

• Effect on pH: Acetate ions decrease hydrogen ion concentration, increasing pH.
• Context with acetic acid: Adding sodium acetate raises the pH by diminishing the acidity.

This action results in an overall pH increase, depicting the nature of acid-base equilibrium at work.

Neutral Salts

Neutral salts, such as sodium chloride \((NaCl)\), stand in a league of their own. When they dissolve in water, they release \(Na^+\) and \(Cl^-\) ions but have no effect on the pH of the solution. This is because neither ion reacts with water, leaving the \(H^+\) and \(OH^-\) ions in the water unaltered.

In scenarios where sodium chloride is introduced to a solution of sodium hydroxide \((NaOH)\), the solution's pH remains stable. Sodium hydroxide is already a strong base, providing \(OH^-\) ions to increase the solution's pH, but the addition of sodium chloride does not influence this concentration. The pH remains constant as there is no change in the hydroxide ion concentration.

• Characteristics: Do not alter \(H^+\) or \(OH^-\) concentrations in solution.
• Example with NaOH: No change in pH upon adding sodium chloride.

This neutrality is definitive, ensuring that neutral salts are crucial when no pH change is desired in a solution.