Problem 4 - General Chemistry: Principles and Modern Applications

Question

\(\mathrm{NH}_{3}(\mathrm{aq})\) conducts electric current only weakly. The same is true for \(\mathrm{CH}_{3} \mathrm{COOH}(\mathrm{aq}) .\) When these solutions are mixed, however, the resulting solution is a good conductor. How do you explain this?

Step-by-Step Solution

Answer

The weak electrolytes \(NH_3\) and \(CH_3COOH\) react together in an acid-base reaction to form ions, \(NH_4^+\) and \(CH3COO^-\), which increase the ion concentration in the solution. This leads to the solution becoming a good conductor of electricity.

Step 1: Understanding Weak Electrolytes

Weak electrolytes are substances that only partially ionize in their aqueous solutions, producing only a small number of ions. \(NH_3(aq)\) and \(CH_3COOH(aq)\) are both examples of weak electrolytes. When these are separate solutions, they generate few ions and conduct electricity only weakly.

Step 2: Acid-Base Reactions

When these two solutions are mixed, an acid-base reaction occurs. \(NH_3\) is a weak base and \(CH_3COOH\) is a weak acid. They react to form ionized entities, that is, \(NH_4^+\) ions and \(CH3COO^-\) ions, which contribute to increased ion concentration in the solution.

Step 3: Increased Conductivity

The increased number of ions in the solution leads to an increased ability to conduct electricity. This is the reason why, when these two weak electrolytes are mixed, the resulting solution becomes a good conductor of electricity.

Key Concepts

Weak Electrolytes

When we talk about weak electrolytes, we are referring to substances that only partially dissociate into ions in water. This means that only a fraction of the molecules split to form ions, while the rest remain as neutral molecules.

• Ammonia (\( \text{NH}_3\text{(aq)} \)) and acetic acid (\( \text{CH}_3\text{COOH} \)) are classic examples of weak electrolytes.
• In their separate solutions, these weak electrolytes generate only a small number of ions.
• As a result, these solutions conduct electricity weakly.

In essence, weak electrolytes do not produce enough ions to carry a significant electric current compared to strong electrolytes like hydrochloric acid (\( \text{HCl} \)), which fully dissociate in solution.

Acid-Base Reactions

In chemistry, an acid-base reaction typically involves the transfer of protons between reactants. When you mix ammonia, a weak base, with acetic acid, a weak acid, they participate in an acid-base reaction.

• Ammonia (\( \text{NH}_3 \)) acts as a proton acceptor (base).
• Acetic acid (\( \text{CH}_3\text{COOH} \)) donates a proton (acid).
• As a result, new ions like\( \text{NH}_4^+ \) (ammonium) and\( \text{CH}_3\text{COO}^- \) (acetate) are formed.

This reaction increases the ionic concentration in the solution, as demonstrated by the equation:\[ \text{NH}_3 + \text{CH}_3\text{COOH} \rightarrow \text{NH}_4^+ + \text{CH}_3\text{COO}^- \]

Ionization

Ionization refers to the process by which neutral molecules are converted into charged ions. This is significant in the context of acid-base reactions and electrolyte solutions.

• For weak electrolytes like ammonia and acetic acid, ionization is incomplete when they are alone in solution.
• Upon mixing, however, the ionization increases due to the acid-base reaction.

Understanding ionization helps clarify why a combination of two weak electrolytes can result in a more conductive mixture. The increase in ions directly correlates with an increase in the solution's ability to conduct electrical current.

Electrical Conductivity in Solutions

Electrical conductivity in solutions is a measure of a solution's ability to conduct electricity, due to the presence of ions. The more ions available in a solution, the better it will conduct electricity.

• Initially, ammonia and acetic acid, being weak electrolytes, provide fewer ions.
• However, when mixed, the generation of\( \text{NH}_4^+ \) and\( \text{CH}_3\text{COO}^- \) ions enhances the conductivity.
• This increase in ionic presence is what transforms a weakly conductive solution into a good conductor.

The solution now acts as a medium that allows ions to move and transmit electric current effectively, explaining why mixing weak electrolytes can significantly enhance electrical conductivity.