Problem 77 - Chemistry and Chemical Reactivity

Question

Identify, from each list below, the compound or compounds that will dissolve in water to give a solution that is only a very weak conductor of electricity. (a) \(\mathrm{NH}_{3}, \mathrm{NaOH}, \mathrm{Ba}(\mathrm{OH})_{2}, \mathrm{Fe}(\mathrm{OH})_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}, \mathrm{Na}_{3} \mathrm{PO}_{4}, \mathrm{HF}, \mathrm{HNO}_{3}\)

Step-by-Step Solution

Answer

(a) \( \mathrm{NH}_{3} \); (b) \( \mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H} \) and \( \mathrm{HF} \).

Step 1: Determine the Nature of Compounds in List (a)

Examine each compound to decide if it is an electrolyte or non-electrolyte. - **\(\mathrm{NH}_{3}\):** Ammonia is a weak base, partially ionizing in water, resulting in a weakly conducting solution.- **\(\mathrm{NaOH}\):** This is a strong base and a strong electrolyte, fully ionizing in water.- **\(\mathrm{Ba(OH)}_{2}\):** Like NaOH, it is a strong base and strong electrolyte, fully ionizing in water.- **\(\mathrm{Fe(OH)}_{3}\):** This is a weakly soluble and weak electrolyte, but mainly considered as a precipitate.

Step 2: Identify Weak Conductors from List (a)

From the analysis:- Both \(\mathrm{NaOH}\) and \(\mathrm{Ba(OH)}_{2}\) are strong electrolytes.- \(\mathrm{Fe(OH)}_{3}\) is not significant due to low solubility.- **Answer:** \(\mathrm{NH}_{3}\) is the weak conductor as it is a weak electrolyte.

Step 3: Determine the Nature of Compounds in List (b)

Examine each compound to decide if it is an electrolyte or non-electrolyte.- **\(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\):** Acetic acid is a weak acid, partially ionizing in water, resulting in a weakly conducting solution.- **\(\mathrm{Na}_{3} \mathrm{PO}_{4}\):** This is a strong electrolyte, fully ionizing in water.- **\(\mathrm{HF}\):** Hydrofluoric acid is a weak acid, weakly ionizing in water, providing weak conductivity.- **\(\mathrm{HNO}_{3}\):** This is a strong acid, fully ionizing in water.

Step 4: Identify Weak Conductors from List (b)

From the analysis:- \(\mathrm{Na}_{3} \mathrm{PO}_{4}\) and \(\mathrm{HNO}_{3}\) are strong electrolytes.- Both \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) and \(\mathrm{HF}\) are weak acids and weak electrolytes.- **Answer:** Both \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) (acetic acid) and \(\mathrm{HF}\) (hydrofluoric acid) are weak conductors of electricity.

Key Concepts

Understanding Ammonia as a Weak Electrolyte

Ammonia (\(\text{NH}_{3}\) ) is a common chemical compound often encountered in household cleaning products and industrial processes. However, one of its key chemical attributes is its behavior as a weak electrolyte.

When ammonia is dissolved in water, it partially ionizes. This means that not all ammonia molecules dissociate into ions. In water, ammonia takes up a proton (\(\text{H}^{+}\)) from water molecules, forming ammonium ions (\(\text{NH}_4^+\)) and hydroxide ions (\(\text{OH}^-\)). The equation for this reaction is represented as:\[\text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-\]

Since only a small fraction of ammonia exists as ions in solution, the resulting conductivity of the solution is low. Therefore, it is considered a weak electrolyte, making ammonia solutions only weakly conductive to electricity.
This partial ionization is also why ammonia is categorized as a weak base.

In summary, the essential points to note about ammonia's behavior include:

• Partial ionization in water, resulting in a low concentration of ions.
• Formation of ammonium and hydroxide ions.
• Weak conductivity due to limited ionization.

Acetic Acid as a Weak Acid and Electrolyte

Acetic acid (\(\text{CH}_3\text{CO}_2\text{H}\) ), known for its presence in vinegar, behaves distinctively in aqueous solutions because it is a weak acid. This means it only partially ionizes when dissolved in water, resulting in fewer free ions to carry electrical current.

When acetic acid comes into contact with water, it donates a proton (\(\text{H}^+\)) to form acetate ions (\(\text{CH}_3\text{COO}^-\)) and hydronium ions (\(\text{H}_3\text{O}^+\)). The chemical representation is:\[\text{CH}_3\text{CO}_2\text{H} + \text{H}_2\text{O} \rightleftharpoons \text{CH}_3\text{COO}^- + \text{H}_3\text{O}^+ \]

The limited amount of ionization suggests that the concentration of ions is low, leading to weak electrical conductivity. It categorizes acetic acid as a weak electrolyte. It's important to note that, despite its weak conductivity, acetic acid solutions still exhibit some of the acidic reactions typical of stronger acids, though to a lesser extent.

Key characteristics of acetic acid include:

• Partial ionization in water, resulting in low ion concentration.
• Formation of hydronium and acetate ions, contributing to weak conductivity.
• Typical behaviors of acids such as donating protons albeit with less reactivity.

Characteristics of Hydrofluoric Acid as a Weak Electrolyte

Hydrofluoric acid (\(\text{HF}\)) is a unique acid, known for its acute chemical reactivity and ability to etch glass. Despite its aggressive behavior in certain reactions, hydrofluoric acid is classified as a weak electrolyte because it only partially ionizes in aqueous solutions.

In water, hydrofluoric acid dissociates into hydrogen ions (\(\text{H}^+\)) and fluoride ions (\(\text{F}^-\)). Unlike strong acids, where nearly all molecules ionize, \(\text{HF}\) retains a significant proportion of its molecules in a unionized form:\[\text{HF} \rightleftharpoons \text{H}^+ + \text{F}^-\]

The limited degree of ionization results in a relatively low ionic concentration, which contributes to its weak conductivity. Even though hydrofluoric acid is weakly ionized, it is highly corrosive due to the unique properties of the \(\text{F}^-\) ion. Its chemistry allows it to penetrate deep into materials, which is why it must be handled with extreme care.

Summary points about hydrofluoric acid include:

• Partial ionization with restrained electric conductivity.
• Formation of hydrogen and fluoride ions in water.
• High reactivity despite being a weak electrolyte and strong corrosiveness.