Question

Indicate whether aqueous solutions of each of the following will contain only ions, only molecules, or mostly molecules and a few ions: a. \(\mathrm{NH}_{4} \mathrm{Cl}\), a strong electrolyte b. ethanol, \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\), a nonelectrolyte c. HCN, hydrocyanic acid, a weak electrolyte

Short answer

a) Only ions. b) Only molecules. c) Mostly molecules, few ions.

Step-by-step solution

Analyze \(\mathrm{NH}_{4} \, \mathrm{Cl}\)

Determine the nature of \(\mathrm{NH}_{4} \, \mathrm{Cl}\). Since it is a strong electrolyte, it will completely dissociate into ions in an aqueous solution. Therefore, \(\mathrm{NH}_{4} \, \mathrm{Cl}\) in water will contain only ions.

Analyze Ethanol (\(\mathrm{C}_{2} \, \mathrm{H}_{5} \, \mathrm{OH}\))

Determine the nature of ethanol. As a nonelectrolyte, ethanol does not dissociate into ions in an aqueous solution. Therefore, \(\mathrm{C}_{2} \, \mathrm{H}_{5} \, \mathrm{OH}\) in water will contain only molecules.

Analyze HCN

Determine the nature of hydrocyanic acid \(\mathrm{HCN}\). As a weak electrolyte, it only partially dissociates in an aqueous solution, leading to mostly molecules with some ions present.

Key concepts

strong electrolytes

Strong electrolytes are substances that completely dissociate into ions when dissolved in water. This means they break apart fully into their constituent ions, leaving no undissociated molecules in the solution.

For instance, when \begin{inl \(\text {NH}_{4} \text {Cl}\begin{inl} (ammonium chloride) is dissolved in water, it separates entirely into \begin{inl \)\text {NH}_{4}^{+}\begin{inl} (ammonium) and \begin{inl \(\text {Cl}^{-}\begin{inl} (chloride) ions. Because it fully dissociates, an aqueous solution of \begin{inl \)\text {NH}_{4} \text {Cl}\begin{inl} contains only ions.

Strong electrolytes are usually ionic compounds like salts (\begin{inl \(\text {NaCl}\begin{inl}, \begin{inl \)\text {KBr}\begin{inl}), strong acids (\begin{inl \(\text {HCl}\begin{inl}, \begin{inl \)\text {H_2SO_4}\begin{inl}), and strong bases (\begin{inl \(\text {NaOH}\begin{inl}, \begin{inl \)\text {KOH}\begin{inl}). These substances are excellent conductors of electricity due to the presence of free-moving ions that carry electric charge through the solution.

• Fully dissociates into ions
• No intact molecules remain
• Excellent conductors of electricity

nonelectrolytes

Nonelectrolytes are substances that do not dissociate into ions when dissolved in water. Instead, they remain intact as molecules and therefore do not produce any ions in the solution. As a result, nonelectrolytes do not conduct electricity.

For example, when ethanol (\begin{inl \(\text {C_2H_5OH}\begin{inl} is dissolved in water, it stays as \begin{inl \)\text {C_2H_5OH}\begin{inl} molecules rather than breaking apart into ions. This means that in an aqueous solution of ethanol, only molecules are present.

Common examples of nonelectrolytes include most organic compounds such as sugars (\begin{inl \(\text {C_6H_12O_6}\begin{inl}, urea (\begin{inl \)\text {(NH_2)_2CO}\begin{inl}, and many alcohols besides ethanol.

• Do not dissociate into ions
• Remain as intact molecules
• Do not conduct electricity

weak electrolytes

Weak electrolytes are substances that only partially dissociate into ions in water. This means that in an aqueous solution, both ions and intact molecules are present. Because of this partial dissociation, weak electrolytes are poor conductors of electricity.

For instance, hydrocyanic acid (\begin{inl \(\text {HCN}\begin{inl} is a weak electrolyte. When \begin{inl \)\text {HCN}\begin{inl} dissolves in water, only a small fraction of the molecules dissociate into \begin{inl \(\text {H}^+\begin{inl} (hydrogen) ions and \begin{inl \)\text {CN}^-\begin{inl} (cyanide) ions, while the majority of molecules remain undissociated. Hence, an aqueous solution of \begin{inl \(\text {HCN}\begin{inl} contains mostly molecules with a few ions.

Common examples of weak electrolytes include weak acids (\begin{inl \)\text {CH_3COOH}\begin{inl} - acetic acid, \begin{inl \(\text {HF}\begin{inl} - hydrofluoric acid) and weak bases (\begin{inl \)\text {NH_3}\begin{inl} - ammonia).

• Partially dissociate into ions
• Contain both ions and molecules
• Poor conductors of electricity