Problem 18 - Chemistry: The Central Science in SI Units, Global Edition

Question

Specify what ions are present upon dissolving each of the following substances in water: (a) \(\mathrm{HIO}_{3},\) (b) \(\mathrm{Ba}(\mathrm{OH})_{2},\) (c) HCN, (d) \(\mathrm{CuSO}_{4}\).

Step-by-Step Solution

Answer

Upon dissolving the given compounds in water, the following ions are formed: (a) \(\mathrm{HIO}_3\): \(\mathrm{H}^+\) and \(\mathrm{IO}_3^-\) (b) \(\mathrm{Ba(OH)_2}\): \(\mathrm{Ba}^{2+}\) and \(\mathrm{OH}^-\) (c) HCN: \(\mathrm{H}^+\) and \(\mathrm{CN}^-\) (in equilibrium) (d) \(\mathrm{CuSO}_4\): \(\mathrm{Cu}^{2+}\) and \(\mathrm{SO}_4^{2-}\)

Step 1: (a) Dissolving \(\mathrm{HIO}_3\)

When \(\mathrm{HIO}_3\) (iodic acid) is dissolved in water, it dissociates into its individual ions: a hydrogen ion (\(\mathrm{H}^+\)) and an iodate ion (\(\mathrm{IO}_3^-\)). The equation for this process is: \[\mathrm{HIO}_3(aq) \rightarrow \mathrm{H}^+(aq) + \mathrm{IO}_3^-(aq)\]

Step 2: (b) Dissolving \(\mathrm{Ba(OH)_2}\)

Barium hydroxide, \(\mathrm{Ba(OH)_2}\), is a strong base and dissociates completely in water. It forms a barium ion (\(\mathrm{Ba}^{2+}\)) and two hydroxide ions (\(\mathrm{OH}^-\)). The equation for this process is: \[\mathrm{Ba(OH)_2}(aq) \rightarrow \mathrm{Ba}^{2+}(aq) + 2 \mathrm{OH}^-(aq)\]

Step 3: (c) Dissolving HCN

Hydrogen cyanide (HCN) is a weak acid, meaning it does not dissociate completely in water. However, when it does dissociate, it forms a hydrogen ion (\(\mathrm{H}^+\)) and a cyanide ion (\(\mathrm{CN}^-\)). The equilibrium reaction is represented as: \[\mathrm{HCN}(aq) \rightleftharpoons \mathrm{H}^+(aq) + \mathrm{CN}^-(aq)\]

Step 4: (d) Dissolving \(\mathrm{CuSO}_4\)

Copper(II) sulfate (\(\mathrm{CuSO}_4\)) is an ionic compound that dissociates completely in water. It forms a copper ion (\(\mathrm{Cu}^{2+}\)) and a sulfate ion (\(\mathrm{SO}_4^{2-}\)). The equation for this process is: \[\mathrm{CuSO}_4(aq) \rightarrow \mathrm{Cu}^{2+}(aq) + \mathrm{SO}_4^{2-}(aq)\] In summary, upon dissolving these compounds in water, the following ions are formed: (a) \(\mathrm{H}^+\), \(\mathrm{IO}_3^-\) (b) \(\mathrm{Ba}^{2+}\), \(\mathrm{OH}^-\) (c) \(\mathrm{H}^+\), \(\mathrm{CN}^-\) (in equilibrium) (d) \(\mathrm{Cu}^{2+}\), \(\mathrm{SO}_4^{2-}\)

Key Concepts

Acid-Base Reactions

In chemistry, acid-base reactions involve the transfer of a proton (\(\mathrm{H}^+\)) from an acid to a base. Acids are proton donors, whereas bases are proton acceptors. These reactions are essential in aqueous solutions, where water often acts as a facilitatory medium.
When \(\mathrm{HIO}_3\), a strong acid, is dissolved in water, it donates a proton to the water molecules, resulting in the formation of hydrogen ions (\(\mathrm{H}^+\)) and iodate ions (\(\mathrm{IO}_3^-\)). Similarly, \(\mathrm{Ba(OH)_2}\) acts as a strong base, releasing hydroxide ions (\(\mathrm{OH}^-\)) that can react with any available \(\mathrm{H}^+\) ions.
Hydrogen cyanide (\(\mathrm{HCN}\)) behaves as a weak acid, meaning it only partially releases \(\mathrm{H}^+\) ions into the solution. This incomplete dissociation results in a state of equilibrium and the formation of cyanide ions (\(\mathrm{CN}^-\)).

Dissociation

Dissociation is the process where a compound breaks down into ions when dissolved in water. This is a critical part of how substances interact in aqueous solutions, influencing conductivity and reaction pathways.
For instance, \(\mathrm{HIO}_3\) completely dissociates into \(\mathrm{H}^+\) and \(\mathrm{IO}_3^-\) ions, demonstrating the behavior of a strong acid. Meanwhile, \(\mathrm{Ba(OH)_2}\) dissociates fully, yielding \(\mathrm{Ba}^{2+}\) and \(\mathrm{OH}^-\) ions, characteristic of strong bases.

• Strong acids and bases dissociate almost entirely in solution.
• Weak acids and bases do not fully dissociate and exist primarily in their molecular form.

This dissociation process is vital for various chemical reactions, influencing properties like pH and electrical conductivity.

Ion Formation

Ion formation occurs when atoms or molecules gain or lose electrons, acquiring a positive or negative charge. This phenomenon is common in aqueous chemistry, where the solvation process facilitates ion separation.
Upon dissolving \(\mathrm{CuSO}_4\) in water, Cupric ions (\(\mathrm{Cu}^{2+}\)) and sulfate ions (\(\mathrm{SO}_4^{2-}\)) are formed. These ions are crucial as they impart distinctive chemical properties and can participate in further reactions.

• The formation and movement of ions in solution enable electrical conductivity.
• Ions play a foundational role in many biochemical and industrial processes.

Understanding ion formation helps us predict how compounds will behave in solution, aiding in the design of various chemical applications.

Equilibrium Reactions

Equilibrium reactions occur when the forward and reverse reactions occur at equal rates, resulting in no net change in the concentration of reactants and products over time. This is a hallmark of weak acids, which do not fully dissociate in water.
Hydrogen cyanide (\(\mathrm{HCN}\)) exemplifies this concept. In an aqueous solution, \(\mathrm{HCN}\) partially dissociates into \(\mathrm{H}^+\) and \(\mathrm{CN}^-\) ions, reaching a point where the rate of dissociation equals the rate of recombination.

• Equilibrium is dynamic; reactions are ongoing but appear stable.
• The equilibrium constant, \(K_{eq}\), quantifies the ratio of products to reactants.

This balance is vital for processes in biological systems and industrial applications, where maintaining conditions near equilibrium can affect efficiency and outcomes.