Problem 15 - Chemistry The Central Science

Question

Specify what ions are present in solution upon dissolving each of the following substances in water: (a) \(\mathrm{ZnCl}_{2}\), (b) \(\mathrm{HNO}_{3}\), (c) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}\) (d) \(\mathrm{Ca}(\mathrm{OH})_{2}\)

Step-by-Step Solution

Answer

Upon dissolving the given substances in water, the following ions are present in the solutions: (a) \( \mathrm{ZnCl}_{2} \): Zinc (II) ions (\(\mathrm{Zn}^{2+}\)) and chloride ions (\(\mathrm{Cl}^-\)) (b) \( \mathrm{HNO}_{3} \): Hydrogen (proton) ions (\(\mathrm{H}^+\)) and nitrate ions (\(\mathrm{NO}_3^-\)) (c) \( \left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} \): Ammonium ions (\(\mathrm{NH}_4^+\)) and sulfate ions (\(\mathrm{SO}_4^{2-}\)) (d) \( \mathrm{Ca}(\mathrm{OH})_{2} \): Calcium ions (\(\mathrm{Ca}^{2+}\)) and hydroxide ions (\(\mathrm{OH}^-\))

Step 1: Dissociation of ZnCl₂

The substance \(\mathrm{ZnCl}_{2}\) is a salt which consists of the zinc (Zn) cation and the chloride (Cl) anion. When it dissolves in water, it dissociates into its respective ions as follows: \[\mathrm{ZnCl}_2 \rightarrow \mathrm{Zn}^{2+} + 2\mathrm{Cl}^-\] So, zinc (II) and chloride ions are present in the solution.

Step 2: Dissociation of HNO₃

The substance \(\mathrm{HNO}_3\) is a strong acid which consists of the hydrogen (H) and nitrate (NO₃) ions. When it dissolves in water, it dissociates completely into its respective ions as follows: \[\mathrm{HNO}_3 \rightarrow \mathrm{H}^+ + \mathrm{NO}_3^-\] So, hydrogen (proton) and nitrate ions are present in the solution.

Step 3: Dissociation of (NH₄)₂SO₄

The substance \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}\) is a salt which consists of the ammonium (NH₄) cation and the sulfate (SO₄) anion. Upon dissolving in water, it dissociates into its respective ions as follows: \[\left(\mathrm{NH}_4\right)_2\mathrm{SO}_4 \rightarrow 2\mathrm{NH}_4^+ + \mathrm{SO}_4^{2-}\] So, ammonium and sulfate ions are present in the solution.

Step 4: Dissociation of Ca(OH)₂

The substance \(\mathrm{Ca}(\mathrm{OH})_{2}\) is a basic salt, which consists of the calcium (Ca) cation and the hydroxide (OH) anion. Upon dissolving in water, it dissociates into its respective ions as follows: \[\mathrm{Ca}(\mathrm{OH})_2 \rightarrow \mathrm{Ca}^{2+} + 2\mathrm{OH}^-\] So, calcium and hydroxide ions are present in the solution.

Key Concepts

Dissociation Process

The dissociation process involves breaking down compounds into their respective ions when dissolved in water. When a substance like a salt, acid, or base is introduced to water, it interacts with water molecules to separate into positive and negative ions.

For example, when zinc chloride (\(\mathrm{ZnCl}_2\)) dissolves in water, it dissociates into zinc ions (\(\mathrm{Zn}^{2+}\)) and chloride ions (\(\mathrm{Cl}^-\)). This separation into ions is crucial in many chemical reactions, as these ions may carry electric charge and participate actively in various processes.

Recognizing which ions are present in a solution helps in understanding the chemical behavior and reaction pathways. Similarly, each compound has its unique pattern of dissociation based on its chemical structure and the nature of the bonds between the atoms.

Strong Acids and Bases

Strong acids and bases are types of compounds that completely dissociate in water, releasing a large number of ions into the solution. This complete dissociation results because the bonds within these molecules are weak enough in water, making them unable to hold the ions together.

Take nitric acid (\(\mathrm{HNO}_3\)) as an example. It is a strong acid that dissociates entirely into hydrogen ions (\(\mathrm{H}^+\)) and nitrate ions (\(\mathrm{NO}_3^-\)) when dissolved in water. Similarly, calcium hydroxide (\(\mathrm{Ca}(\mathrm{OH})_2\)) is a strong base, breaking apart into calcium ions (\(\mathrm{Ca}^{2+}\)) and hydroxide ions (\(\mathrm{OH}^-\)).

The ability of strong acids and bases to conduct electricity and react rapidly with other substances originates from this high level of ionization. Understanding these properties is key for predicting and harnessing the behavior of these species in chemical reactions.

Cations and Anions

In aqueous solutions, ions fall into two categories: cations and anions. Cations are positively charged ions, whereas anions are negatively charged. This charge difference leads to important interactions in solutions, making cations and anions crucial players in many chemical processes.

For instance, in the dissociation of ammonium sulfate (\((\mathrm{NH}_4)_2\mathrm{SO}_4\)), it breaks down into ammonium cations (\(\mathrm{NH}_4^+\)) and sulfate anions (\(\mathrm{SO}_4^{2-}\)). Cations like ammonium have a positive charge, meaning they have more protons than electrons. Anions like sulfate have a negative charge because they have more electrons than protons.

These ions in solution create dynamic environments where electrical neutrality is maintained by equal amounts of positive and negative charges. Identifying cations and anions in a solution helps predict interactions such as precipitation, neutralization, and forming complex ions.