Problem 16 - Chemistry: The Central Science

Question

Specify what ions are present upon dissolving each of the following substances in water: (a) \(\mathrm{MgI}_{2}\), (b) \(\mathrm{Al}\left(\mathrm{NO}_{3}\right)_{3}\) (c) \(\mathrm{HClO}_{4}\) (d) \(\mathrm{NaCH}_{3} \mathrm{COO}\).

Step-by-Step Solution

Answer

Upon dissolving each substance in water, the following ions are formed: (a) \(\mathrm{Mg}^{2+}\) and \(\mathrm{I}^{-}\) (b) \(\mathrm{Al}^{3+}\) and \(\mathrm{NO}_{3}^{-}\) (c) \(\mathrm{H}^{+}\) and \(\mathrm{ClO}_{4}^{-}\) (d) \(\mathrm{Na}^{+}\) and \(\mathrm{CH}_{3}\mathrm{COO}^{-}\).

Step 1: (a) Dissolving \(\mathrm{MgI}_{2}\) in water

When magnesium iodide (\(\mathrm{MgI}_{2}\)) is dissolved in water, it dissociates into its respective ions: \(\mathrm{MgI}_{2} \rightarrow \mathrm{Mg}^{2+} + 2 \mathrm{I}^{-}\) The ions formed are magnesium (\(\mathrm{Mg}^{2+}\)) and iodide (\(\mathrm{I}^{-}\)) ions.

Step 2: (b) Dissolving \(\mathrm{Al}\left(\mathrm{NO}_{3}\right)_{3}\) in water

When aluminum nitrate (\(\mathrm{Al}\left(\mathrm{NO}_{3}\right)_{3}\)) is dissolved in water, it dissociates into its constituent ions: \(\mathrm{Al}\left(\mathrm{NO}_{3}\right)_{3} \rightarrow \mathrm{Al}^{3+} + 3 \mathrm{NO}_{3}^{-}\) The ions formed are aluminum (\(\mathrm{Al}^{3+}\)) and nitrate (\(\mathrm{NO}_{3}^{-}\)) ions.

Step 3: (c) Dissolving \(\mathrm{HClO}_{4}\) in water

When perchloric acid (\(\mathrm{HClO}_{4}\)) is dissolved in water, it dissociates into its respective ions: \(\mathrm{HClO}_{4} \rightarrow \mathrm{H}^{+} + \mathrm{ClO}_{4}^{-}\) The ions formed are hydrogen (\(\mathrm{H}^{+}\)) and perchlorate (\(\mathrm{ClO}_{4}^{-}\)) ions.

Step 4: (d) Dissolving \(\mathrm{NaCH}_{3}\mathrm{COO}\) in water

When sodium acetate (\(\mathrm{NaCH}_{3}\mathrm{COO}\)) is dissolved in water, it dissociates into its constituent ions: \(\mathrm{NaCH}_{3}\mathrm{COO} \rightarrow \mathrm{Na}^{+} + \mathrm{CH}_{3}\mathrm{COO}^{-}\) The ions formed are sodium (\(\mathrm{Na}^{+}\)) and acetate (\(\mathrm{CH}_{3}\mathrm{COO}^{-}\)) ions.

Key Concepts

Dissolution Process

When a substance dissolves in water, it breaks down into its individual particles. This is known as the dissolution process. Understanding this process is key to grasping the behavior of ionic compounds in solutions. Essentially, when an ionic compound dissolves, the forces holding the ions together in the solid are overcome by the interaction with water molecules.

• Water molecules are polar, meaning they have a slight positive and negative charge on different ends.
• As the compound enters the water, the water molecules surround and interact with the ions.
• The positive end of water interacts with negatively charged ions, while the negative end of water interacts with positively charged ions.
• This interaction helps break the ionic bonds in the compound, allowing individual ions to disperse throughout the solution.

This dissociation process allows the solutes to exist freely in aqueous solutions, making them available for reactions.

Ion Formation

Ion formation occurs when an ionic compound dissolves in water, separating into positively and negatively charged ions. These compounds are made of metals and non-metals that form a lattice structure. Here's a closer look at how ions emerge from this process:

• For example, magnesium iodide (\(\mathrm{MgI}_{2}\)) breaks into magnesium ions (\(\mathrm{Mg}^{2+}\)) and iodide ions (\(\mathrm{I}^{-}\)).
• During dissolution, electrolytes separate into their respective ions through a process called dissociation.
• This involves the ion-dipole interactions where water molecules stabilize the ions, thus releasing them into the solution.
• As water surrounds each ion, it prevents them from rearranging back into a solid form.

This is why when ionic solids dissolve, they generate solutions that can conduct electricity due to the movement of these charged particles.

Aqueous Solutions

Aqueous solutions form when substances dissolve in water, creating a homogeneous mix where water acts as the solvent. This environment is crucial for many chemical reactions and biological processes. Here's what happens in an aqueous solution:

• Water molecules interact with solute particles, allowing them to be evenly dispersed throughout the liquid.
• The ions from dissolved compounds make the solution conductive due to their charge.
• This dispersion also means that reactions occur more readily because the molecules and ions are free to move and collide.
• For example, sodium acetate dissolves in water, forming sodium ions (\(\mathrm{Na}^{+}\)) and acetate ions (\(\mathrm{CH}_{3}\mathrm{COO}^{-}\)).

Understanding aqueous solutions is essential as they form the basis for various chemical processes both in the laboratory and within living organisms.