Question

Predict which member of each pair produces the more acidic aqueous solution: (a) \(\mathrm{K}^{+}\) or \(\mathrm{Cu}^{2+}\), (b) \(\mathrm{Fe}^{2+}\) or \(\mathrm{Fe}^{3+}\) (c) \(\mathrm{Al}^{3+}\) or \(\mathrm{Ga}^{3+}\)

Short answer

In summary, the more acidic aqueous solutions are produced by the following ions: (a) Cu2+ (b) Fe3+ and (c) Ga3+. This is based on their higher charge densities compared to their counterparts in each pair.

Step-by-step solution

Predict the acidity based on charge density

To predict the acidic strength of the ions, we should consider their charge densities. Ions with higher charge densities will produce a more acidic solution because they attract water molecules more strongly, causing them to donate protons.

(a) Comparing K+ and Cu2+

K+ ion has a charge of +1 and a larger atomic radius, while Cu2+ ion has a charge of +2 and a smaller atomic radius. Cu2+ has a higher charge density than K+ ion, so the Cu2+ aqueous solution should be more acidic than the K+ aqueous solution.

(b) Comparing Fe2+and Fe3+

Fe2+ ion has a charge of +2 and Fe3+ ion has a charge of +3, both of them have the same atomic radius because they are isotopes. Fe3+ has a higher charge density than Fe2+, so the Fe3+ aqueous solution should be more acidic than the Fe2+ aqueous solution.

(c) Comparing Al3+ and Ga3+

Al3+ ion and Ga3+ ion both have a charge of +3. Al has a larger atomic radius than Ga, so the charge density of Al3+ is lower than that of Ga3+. The Ga3+ aqueous solution should be more acidic than the Al3+ aqueous solution. In summary, the more acidic ions in each pair are: Cu2+, Fe3+, and Ga3+.

Key concepts

Charge Density

Charge density is a measure of the amount of electric charge in a given volume or area. It influences how strongly an ion can attract and interact with its surrounding water molecules. The higher the charge density, the more effective the ion is at pulling water molecules close. This interaction is crucial in understanding the acidity of solutions.
When ions with high charge densities attract water molecules, they cause these water molecules to lose protons more easily. This results in the increase of hydrogen ions (\(\mathrm{H}^+\)) in the solution, making it more acidic. Consider the charge and size when computing charge density:

• High charge (e.g. +3) leads to higher charge density.
• Small atomic radius also increases charge density.

Therefore, ions like (\(\mathrm{Fe}^3+\) or \(\mathrm{Cu}^2+\) ) which have high charges and relatively small radii, have higher charge densities, resulting in more acidic solutions.

Metal Ions

Metal ions are atoms of metal elements that have lost some electrons and have a positive charge. They are prevalent in chemistry, known for forming ionic compounds and existing in different charged states such as +1, +2, or +3. These charges significantly influence their chemical behavior, especially in aqueous solutions.
In aqueous chemistry, metal ions can interact with water molecules in different ways. Ions with multiple charges, such as (\(\mathrm{Cu}^2+\)) or (\(\mathrm{Fe}^3+\)), are most likely to form acidic solutions compared to those with a single charge, like (\(\mathrm{K}^+\)). This is because the larger charge increases the strength of interaction with water, causing water to release protons more readily.
Understanding the behavior of metal ions is essential in predicting the acidity level of solutions. The ability to form stronger interactions with water often results in solutions that tend to be more acidic.

Aquatic Chemistry

Aquatic chemistry studies chemical processes that occur in water, taking into account factors like solubility, reactions with dissolved gases, and acid-base behavior of ions. The presence of metal ions in water influences these processes significantly.
The acidity of water is a common focus in aquatic chemistry, as it affects the environment and biological processes. An ion with a higher charge density will make water more acidic, affecting plants, animals, and chemical reactions. The interaction of ions like (\(\mathrm{Fe}^3+\)) or (\(\mathrm{Ga}^3+\)) with water can demonstrate the principles of aquatic chemistry, emphasizing their impact on the surrounding environment by increasing acidity levels.
When studying aquatic chemistry, it is vital to understand how charge density and the behavior of metal ions impact the acidity of water. Recognizing these interactions helps in predicting and explaining changes in the aquatic environments.