Question

Which of the following compounds is amphoteric? (a) \(\mathrm{Cr}(\mathrm{OH})_{2}\) (b) \(\mathrm{Fe}(\mathrm{OH})_{2}\) (c) \(\mathrm{Cr}(\mathrm{OH})_{3}\) (d) \(\mathrm{Fe}(\mathrm{OH})_{3}\)

Short answer

(c) \(\mathrm{Cr(OH)}_{3}\) is amphoteric.

Step-by-step solution

Understanding Amphoteric Compounds

Amphoteric compounds can react with both acids and bases. We need to identify which of the given hydroxides have this property.

Analyzing Each Compound

- (a) \(\mathrm{Cr(OH)}_{2}\): Chromium(II) hydroxide. This typically does not show amphoteric properties. (b) \(\mathrm{Fe(OH)}_{2}\): Iron(II) hydroxide. It normally behaves as a base.(c) \(\mathrm{Cr(OH)}_{3}\): Chromium(III) hydroxide. Known to be amphoteric, reacting with both acids (to form \(\mathrm{Cr}^{3+}\)) and bases (to form \([\mathrm{Cr(OH)}_{4}]^-\)).(d) \(\mathrm{Fe(OH)}_{3}\): Iron(III) hydroxide. It typically acts as a base.

Identifying the Amphoteric Compound

The compound that exhibits amphoteric behavior by reacting as both an acid and a base is \(\mathrm{Cr(OH)}_{3}\).

Key concepts

Chromium(III) hydroxide

Chromium(III) hydroxide, represented as \(\mathrm{Cr(OH)}_3\), is a fascinating compound because it exhibits amphoteric properties. Simply put, this means that it can react with both acids and bases, which is quite unique. When \(\mathrm{Cr(OH)}_3\) is mixed with an acid, it behaves like a base, giving off hydroxide ions (OH\(^-\)). On the other hand, when it interacts with a base, it acts like an acid, donating protons to form complex ions. This dual behavior makes chromium(III) hydroxide an amphoteric compound, which is useful in various chemical reactions and processes. In acidic solutions, \(\mathrm{Cr(OH)}_3\) dissolves to form \(\mathrm{Cr^{3+}}\) ions, a classic behavior of a base. Conversely, in basic solutions, it can form a complex \( \left[\mathrm{Cr(OH)}_{4}\right]^{-} \), demonstrating its acidic nature. The ability to interchange roles between acid and base is a remarkable feature specific to amphoteric compounds like chromium(III) hydroxide.

Acid-base reactions

Acid-base reactions are fundamental in chemistry and involve the transfer of protons between reactants. An acid is a substance that donates protons (\(\mathrm{H}^+\)), while a base is one that accepts protons. Amphoteric compounds, such as chromium(III) hydroxide, uniquely participate in these reactions by acting as either an acid or a base, depending on the conditions. Let's break it down further:

• When chromium(III) hydroxide reacts with an acid, it behaves like a base; this is because it accepts protons to form \(\mathrm{Cr^{3+}}\) ions.
• Alternatively, when it is introduced to a base, it donates hydroxide ions to form complex ions like \( \left[\mathrm{Cr(OH)}_{4}\right]^{-} \), acting as an acid.

This transformation indicates how the same substance can adapt its role based on the chemical environment, making acid-base reactions involving amphoteric compounds particularly versatile. Understanding these reactions is crucial for predicting the behavior of compounds in various chemical settings.

Hydroxides in chemistry

Hydroxides are compounds that contain the OH\(^-\) ion, and they play an essential role in chemistry as bases. However, their behavior in chemical reactions varies. Most hydroxides are basic, meaning they tend to accept protons and produce water in reactions with acids. Amphoteric hydroxides are unique because, unlike most hydroxides, they can also react with bases. This dual capability allows them to:

• Act like a conventional base in acidic environments, forming water and ions.
• Behave like an acid when mixed with other bases, forming complex ions.

Chromium(III) hydroxide is a prime example of an amphoteric hydroxide. Its ability to engage in reactions as either an acid or a base makes it versatile in various industrial and laboratory applications, such as wastewater treatment and in metal finishing processes. Recognizing the properties of different hydroxides, including the rare amphoteric ones, is vital for understanding and performing chemical reactions effectively.