Amphoteric Species: Videos & Practice Problems

Amphoteric substances, such as water, have the unique ability to act as either an acid or a base depending on their environment. Acids are defined as donors of H⁺ ions (protons), while bases accept H⁺ ions. Amphoteric species often have a hydrogen atom at the start and a negative charge at the end of their structure, though water, a common amphoteric substance, does not fit this pattern. Instead, water can donate an H⁺ ion to become OH⁻ when acting as an acid, or accept an H⁺ ion to form H₃O⁺ when acting as a base. This dual behavior is essential in understanding acid-base reactions and the concept of proton transfer in aqueous solutions.

$H⁺$ ${\mathrm{OH}}_{-}$ $H_{3}O⁺$

Amphoteric Species

An amphoteric substance, also known as an amphiprotic substance, is capable of acting as either an acid or a base depending on its environment. In the context of acid-base chemistry, an acid is defined as a substance that donates a hydrogen ion (H⁺), also referred to as a proton, when dissolved in a solvent. Conversely, a base is a substance that accepts an H⁺ ion in a solution.

Many amphoteric species can be identified by their chemical structure, typically featuring an H⁺ ion at the beginning of the compound and a negative charge at the end. A notable exception to this pattern is water (H₂O), which serves as a prime example of an amphoteric species that does not strictly conform to the aforementioned description.

Water can demonstrate its amphoteric nature through two distinct reactions. In the first reaction, water acts as an acid when it donates an H⁺ ion to a fluoride ion (F^-). The reaction can be represented as follows:

H₂O + F^- → HF + OH^-

Here, water donates an H⁺ ion, resulting in the formation of hydrofluoric acid (HF) and leaving behind a hydroxide ion (OH^-).

In another scenario, water behaves as a base when it accepts an H⁺ ion from a strong acid, such as hydrochloric acid (HCl). This reaction can be expressed as:

H₂O + HCl → H₃O⁺ + Cl^-

In this case, water accepts an H⁺ ion, forming the hydronium ion (H₃O⁺) and leaving behind a chloride ion (Cl^-). It is important to note that H⁺ and H₃O⁺ are often used interchangeably in acid-base chemistry, as the presence of H⁺ in solution typically leads to the formation of hydronium ions.

In summary, amphoteric species, such as water, can act as either acids or bases depending on their surroundings. Identifying these species often involves recognizing the presence of hydrogen at the beginning of the compound and a negative charge at the end, which serves as a useful indicator of their amphoteric nature.

Amphoteric Compounds Example

In the context of acid-base chemistry, an amphoteric species is one that can act as both an acid and a base, depending on the surrounding conditions. A prime example of an amphoteric substance is water, which can donate a proton (acting as an acid) or accept a proton (acting as a base).

When identifying amphoteric compounds, a useful guideline is that they typically begin with a hydrogen atom and possess a negative charge at the end. In this case, the bisulfite ion (or hydrogen sulfite ion), represented as HSO_3^−, fits this description perfectly. It has a hydrogen at the beginning and a negative charge, allowing it to behave as either an acid or a base depending on the reaction environment.

Thus, among the given options, the correct choice that exemplifies an amphoteric species is option C, the bisulfite ion. This ion can donate a proton to form sulfite SO_3^{2−} (acting as a base) or accept a proton to form sulfuric acid H_2SO_3 (acting as an acid), demonstrating its amphoteric nature.

Which of the following species is amphiprotic within an aqueous solvent?  
a) Br^–                           b) HC₂H₃O₂                   c) SO₄^2–                        d) HSe^–                        e) H₃­O⁺

• A. Br^–
• B. HC₂H₃O₂
• C. SO₄^2–
• D. HSe^–
• E. H₃O⁺

Answer: (D)

Here’s what students ask on this topic:

Which of the following is an amphoteric species?

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An amphoteric species is one that can act as both an acid and a base, depending on the circumstances and the reaction it is involved in. This behavior is described by the Bronsted-Lowry theory, where an acid is a proton (<sup>H+</sup>) donor and a base is a proton acceptor.

Common examples of amphoteric species include water (<sub>H2O</sub>), which can donate a proton to become hydroxide (<sub>OH<sup>-</sup></sub>) or accept a proton to become hydronium (<sub>H3O<sup>+</sup></sub>), and the hydroxide ion (<sub>OH<sup>-</sup></sub>), which can also act as either an acid or a base. Other examples are aluminum hydroxide (<sub>Al(OH)3</sub>) and zinc oxide (<sub>ZnO</sub>), which can react with both acids and bases to form salts and water. Amphoteric behavior is also seen in amino acids, which contain both an acidic carboxyl group and a basic amino group, allowing them to react with both acids and bases.

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How can amphoteric species be identified?

Amphoteric species are substances that can behave as either acids or bases in chemical reactions. To identify an amphoteric species, you should look for compounds that have the ability to donate and accept protons (H⁺ ions). A classic example of an amphoteric substance is water (H₂O), which can donate a proton to become a hydroxide ion (OH⁻) or accept a proton to form a hydronium ion (H₃O⁺).

Most commonly, amphoteric species are oxides or hydroxides of metalloids and some metals, particularly those located near the diagonal band of metalloids in the periodic table. For example, aluminum hydroxide (Al(OH)₃) and zinc oxide (ZnO) are amphoteric; they can react with both acids and bases.

Another way to identify amphoteric species is by considering their chemical reactions. If a substance reacts with both strong acids to form salts and water, and with strong bases to form salts and water, it is likely amphoteric. This dual reactivity is a hallmark of amphoteric compounds.

Which of the following species is not amphoteric?

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To determine if a species is amphoteric, you need to consider whether it can act as both an acid and a base according to the Bronsted-Lowry theory. Amphoteric species can donate and accept protons. Common amphoteric substances include water (<sub>H2O</sub>), amino acids, and many metal oxides and hydroxides, such as aluminum hydroxide (<sub>Al(OH)3</sub>) and zinc oxide (<sub>ZnO</sub>).

However, without a list of specific species to choose from, I can't pinpoint which one is not amphoteric. In general, simple ions like <sub>Na<sup>+</sup></sub> or <sub>Cl<sup>-</sup></sub> are not amphoteric because they cannot donate and accept protons. Similarly, strong acids like <sub>HCl</sub> or strong bases like <sub>NaOH</sub> are not amphoteric because they are fully dissociated in water and do not have the capacity to act in the opposite manner (i.e., <sub>HCl</sub> can only donate protons, and <sub>NaOH</sub> can only accept protons). To identify a non-amphoteric species, look for substances that cannot both donate and accept a proton.

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