Amphoteric Species - Online Tutor, Practice Problems & Exam Prep

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.

$H⁺$ ${\mathrm{OH}}_{-}$ $H_{3}O⁺$

Intro to Amphoteric Species

Here we're going to say that an amphoteric, also called an amphiprotic substance, is one that can act as an acid or base based on its environment. So in acid, we're going to say an acid here is a substance that donates or gives away an H ion, also referred to as a proton when dissolved in a solvent. A base, on the other hand, is a substance that accepts an H proton when dissolved in a solvent.

Now we're going to say many amphoteric species possess an H ion at the beginning of the compound and a negative charge at the end. So this is a way of spotting many types of amphoteric species. Now a big exception to this is water. Water is a prime example of an amphoteric species that doesn't exactly fit the description. If we take a look here, we have water behaving in two different reactions.

In the first one, it acts as an acid when it reacts with the fluoride ion. Here, if it's acting as an acid, it's going to donate an H plus to F-, F-, H plus. Combined they have opposite charges so they can't allow to give us HF. We lose an H from the water, so what's going to be left behind is OH minus. Now in another equation we have the strong acid, hydrochloric acid. In this one, water will behave as a base and accept an H plus from the acid.

Now here to make things a little bit visually easier to see, so just imagine H plus going here and H plus went here so H plus went to the water, so H plus left HCL. So what's left behind is CL minus. H is combining with the neutral water to give us H₃O Plus. Now here, remember H₃O represents the hydronium ion. To this point, we've been saying that H and H₃O plus are very similar to each other. They're synonymous, interchangeable.

So when we're releasing H Plus, we're basically saying that H plus can be donated to any of the water molecules within our solution, thereby creating H₃O Plus. That's why we kind of say that they can represent the same thing. So water is a great example of an amphoteric species. If we take a look at others, based on the description that we set up above, they all have a hydrogen in the beginning and they all have a negative charge. So that's a good giveaway that the rest of these examples below represent amphoteric species.

All right, so keep that in mind. Amphoteric species, based on what's around them, they can act as an acid or as a base. All right, so now that we've done this, click on to the next video and let's talk a little bit more about amphoteric species with an example problem.

Amphoteric Species Example

Which of the following compounds is amphoteric when placed in water? We know that water itself is a prime example of an apothecary species. It's not present here, so fall back on our other definition. For an amphoteric species, they begin with a hydrogen and they possess a negative charge at the end.

If we take a look, which choice fits this description? If we take a look, the answer has to be option C because with option C we have a hydrogen at the beginning and we have a negative charge at the end. Here this represents bisulfite ion or hydrogen sulfide ion. It's an apoteric species.

Based on what's around it, it could behave as an acid or a base. So in this particular question we have option C as our final answer.

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⁺

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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