Amphiprotic Substances: Definition & Examples

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  • 0:01 Defining the Term Amphiprotic
  • 3:03 Examples of…
  • 4:30 Lesson Summary
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Lesson Transcript
Instructor: Danielle Reid

Danielle has taught middle school science and has a doctorate degree in Environmental Health

In acid-base chemistry, did you know water is a very common amphiprotic substance? Explore this lesson to learn what amphiprotic is and identify different compounds classified as amphiprotic.

Defining the Term Amphiprotic

Substances from amino acids to proteins, and yes even our friend water, are all known to be amphiprotic. By definition, something that is amphiprotic is a molecule (or compound) that can both donate and accept a proton. With this dual ability, think of amphiprotic compounds as your favorite BOGO (buy one get one) deal at a clothing store. Yes, with amphiprotic compounds you get two features for the price of one!

Now, be careful not to confuse amphiprotic with the term amphoteric. Although very similar, think of amphiprotic as the little brother to its big sister amphoteric. Amphoteric refers to substances that can act as either an acid or base. There are different types of amphoteric species; amphiprotic substances happen to be one type.

In order to enter into the amphiprotic club, there are a few requirements a compound must have:

  1. The compound must have a hydrogen atom. This is so that it is able to donate a proton (H).
  2. The compound must be able to accept protons (H+) from other compounds.

Again, the process of donating and accepting go hand in hand. A compound must be able to carry out both processes in order to be a member of the amphiprotic club.

Now you may be thinking, where would you see an amphiprotic compound in action? Great question! Recall that amphiprotic compounds fall under the subject of acid-base chemistry. Hence, with these types of compounds we are concerned with acid-base reactions typically in aqueous solutions. Just as a quick review, an acid is a compound that can donate a proton (hydrogen ion - H+) in an aqueous solution. A base is a compound that that can donate a hydroxide ion (OH-) in an aqueous solution.

Let's use our friend water to describe the function of amphiprotic compounds.

Shown is an acid-base reaction using water.

water eq 1

water

As we can see from both equations, water meets the requirements to be an amphiprotic club member. That is, in the first reaction it can readily donate a proton. This is shown on the product side where a proton (H+) is present, ready, and willing to bind with a molecule.

In the second equation, water is able to accept a proton. This is identifiable in the second reaction given the hydronium ion (H3O+) formed in the product. A hydronium ion is an ion formed when a molecule of water is protonated (i.e. has a proton added to it). Do you notice anything unique with both reactions that support the claim water is amphiprotic? Well, water is present in both reactions on the reactant side.

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