Video: Defining Lewis Acids

Which of the following statements best defines a Lewis acid? [A] A substance that can donate a pair of electrons [B] A substance that can accept a pair of electrons [C] A substance that can donate an H⁺ ion [D] A substance that can accept an H⁺ ion [E] A substance that produces OH⁻ ions

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Video Transcript

Which of the following statements best defines a Lewis acid? (A) A substance that can donate a pair of electrons, (B) a substance that can accept a pair of electrons, (C) a substance that can donate an H+ ion, (D) a substance that can accept an H+ ion, or (E) a substance that produces OH− ions.

The first thing that’s important to identify is that we’re not looking for a simply correct answer. We’re looking for the best statement out of the five. Lewis described acidity and basicity in terms of the acceptance or donation of lone pairs of electrons. He describes substances like ammonia with reactive lone pairs as bases and electron-deficient substances, like BH3, as acids. When the two react, a bond is formed between the area with the lone pair and the electron-deficient area.

The first statement suggests that a Lewis acid is a substance that can donate a pair of electrons. This corresponds to the description of a Lewis base, not a Lewis acid. The second statement suggests that a Lewis acid accepts a pair of electrons. This fits nicely with what we’ve already seen. So, let’s hold on to this answer and check the other three.

Our third option is that a Lewis acid is a substance that can donate a hydrogen ion. These would be substances like hydrochloric acid and nitric acid. Here’s the reaction of a common base sodium hydroxide with hydrochloric acid, forming sodium chloride and water. This reaction does conform to the description of Lewis acids and bases because a lone pair from the hydroxide ion is attacking the hydrogen ion.

Statement (C) is not our correct answer because even though a substance that can donate a hydrogen ion is a Lewis acid, not all Lewis acids donate hydrogen ions. So, this statement is not the best description. Instead, this type of acid is commonly referred to as a Brønsted–Lowry acid. The next statement changes the game by talking about accepting hydrogen ions. This is a good definition of a Brønsted–Lowry base and not a Lewis acid.

Finally, the last statement suggests that a Lewis acid is a substance that produces OH− ions, hydroxide ions. A good example of this is the addition of solid sodium hydroxide to water, producing hydroxide ions in solution. Sodium hydroxide is an Arrhenius base. It dissociates in water to produce hydroxide ions. The general term that covers statement (E) is alkali because there are some substances that are not Arrhenius bases that still react with water to produce hydroxide ions. Either way, this is definitely not an acid and definitely not a Lewis acid, meaning of the five statements we’ve been given, the one that best defines a Lewis acid is a substance that can accept a pair of electrons.

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