Video Transcript
Which of the following best describes the difference between a Brønsted–Lowry base and a Lewis base? (A) A Lewis base is a proton donor, while a Brønsted–Lowry base is a species that can accept an electron pair or more. (B) A Lewis base is a proton acceptor, while a Brønsted–Lowry base is a species that can donate an electron pair or more. (C) A Brønsted–Lowry base is a proton donor, while a Lewis base is a species that can accept an electron pair or more. (D) A Brønsted–Lowry base is a proton acceptor, while a Lewis base is a species that can donate an electron pair or more. Or (E) a Brønsted–Lowry base is an OH− ion acceptor, while a Lewis base is a species that can donate an electron pair or more.
The options from (A) to (E) take up quite a lot of space. So let’s rewrite them in the form of a table, where a species that can accept an electron pair or more can be described as an electron pair acceptor, where an electron is represented by e−. Likewise, we can rewrite a species that can donate an electron pair or more as an electron pair donor.
Now that we’ve created some more space, we can look at the definition of a Lewis base. Gilbert and Lewis defined acidity and basicity in terms of accepting or donating lone pairs, where a lone pair is a pair of valence or outer shell electrons not shared in a covalent bond. For example, NH3, or ammonia, has a lone pairs of electrons. They can be donated to an electron-deficient species, such as BH3, or borane. This forms a bond between the two species. The bond is represented using a coordinate covalent bond arrow. The bond forms between the area with the lone pair and the electron-deficient area. So ammonia can be described as a lone pair or electron pair donor. And borane can be described as a lone pair or electron pair acceptor. In this example, borane can be described as a Lewis acid and ammonia can be described as a Lewis base.
So we have worked out that the Lewis base can be described as an electron pair donor. If we look at all of the options given for the definition of a Lewis base, we find that only two of them have the definition electron pair donor. These two options are (D) and (E). So either option (D) or (E) will be the answer to this question, but not (A), (B), or (C).
The other definition we need to look at is for a Brønsted–Lowry base. This definition was devised by Johannes Nicolaus Brønsted and Thomas Martin Lowry. Brønsted and Lowry defined acidity and basicity in terms of accepting or donating hydrogen ions, where a hydrogen ion can be represented by H+ and is equivalent to a proton.
Let’s look at an example. If we react HCN, or hydrogen cyanide, with water, the hydrogen cyanide will donate a hydrogen ion to the water molecule. In doing so, the hydrogen cyanide becomes CN−, a cyanide ion, and the water molecule becomes H3O+, a hydronium ion. So hydrogen cyanide can be described as a hydrogen ion or proton donor, and water can be described as a hydrogen ion or proton acceptor. The hydrogen cyanide can be described as a Brønsted–Lowry acid. And for this reaction, the water molecule can be described as a Brønsted–Lowry base. Therefore, a Brønsted–Lowry base is defined as a proton acceptor.
We have already ruled out options (A), (B), and (C) as an answer. So, of options (D) and (E), the only option that defines a Brønsted–Lowry base as a proton acceptor is option (D). So we know the answer to the question must be option (D), where a Lewis base is defined as an electron pair donor and a Brønsted–Lowry base is defined as a proton acceptor. We only used this table form to create space. So let’s rewrite the original options for this question.
We now know that the answer to the question “Which of the following best describes the difference between a Brønsted–Lowry base and a Lewis base?” is (D). A Brønsted–Lowry base is a proton acceptor, while a Lewis base is a species that can donate an electron pair or more.
