Chapter 15: Problem 96
All Brønsted acids are Lewis acids, but the reverse is not true. Give two examples of Lewis acids that are not Brönsted acids.
Short Answer
Expert verified
Two examples of Lewis acids that are not Brønsted acids are boron trifluoride (BF3) and ferric chloride (FeCl3).
Step by step solution
01
Knowledge of Acid Definitions
First, understand the difference between Brønsted and Lewis acid. A Brønsted acid is a substance that donates a proton (or hydrogen ion H+) in a reaction, whereas a Lewis acid is defined as any substance that accepts an electron pair.
02
Identification of Brønsted Acids
A Brønsted acid must have a removable (acidic) proton, something a Lewis acid doesn't need.
03
Identification of Lewis Acids that are not Brønsted Acids
Consider the substances that do not have a proton to donate, i.e., cannot act as a proton donor, but can accept an electron pair to form a chemical bond. Here are two examples of these substances: boron trifluoride (BF3) and ferric chloride (FeCl3). Neither of them has an H+ proton to donate, so they are not considered Brønsted acids. However, they both can accept electron pairs, so they are considered Lewis acids.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Acid-Base Theory
Understanding the fundamentals of acid-base theory is crucial for mastering chemistry. In the simplest terms, this theory classifies substances based on their ability to donate or accept protons (H+) or electrons during chemical reactions.
The most well-known definition was proposed by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923, which focuses on the transfer of protons. According to Brønsted-Lowry theory, an acid is a substance that can donate a proton to another substance, which is the base. Conversely, the base is defined as a substance that can accept a proton.
An extension of the acid-base theory was presented by Gilbert N. Lewis in 1923, which includes more substances that do not fall under the Brønsted-Lowry definition. Lewis acid-base theory broadened the concept by suggesting that an acid is not only limited to proton donation but can also be an electron pair acceptor, while a Lewis base is an electron pair donor.
This nuanced understanding demonstrates that while all Brønsted acids are indeed Lewis acids due to their ability to accept electron pairs (through proton donation), there exist Lewis acids that do not qualify as Brønsted acids since they cannot donate a proton.
The most well-known definition was proposed by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923, which focuses on the transfer of protons. According to Brønsted-Lowry theory, an acid is a substance that can donate a proton to another substance, which is the base. Conversely, the base is defined as a substance that can accept a proton.
An extension of the acid-base theory was presented by Gilbert N. Lewis in 1923, which includes more substances that do not fall under the Brønsted-Lowry definition. Lewis acid-base theory broadened the concept by suggesting that an acid is not only limited to proton donation but can also be an electron pair acceptor, while a Lewis base is an electron pair donor.
This nuanced understanding demonstrates that while all Brønsted acids are indeed Lewis acids due to their ability to accept electron pairs (through proton donation), there exist Lewis acids that do not qualify as Brønsted acids since they cannot donate a proton.
Electron Pair Acceptors
When it comes to electron pair acceptors, the Lewis acid definition captures our attention. Lewis acids are characterized by their ability to accept an electron pair from a Lewis base, which in turn donates the electron pair.
This trait allows Lewis acids to participate in a coordinate covalent bond where both shared electrons come from the Lewis base. Such a type of bond formation extends the realm of acid-base reactions to include a broader range of chemical species beyond those capable of donating protons.
A classic example of a Lewis acid is aluminum chloride (AlCl3), which does not have hydrogen ions to donate but readily accepts electron pairs due to the presence of vacant orbitals in its atomic structure. This electron pair acceptance enables AlCl3 to form additional bonds, acting as an acid even in the absence of any available protons to donate.
This trait allows Lewis acids to participate in a coordinate covalent bond where both shared electrons come from the Lewis base. Such a type of bond formation extends the realm of acid-base reactions to include a broader range of chemical species beyond those capable of donating protons.
A classic example of a Lewis acid is aluminum chloride (AlCl3), which does not have hydrogen ions to donate but readily accepts electron pairs due to the presence of vacant orbitals in its atomic structure. This electron pair acceptance enables AlCl3 to form additional bonds, acting as an acid even in the absence of any available protons to donate.
Proton Donors
Proton donors are at the core of the Brønsted-Lowry acid-base theory. They are identified by their capacity to release a hydrogen ion, which carries a single positive charge, during a chemical reaction. The interaction typically involves a proton donor (the Brønsted acid) and a proton acceptor (the Brønsted base).
Hydrochloric acid (HCl) is an exemplary Brønsted acid; when dissolved in water, it donates a proton to a water molecule, producing hydronium ion (H3O+) and chloride ion (Cl-). The substance that accepts the proton – water in this case – is the Brønsted base.
The strength of a Brønsted acid can be determined by its tendency to donate a proton, which is often represented by its dissociation constant, Ka. Stronger acids possess higher Ka values, indicating a greater propensity to act as proton donors in chemical reactions. Proton donation is a key feature in many biological and environmental processes, underscoring the importance of these substances in both science and daily life.
Hydrochloric acid (HCl) is an exemplary Brønsted acid; when dissolved in water, it donates a proton to a water molecule, producing hydronium ion (H3O+) and chloride ion (Cl-). The substance that accepts the proton – water in this case – is the Brønsted base.
The strength of a Brønsted acid can be determined by its tendency to donate a proton, which is often represented by its dissociation constant, Ka. Stronger acids possess higher Ka values, indicating a greater propensity to act as proton donors in chemical reactions. Proton donation is a key feature in many biological and environmental processes, underscoring the importance of these substances in both science and daily life.