Question

If a substance is a Lewis acid, is it necessarily a Brønsted-Lowry acid? Is it necessarily an Arrhenius acid? Explain.

Short answer

A substance being a Lewis acid, which is an electron-pair acceptor, does not necessarily mean it is a Brønsted-Lowry acid or an Arrhenius acid. The key differences are that Brønsted-Lowry acids donate protons in reactions, and Arrhenius acids increase the concentration of H⁺ ions in aqueous solutions. Some Lewis acids may not exhibit these behaviors, making it possible for a substance to be a Lewis acid without being a Brønsted-Lowry or Arrhenius acid.

Step-by-step solution

Defining Lewis, Brønsted-Lowry, and Arrhenius acids

A Lewis acid is a substance that can accept an electron pair from a Lewis base to form a coordinate covalent bond. In other words, it is an electron-pair acceptor. A Brønsted-Lowry acid is a substance that can donate a proton (H⁺) to a Brønsted-Lowry base in a reaction. So, it is a proton donor. An Arrhenius acid is a substance that, when dissolved in water, increases the concentration of H⁺ ions (or H₃O⁺, hydronium ions) in the solution. These are typically compounds containing hydrogen, such as hydrogen halides (e.g., HCl, HBr, HI).

Comparing Lewis acids to Brønsted-Lowry acids

Now that we have defined each type of acid, let's compare Lewis acids to Brønsted-Lowry acids. While both types of acids can accept electron pairs to form coordinate covalent bonds, the key difference is that Brønsted-Lowry acids include the donation of a proton in the reaction. It is possible for a substance to be a Lewis acid without being a Brønsted-Lowry acid, as it may not necessarily donate a proton during the reaction. Therefore, a substance being a Lewis acid does not necessarily mean it is a Brønsted-Lowry acid.

Comparing Lewis acids to Arrhenius acids

Next, let's compare Lewis acids to Arrhenius acids. Arrhenius acids specifically increase the concentration of H⁺ ions in an aqueous solution. Some Lewis acids may not necessarily do this, as they may accept electron pairs from Lewis bases without releasing H⁺ ions into the solution. Therefore, a substance being a Lewis acid does not necessarily mean it is an Arrhenius acid.

Conclusion

In conclusion, a substance being a Lewis acid is not sufficient to guarantee that it is also a Brønsted-Lowry or an Arrhenius acid. The key differences lie in whether the substance donates a proton (for Brønsted-Lowry) or increases the concentration of H⁺ ions in an aqueous solution (for Arrhenius).