Chapter 16.1, Problem 1RQ

Interpretation Introduction

Interpretation: To account for the difference in definition of acid in Arrhenius and Brønsted-Lowry theory.

Concept introduction: An Arrhenius acid is a substance that dissociates in water to form hydrogen ions ( $H^{+}$ ). Conversely we can say an acid increases the concentration of $H^{+}$ ions in an aqueous solution.

A Brønsted-Lowry acid is a hydrogen ion ( $H^{+}$ ) donor.

Answer to Problem 1RQ

A Bronsted-Lowry acid, like an Arrhenius acid, is a compound that breaks down to give an $H^{+}$ in solution. The only distinction between Bronsted-Lowry acid and Arrhenius acid is that the solution does not have to be water.

Explanation of Solution

Brønsted-Lowry theory of acid and bases is an extension of Arrhenius definition of acids and bases, as a substance no longer needed to be composed of hydrogen ( $H^{+}$ ) or hydroxide ( $O{H}^{-}$) ions for getting tagged as an acid or base. For exmaple, consider the following chemical equation:

  $HCl(aq)+N{H}_3\left(aq\right)\iff C{l}^{-}\left(aq\right)+N{H}_4{}^{+}\left(aq\right)$

Here, hydrochloric acid ( $HCl$ ) "donates" a proton ( $H^{+}$ ) to ammonia ( $N{H}_3$ ) which "accepts" it, forming a positively charged ammonium ion ( $N{H}_4{}^{+}$ ) and a negatively charged chloride ion ( $C{l}^{-}$ ). Therefore, $HCl$ is a Brønsted-Lowry acid (donates a proton) while $N{H}_3$ is a Brønsted-Lowry base (accepts a proton).

Also, $C{l}^{-}$  is called the conjugate base of the acid $HCl$ and $N{H}_4{}^{+}$  is called the conjugate acid of the base $N{H}_3$ .

Conclusion

A Bronsted-Lowry acid, like an Arrhenius acid, is a compound that breaks down to give an $H^{+}$ in solution. The only distinction between Bronsted-Lowry acid and Arrhenius acid is that the solution does not have to be water.