Chapter 9, Problem 21P

Which of the following species can be Brønsted—Lowry acids?
a. $\text{HBr}$
b. ${\text{Br}}_{\text{2}}$
c. ${\text{AIC1}}_{\text{3}}$
d. $\text{HCOOH}$
e. ${\text{NO}}_{\text{2}}{}^{\text{-}}$
f. ${\text{HNO}}_{\text{2}}$

Interpretation Introduction

Interpretation:

The species that can form Bronsted-Lowry acids should be selected from the following:

  ${\text{HBr; Br}}_{\text{2}}{\text{; AlCl}}_{\text{3}}{\text{; HCOOH; NO}}_{\text{2}}{}^{\text{-}}{\text{; HNO}}_{\text{2}}$

Concept Introduction:

A substance is said to be acidic if it can give hydrogen ions ( ${\text{H}}^{\text{+}}$ ) whereas a base is said to be a substance that can give ${\text{OH}}^{\text{-}}$ ion. This theory of acid and base is called the Arrhenius theory of acid and base.

Another theory that is purposed by Bronsted and Lowry is said to be Bronsted-Lowry acid-base theory. It states that acid can give ${\text{H}}^{\text{+}}$ ions whereas a base can accept the ${\text{H}}^{\text{+}}$ ion in its solution. Hence this theory is entirely based on the presence of ${\text{H}}^{\text{+}}$ ion in the given substance. An acid forms conjugated base of it after giving ${\text{H}}^{\text{+}}$ ion.

The acidic strength can be determined with the help of number of H+ and also with the help of strength of conjugated base. A strong Bronsted acid forms a weaker conjugated base.

Answer to Problem 21P

Correct answer: Option a, d and f ; ${\text{HBr, HCOOH, and HNO}}_{\text{2}}$ are examples of Bronsted-Lowry acids.

Explanation of Solution

Reason for correct option:

The Bronsted-Lowry acid can give ${\text{H}}^{\text{+}}$ ions whereas a Bronsted-Lowry base can accept the ${\text{H}}^{\text{+}}$ ion in its solution. An acid forms conjugated base of it after giving ${\text{H}}^{\text{+}}$ ion. ${\text{HBr, HCOOH, and HNO}}_{\text{2}}$ are examples of Bronsted Lowry acids as they give ${\text{H}}^{\text{+}}$ ions in their solutions.

  $\begin{array}{l}{\text{HBr}}_{\text{(aq)}}{\text{+ H}}_{\text{2}}{\text{O}}_{\text{(l) }}\underset{}{\to }{\text{ H}}_{\text{3}}{\text{O}}^{\text{+}}{}_{\text{(aq)}}{\text{ + Br}}^{\text{-}}{}_{\text{(aq)}}\\ {\text{HCOOH}}_{\text{(aq)}}{\text{+ H}}_{\text{2}}{\text{O}}_{\text{(l) }}\underset{}{\to }{\text{ H}}_{\text{3}}{\text{O}}^{\text{+}}{}_{\text{(aq)}}{\text{ + HCOO}}^{\text{-}}{}_{\text{(aq)}}\\ {\text{HNO}}_{\text{2}}{}_{\text{(aq)}}{\text{+ H}}_{\text{2}}{\text{O}}_{\text{(l) }}\underset{}{\to }{\text{ H}}_{\text{3}}{\text{O}}^{\text{+}}{}_{\text{(aq)}}{\text{ + NO}}_{\text{2}}{{}^{\text{-}}}_{\text{(aq)}}\end{array}$

Conclusion

Reasons for incorrect options:

${\text{Br}}_{\text{2}}{\text{, AlCl}}_{\text{3}}{\text{, and NO}}_{\text{2}}{}^{\text{-}}$ are not examples of Bronsted Lowry acids. Here ${\text{NO}}_{\text{2}}{}^{\text{-}}$ is an example of Bronsted Lowry baseas it has tendency to accept ${\text{H}}^{\text{+}}$ ions.

  ${\text{NO}}_{\text{2}}{{}^{\text{-}}}_{\text{(aq) }}{\text{ + H}}_{\text{2}}{\text{O}}_{\text{(l) }}\underset{}{\to }{\text{ HNO}}_{\text{2}}{}_{\text{(aq) }}{\text{+ OH}}^{\text{-}}{}_{\text{(aq) }}$

  ${\text{Br}}_{\text{2}}{\text{ and AlCl}}_{\text{3}}$ are neither Bronsted Lowry acid nor Bronsted Lowry base.