Chapter U4.18, Problem 2E

Interpretation Introduction

Interpretation: The similarities and differences between the Arrhenius theory and the Bronsted-Lowry theory needs to be explained.

Concept Introduction: Both the Arrhenius and the Bronsted-Lowry theories offer modern definition of an acid and a base based on the ions produced in the aqueous solutions.

Answer to Problem 2E

The Arrhenius and the Bronsted-Lowry theory offer similar definitions of an acid; but the definitions of a base are different in the two theories.

Explanation of Solution

According to Arrhenius, an acid is a substance which produces hydronium ion, ${\text{H}}_3{\text{O}}^{+}$ in aqueous solution. A base is defined by Arrhenius to produce hydroxide ion, ${\text{OH}}^{-}$ in solution. Thus, any substance that produces ${\text{H}}_3{\text{O}}^{+}$ in aqueous solution is an Arrhenius acid. Similarly, a solution that produces ${\text{OH}}^{-}$ in solution is an Arrhenius base. While the Arrhenius can easily define substances like $\text{NaOH}$ , $\text{KOH}$ as bases, there are many bases like ${\text{S}}^{2-}$ , ${\text{CO}}_3^{2-}$ , ${\text{Cl}}^{-}$ , etc. which cannot be defined as base by the Arrhenius theory.

The Bronsted-Lowry theory defines an acid as a substance that can produce hydronium ion, ${\text{H}}_3{\text{O}}^{+}$ in solution. The Bronsted-Lowry theory actually defines an acid as a proton ( ${\text{H}}^{+}$ ) donor, but since a bare proton cannot exist in solution and always combines with ${\text{H}}_2\text{O}$ , hence, a Bronsted-Lowry acid is defined a hydronium ion ( ${\text{H}}_3{\text{O}}^{+}$ ) donor. Thus, both the Arrhenius and the Bronsted-Lowry theories state that an acid produces ${\text{H}}_3{\text{O}}^{+}$ in solution. Hence, mineral acids like $\text{HCl}$ , ${\text{HNO}}_3$ , $\text{HF}$ , etc. are all both Arrhenius and Bronsted-Lowry theories.

The definition of a base as per the Bronsted-Lowry theory is different. A Bronsted-Lowry base is a hydronium ion acceptor. Thus, any molecule or ion that accepts ${\text{H}}_3{\text{O}}^{+}$ is a Bronsted-Lowry base. Therefore, ${\text{OH}}^{-}$ is an Arrhenius base as well as a Bronsted-Lowry base because ${\text{OH}}^{-}$ accepts ${\text{H}}_3{\text{O}}^{+}$ to form water, ${\text{H}}_2\text{O}$ . However, substances like ${\text{Cl}}^{-}$ , ${\text{NO}}_3^{-}$ , etc. are not Arrhenius bases. However, these anions accept ${\text{H}}_3{\text{O}}^{+}$ and can be defined by the Bronsted-Lowry theory as a base.

Conclusion

Both the theories have similar definitions of an acid, but different definitions of a base.