Chapter U4.18, Problem 1E

Interpretation Introduction

Interpretation: The term acid and a base in terms of the Arrhenius theory needs to be explained.

Concept Introduction: The Arrhenius theory offered the first modern description of an acid or a base. The Arrhenius theory is based on the presence of ions produced by a substance in an aqueous solution.

Answer to Problem 1E

An Arrhenius acid is a substance that dissociates in water to produce proton, ${\text{H}}^{+}$ . A bare proton cannot exist in an aqueous solution and combines with a neutral water molecule to form the hydronium ion, ${\text{H}}_3{\text{O}}^{+}$ . Thus, an Arrhenius acid is a hydronium ion generator.

An Arrhenius base is a substance that ionizes in water to generate hydroxide ion, ${\text{OH}}^{-}$ .

Explanation of Solution

An Arrhenius acid produces hydronium ion in an aqueous solution. Thus, substances like hydrochloric acid $\left(\text{HCl}\right)$ , sulfuric acid $\left({\text{H}}_2{\text{SO}}_4\right)$ , acetic acid $\left({\text{HC}}_2{\text{H}}_3{\text{O}}_2\right)$ which produce ${\text{H}}_3{\text{O}}^{+}$ ions in solution are Arrhenius acids.

$\begin{array}{l}\text{HCl}\left(\text{aq}\right)+{\text{H}}_2\text{O}\left(\text{l}\right)\to {\text{H}}_3{\text{O}}^{+}\left(\text{aq}\right)+{\text{Cl}}^{-}\left(\text{aq}\right)\\ {\text{H}}_2{\text{SO}}_4\left(\text{aq}\right)+{\text{2 H}}_2\text{O}\left(\text{l}\right)\to 2{\text{ H}}_3{\text{O}}^{+}\left(\text{aq}\right)+{\text{SO}}_4^{2-}\left(\text{aq}\right)\\ {\text{HC}}_2{\text{H}}_3{\text{O}}_2\left(\text{aq}\right)+{\text{H}}_2\text{O}\left(\text{l}\right)\to {\text{H}}_3{\text{O}}^{+}\left(\text{aq}\right)+{\text{C}}_2{\text{H}}_3{\text{O}}_2^{-}\left(\text{aq}\right)\end{array}$

An Arrhenius base is a substance that gives hydroxide ions in water. Thus, sodium hydroxide $\left(\text{NaOH}\right)$ and calcium hydroxide $\left[\text{Ca}{\left(\text{OH}\right)}_2\right]$ are both bases since they produce ${\text{OH}}^{-}$ ions.

$\begin{array}{l}\text{NaOH}\left(\text{aq}\right)\to {\text{Na}}^{+}\left(\text{aq}\right)+{\text{OH}}^{-}\left(\text{aq}\right)\\ \text{Ca}{\left(\text{OH}\right)}_2\left(\text{aq}\right)\to {\text{Ca}}^{2+}\left(\text{aq}\right)+{\text{2 OH}}^{-}\left(\text{aq}\right)\end{array}$

As per the Arrhenius theory, ammonia, $$

${\text{NH}}_3$ is also a base since it produces ammonium hydroxide, ${\text{NH}}_4\text{OH}$ which ionizes to give ${\text{OH}}^{-}$ in solution.

${\text{NH}}_3\left(\text{aq}\right)+{\text{H}}_2\text{O}\left(\text{l}\right)\to {\text{NH}}_4\text{OH}\to {\text{ NH}}_4^{+}\left(\text{aq}\right)+{\text{OH}}^{-}\left(\text{aq}\right)$

Conclusion

An Arrhenius acid gives ${\text{H}}_3{\text{O}}^{+}$ ions in solution while an Arrhenius base produces ${\text{OH}}^{-}$ in solution.