# Arrhenius Acid-Base

According to the Arrhenius concept of acid, "an acid is a substance that, when dissolved in water, increases the concentration of hydronium ion. Is $\ce{HSO4-}$ in the following chemical equation an Arrhenius acid, Brønsted–Lowry acid, or both?

$$\ce{HSO4- + H2O -> SO4- + H3O+}$$

My chemistry teacher says only molecules can be Arrhenius acids and bases, is his statement true?

• Second statement is false. $\ce{NaOH}$ is not a molecule but is an Arrhenius base. – Zhe Dec 5 '16 at 18:13
• So is hydrogen sulfate ion also an Arrhenius acid when reacting with water? – alend ahmed Dec 5 '16 at 18:28
• Yes. It has a proton to give. – Zhe Dec 5 '16 at 18:35

Arrhenius acid-base theory is the first modern definition of acids and bases, and defines them as species that dissociate into $\ce{H+}$ and $\ce{OH-}$ ions in aqueous solution, respectively. An example of each:

$$\ce{HA ->H+(aq) + A-(aq)}$$

$$\ce{BOH(s) -> B+(aq) + OH-(aq)}$$

Brønsted–Lowry acid–base theory was proposed some time after Arrhenius acid-base theory to give a more general definition of acid-base reactions. This definition does not constrain acid-base reactions to aqueous solution. Also, it instead defines acids and bases on how they react with one another, acids defined as species that donate $\ce{H+}$, and bases defined as species capable of accepting $\ce{H+}$. The characteristic reaction:

$$\ce{HA +B <=>HB+ + A-}$$

As you can see, all Arrhenius acids will also be classified as Brønsted–Lowry acids, but there are far more many Brønsted–Lowry bases than Arrhenius bases.

• Yes I fully understand both concept, the only confused pp – alend ahmed Dec 5 '16 at 18:23
• I fully understand both concepts, my only confusion was from my teacher's statement where he said only molecules can be Arrhenius acids and base, is the hydrogen sulfate ion an Arrhenius acid when reacting with water or not! – alend ahmed Dec 5 '16 at 18:26
• The term species would include molecules as well as ions. – ringo Dec 6 '16 at 2:08