# Which of these follows the Arrhenius definition?

## a) Dissociates in water to produce ${\text{OH}}^{-}$ in solution. b) Donates a proton c) Releases protons into aqueous solution d) A proton acceptor

May 26, 2016

I got only (c), because it follow the Arrhenius definition 100% with no ambiguity or vagueness.

ARRHENIUS DEFINITION

The Arrhenius definition of acids and bases states that:

• An acid releases protons (${\text{H}}^{+}$, or hydrogen ions) into aqueous solution, or increases hydrogen ion concentration in aqueous solution.
• A base releases hydroxide ions (${\text{OH}}^{-}$) into aqueous solution, or increases hydroxide concentration in aqueous solution.

Unfortunately, the Arrhenius definition does not include acid/base behavior in acids/bases that do not obey the above classifications.

Other acid/base classifications that should be familiar are the Bronsted-Lowry and Lewis definitions.

BRONSTED-LOWRY DEFINITION

• A Bronsted acid donates protons to a Bronsted base.
• A Bronsted base accepts protons from a Bronsted acid.

LEWIS DEFINITION

• A Lewis acid accepts electron pairs from a Lewis base.
• A Lewis base donates electron pairs to a Lewis acid.

How I keep this straight is that I use ammonia ($: {\text{NH}}_{3}$) as the prime example of a Lewis base, capable of donating electrons. So, I know that accepting protons makes something a Lewis acid.

From here, I recall that when you donate electrons, you do so with a purpose: to make a bond. You can bond with either ${\text{H}}^{+}$, or something else.

When a Lewis base donates an electron pair to get ${\text{H}}^{+}$, it accepts a proton and thus is a Bronsted base. So, if a Lewis base acquires a proton, it is also a Bronsted base.

Then I associate Lewis acids with Bronsted acids by process of elimination! :)

So, I think we have enough context now.

a) By definition, this describes an Arrhenius base, not an acid. Example:

color(red)(stackrel("Arrhenius Base")overbrace("KOH"(s)) stackrel("H"_2"O"(l)" ")(->) "K"^(+)(aq) + stackrel("Released hydroxide")overbrace("OH"^(-)(aq)))

b) Donating a proton describes a Bronsted acid, not an Arrhenius acid, unless a proton is donated to water (in which case it actually increases the hydrogen ion concentration in aqueous solution as a result).

color(red)(stackrel("Arrhenius Acid")stackrel("Lewis Acid")(stackrel("Bronsted Acid")overbrace("HA"(aq))) + stackrel("Lewis Base")(stackrel("Bronsted Base")overbrace("H"_2"O"(l))) -> stackrel("Conjugate acid")overbrace("H"_3"O"^(+)(aq)) + stackrel("Conjugate base")overbrace("A"^(-)(aq)))

where ${\text{H"_3"O}}^{+}$ is another way to represent ${\text{H}}^{+}$.

But since this is not specific enough of a wording, we cannot say that (b) is always true, so we cannot accept (b).

c) !This is an Arrhenius acid! It releases protons into aqueous solution, increasing the ${\text{H}}^{+}$ concentration. An example, like the previous one, shows:

color(blue)(stackrel("Bronsted Acid")stackrel("Lewis Acid")stackrel("Arrhenius Acid")overbrace("HCl"(aq))) $\textcolor{b l u e}{+ \stackrel{\text{Lewis Base")stackrel("Bronsted Base")(overbrace("H"_2"O} \left(l\right)}{\to}}$ color(blue)(stackrel("Released protons")stackrel("written here as H"_3"O"^(+))overbrace("H"_3"O"^(+)(aq)) + "Cl"^(-)(aq))

d) A proton acceptor is similar to (b): it is not specific enough, and it instead defines a Bronsted base. Example:

color(red)(stackrel("Lewis Base")(stackrel("Bronsted Base")overbrace("A"^(-))) + stackrel("Arrhenius Base")stackrel("Lewis Acid")(stackrel("Bronsted Acid")overbrace("H"_2"O")) -> stackrel("Conjugate base")overbrace("OH"^(-)) + stackrel("Conjugate acid")overbrace("HA"))

In this case, the Bronsted base turned out to not also be an Arrhenius base. Instead, the Arrhenius base was water, because it was the source for the increase in ${\text{OH}}^{-}$ concentration, caused by ${\text{A}}^{-}$. Nothing here is an Arrhenius acid.