Question

Which of these follows the Arrhenius definition?

`a)` Dissociates in water to produce `"OH"^(-)` in solution.
`b)` Donates a proton
`c)` Releases protons into aqueous solution
`d)` A proton acceptor

Answer 1

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

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ARRHENIUS DEFINITION

The Arrhenius definition of acids and bases states that:

• An acid releases protons (`"H"^(+)`, or hydrogen ions) into aqueous solution, or increases hydrogen ion concentration in aqueous solution.
• A base releases hydroxide ions (`"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 (`:"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 `"H"^(+)`, or something else.

When a Lewis base donates an electron pair to get `"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! :)

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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 `"H"_3"O"^(+)` is another way to represent `"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 `"H"^(+)` concentration. An example, like the previous one, shows:

`color(blue)(stackrel("Bronsted Acid")stackrel("Lewis Acid")stackrel("Arrhenius Acid")overbrace("HCl"(aq)))` `color(blue)(+ stackrel("Lewis Base")stackrel("Bronsted Base")(overbrace("H"_2"O"(l)) -> )` `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 `"OH"^(-)` concentration, caused by `"A"^(-)`. Nothing here is an Arrhenius acid.