Architecture

Solved Which Of The Following Correctly Describes An Arrhenius Acid?

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

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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.

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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! ๐Ÿ™‚

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”^(+)#.

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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.

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