Asked by Mark


my ap chem teacher has been absent for the past two weeks and part of the notes her sub gave us to copy says that

In the equation
HC2H3O2 + H20 <--> C2H3O2(- charge) + H3O(+ charge)

HC2H3O2 is an acid
H2O is a base
C2H3O2(- charge) is the conjugate base
and
H3O(+ charge) is the conjugate acid

but how would i know what each of those were?
or.. hm i don't know how to phrase this...
why is this so??

why is HC2H3O2 an acid?
what makes HC2H3O2 an acid?

and why is H2O a base?
what makes this a base?
Answered by DrBob222
According to the Bronsted-Lowry acid/base theory, an acid is a proton donor. A base is a proton acceptor. I'm going to write acetic acid like this CH3COOH (that's the same HC2H3O2 you have). The H on the right side is the acidic H; the others are not.
So in
CH3COOH + H2O ==> CH3COO^- + H3O^+
You see the CH3COOH has donated a proton to H2O. So CH3COOH is the acid, CH3COO^- is the conjugate base. The H2O molecule accepted the proton; therefore, it is a base and it formed the H3O^+ which is the conjugate acid.

What can act as both an acid and a base. In the above example it is a base. In the following example it is an acid.
NH3 + HOH ==> NH4^+ + OH^-
The NH3 is a base because it accepta a proton. That makes NH4^+ the conjugate acid. HOH donates a proton to the NH3; therefore, H2O is an acid. The OH^- is the conjugate base.
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