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?

1 answer

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.