NaCO3 as Acid and Base
Why is NaCO3 both an acid and a base?
First, the formula for the species above (NaCO3) is incorrect. It should
be (Na2CO3), (NaHCO3), and (H2CO3). That is, the carbonate anion is
di-valent (CO3)(-2). In water these equilibria occur simultaneously.
So if acid (H+) is added to the solution, some Na2CO3 is converted to NaHCO3
and H2CO3. The relative amounts being determined by how much acid (H+) is
added. On the other hand if base (OH)(-1) is added to the solution, some
H2CO3 is converted to HCO3(-1) + H2O, and (OH)(-1) + HCO3(-1) ---> H2O +
CO3(-2), again, the relative amounts depend upon how much (H+) or (OH-) is
added. So a solution of the sodium salts of carbonate can react with either
an acidic (H+) [meaning it is a base] or with (OH-) [meaning it is an acid.
A detail: NaCO3 is an unbalanced formula.
CO3(2-) needs two (+)'s, and Na(+) only has one.
So you probably meant Na2CO3 or NaHCO3. (adding an Na(+) or an H(+)).
This is important because the H does a lot of the work.
Na2CO3 is only a base:
CO3(2-) + H2O <--> HCO3(-) + OH(-)
CO3(2-) + H(+) --> HCO3(-)
HCO3(-) + H(+) --> H2CO3 <--> H2O + CO2(gas)
NaHCO3 is both acid and base like you mentioned. (Do not forget the H.)
In neutral solutions it ionizes to Na(+) + HCO3(-).
The HCO3(-) can either donate its proton (acid) or accept another (base).
in strong basic solution:
HCO3(-) + OH(-) --> CO3(2-) + H2O
in strong acidic solution:
HCO3(-) + H(+) --> H2CO3(temporary) <--> H2O + CO2(gas).
When a substance adds acid to basic solutions and base to acid solutions,
such as NaHCO3 does, it is called a buffer.
It tends to damp out either extreme, tries drag them to the pH it
considers "the middle".
Buffers are good to have around in case of spills of strong acids or bases.
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Update: June 2012