Weak, Strong Acids and Neutralization
Date: Fall 2009
Hello there, I am simply wondering if a weak acid will
neutralize the same amount of strong base as a strong acid with the
same concentration? We had a question like this in Chemistry, asking
how much 0.1 mol/L ethanoic acid (CH3COOH) is needed to neutralize
20mL of NaOH when 25mL of 0.1 mol/L HCl is needed to neutralize the
same amount of NaOH.
In the case of neutralizing sodium hydroxide with acetic
(systematically, "ethanoic") acid, you will make sodium
acetate. When neutralizing sodium hydroxide with hydrochloric acid,
you will make sodium chloride. The sodium acetate is itself a weak
base, unlike sodium chloride. It will take 25 mL of acid solution
to reach the endpoint in either case.
I am not terrifically comfortable with the term "neutralize." At
their endpoints, the two solutions will not have exactly the same
pH. You would need to add more acetic acid to get the pH down to
the level of the endpoint with hydrochloric acid.
Richard Barrans, Ph.D., M.Ed.
University of Wyoming
Remember that the only difference between a weak and strong acid is
that a weak acid forms an equilibrium with its ions whereas a strong
acid fully dissociates to its ions.
Keeping this in mind then, if a weak acid were to partially
dissociate (form an equilibrium), then any H+ (or H3O+) present in
solution will react with the base. But when it does so, according to
the principles described by Le Chatelier, more of the acid will
dissociate to form H+, and this will react with additional base.
This shifting of the equilibrium to form more H+ will continue until
all of the weak base is consumed.
This means then that in a reaction of an acid, HA, it doesn't matter
if the HA dissociates or equilibrates, all the H will be reacted.
Greg (Roberto Gregorius)
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Update: June 2012