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Submitted by Sunil on Fri, 04/04/2008 - 20:32

Can someone please explain how conjugate acids and bases work. I know how you can find the conjugate acid of a base and conjugate base of an acid, but I am not sure how you can use those to predict products in your aqueous reactions. Can someone provide a detailed explanation please?

Conjugate acid base pairs are made up of an acid that can donate a proton (HB) and it's conjugate base (B-).  The difference between the acid and the base is the extra H+.  The acid has one and the base doesn't.

The conjugate bases of strong acid such as HCl are very weak bases.  Since the acid so readily donates the H+ ion, the base is not going to readily accept it back.

Conjugate acid base pairs are most important when dealing with weak acids and bases.  For instance, acetic acid (HC2H3O2) is a weak acid that partially ionizes in water.  The acetate ion (C2H3O2-) will readily accept protons to return to the acid (that action is what makes it a base.

The chemical properties of the conjugate acid base pair are most significant in buffers.  Buffers are solutions of conjugate acid base pairs that are excellent for maintaining fairly constant pH values even when acids or bases are added to them.

These buffers work because if a strong acid is added to the mixture, the conjugate base absorbs most of the extra H+ ions from the acid, and prevents the pH from changing significantly.

Likewise, if a stong base is added to a buffer system the conjugate acid part of the buffer would give up  H+ ions to neutralize most of the additional OH- ion.

Hope that helps

Submitted by spock on Fri, 04/04/2008 - 21:03 Permalink

Spock has done a brilliant job explaining the concept of conj acids and bases. In case you're more of a visual learner, heed the example:

HCl------->  H+    +    Cl-
acid          conj acid      conj base

so why is H+ the conj acid and Cl- the conj base and not the other way around?

Well, let's review: According the definition of a Bronsted Lowry acids and bases, any substance that can donate a hydrogen ion (H+) is a bronsted lowry acid, and any substance that can accept a hydrogen ion is a bronsted lowry base. Thus, acids  must come in conjugate pairs of a base and an acid.

The H+ will always be the conjugate acid: it can donate itself to a B/L base to form an acid/base conjugate pair (which is the undissociated acid or base, such as HCl)

Therefore, H+ is the conjugate acid and Cl- is the conjugate base.

I think the thing that gets most students confused is the word "conjugate".
Conjugate just means "together"

An exceptionally simplified summary of the matter is:
An acid consists of two parts: one "acidic part", one "basic" part. The acidic part is the conjugate acid. The basic part is the conjugate base.

Submitted by kyle1990 on Tue, 04/22/2008 - 15:10 Permalink