Conjugate Acids and Bases

[Jessica Castellanos]

How do you identify conjugate acids and bases in a chemical reaction? How do they differ from their respective acids and bases?

[sarahforman_Dis2I]

How do you identify conjugate acids and bases in a chemical reaction? How do they differ from their respective acids and bases?

A conjugate acid/base in a reaction is what is left over after the proton is donated or accepted from water. For example, in the dissociation of HCl, Cl- and H3O+ ions are left over after the dissociation. This means that Cl- is the conjugate BASE to the acid HCl.

Here is another example with a weak base NH3. NH3 partially dissociates to form NH4+ and OH-. The NH4+ could theoretically donate another proton back to the OH- ion, meaning that it could act as an acid. This means that NH4+ is the conjugate acid of the base NH3.
I hope this helps

[Paul Hage 2G]

Conjugate acids and bases will typically have either one less H+ ion or one more H+ ion than their reacting counterparts.
For example, in the reaction HCl + H2O --> Cl- + H3O+, you can immediately notice that on the reactants side, HCl has one more proton than the Cl- on the products side and the Cl- on the products side has one less H+ ion than the HCl on the reactants side. Looking at the forward reaction (which is technically all there is because HCl is a strong acid), we see that the HCl molecule must donate an H+ to result in the Cl- that is on the products side. Therefore, it is an acid. The conjugate base would be the Cl- because this is the result of the acid (HCl) its H+ ion.

[Sue Bin Park 2I]

the term 'conjugate acid/base' is arbitrary and based on whatever we want to be focusing on. if HA + H2O <-> A- + H3O+ is our reaction, we could call HA our acid and A- our conjugate base, or A- our base and HA our conjugate acid. the word 'conjugate' just establishes the fact that one species is related to the other by a difference in protonation.

[kristi le 2F]

it is also helpful to know that if there is a strong acid, it will have a weak conjugate base and a strong base will have a weak conjugate acid. Therefore, by knowing the strength of the conjugate, we can determine the strength of the acid or base.