Chemistry 30

1.5 Conjugate Acid-Base Pairs

As you ponder the Brønsted-Lowry theory of acids and bases there are some important things to think about. Once again we'll examine ammonia:

Consider how NH3 changes to: NH4+	NH3 → NH4+
The formulas differ by a single hydrogen; NH3 gains an H+ to become NH4+
Consider how H2O (or HOH) changes to OH–:	HOH → OH–
Again the formulas differ only by a single hydrogen; H2O  lost a H+ forming OH–
Now consider these two changes as reversible reactions. What if the reaction proceeds in the opposite direction:
NH4+ can change back to NH3:	NH4+ → NH3
OH– can change back into H2O:	OH– → HOH

Putting these observations together we see that

• ammonia acts as a base because it can combine with a hydrogen ion. It's partner ammonium is now an acid, for it has a hydrogen ion that it can give up; once it does it is converted back into ammonia.
  
  
• water acts as an acid because it gives away a hydrogen ion to ammonia. Once it has lost the hydrogen ion and becomes hydroxide, the hydroxide in turn can act as a base and accept a hydrogen ion from ammonium.

What we have here are conjugate acid-base pairs

Conjugate acid-base pairs differ from each other by the presence or absence of a single hydrogen ion (proton). Every acid has a conjugate base, and every base has a conjugate acid.

The conjugates will always be listed on the product side of the reaction.