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HCl + H2O ⇌ H3O + + Cl¯<br />
HCl - this is an acid, because it has a proton available to be transferred.<br />
H2O - this is a base, since it gets the proton that the acid lost.<br />
Now, here comes an interesting idea:<br />
H3O + - this is an acid, because it can give a proton.<br />
Cl¯ - this is a base, since it has the capacity to receive a proton.<br />
Notice that each pair (HCl and Cl¯ as well as H2O and H3O + differ by one proton (symbol = H + ). These<br />
pairs are called conjugate pairs.<br />
HNO3 + H2O ⇌ H3O + + NO3¯<br />
The acids are HNO3 and H3O + and the bases are H2O and NO3¯.<br />
Remember that an acid-base reaction is a competition between two bases (think about it!) for a<br />
proton. If the stronger of the two acids and the stronger of the two bases are reactants (appear on the<br />
left side of the equation), the reaction is said to proceed to a large extent.<br />
Here are some more conjugate acid-base pairs to look for:<br />
H2O and OH¯<br />
HCO3¯ and CO3 2¯<br />
H2PO4¯ and HPO4 2¯<br />
HSO4¯ and SO4 2¯<br />
NH4 + and NH3<br />
CH3NH3 + and CH3NH2<br />
HC2H3O2 and C2H3O2¯<br />
B. Reactions that proceed to a small extent:<br />
If the weaker of the two acids and the weaker of the two bases are reactants (appear on the left side<br />
of the equation), the reaction is said to proceed to only a small extent:<br />
HC2H3O2 + H2O ⇌ H3O + + C2H3O2¯<br />
NH3 + H2O ⇌ NH4 + + OH¯<br />
Identify the conjugate acid base pairs in each reaction.<br />
HC 2H 3O 2 and C 2H 3O 2¯<br />
is one conjugate pair.<br />
H 2O and H 3O + is the other.<br />
NH 3 and NH 4<br />
+<br />
is one pair.<br />
H 2O and OH¯ is the other.<br />
Notice that H 2O in the first equation is acting as a base and in the second equation is acting as an acid.
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