Acids and Bases: Molecular Structure and Behavior
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Conjugate Acid-Base Pairs
Below
are some molecular representations of various acids (the water molecules
have been left out). Which best represents an aqueous solution of hydrogen
cyanate (HOCN, a weak acid)?
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HOCN
is a weak acid and will partially ionize in aqueous solutions.
Good!
Since HOCN is a weak acid, both HOCN and OCN- will be in
solution.
HOCN
is a weak acid and will only partially ionize in aqueous solutions.
The chemical equation representing the reactions occuring in a hydrogen cyanate solution is:
The double arrow in the above equation represents an equilibrium. This means in the solution, both the forward and backward reactions are occurring. It is important to realize that in this solution, reactions are always occuring; reactions do not stop after equilibrium is reached. Under the magnifying glass below you can see a representation of the molecules present in this solution.
Click
on magnifying glass to see a reaction occur. Clicking again will show
another reaction.
In this representation, you can observe the acidic nature of the HOCN molecule, that is you can see it donate a proton to water. The reverse reaction shows the basic nature of the OCN- ion; you can see it accept a proton from the hydronium ion.
HOCN and OCN- are an example of a conjugate acid-base pair. The only difference between the two is a proton (H+).
All acids have a conjugate base and all bases have a conjugate acid.
From
the list of molecule/ion pairs below, click on those that are conjugate
acid-base pairs.
| H3O+
/ OH- |
NH4+
/ NH3 |
|
| OH-
/ O2- |
H2SO4
/ SO42- |
|
| H2S
/ S2- |
HOBr
/ OBr- |
