Wize High School Grade 12 Chemistry Textbook > Acids and Bases
Strength of Acids and Bases
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Relationships Between the Strength of an Acid/Base and its Conjugate Base/Acid
The stronger an acid is, the weaker its conjugate base will be.
- HCl is a strong acid; this tells us that Cl- will be a very weak/strong:weakbase.
- Cl- is so weak that it is actually a completely ineffective base.
- On the other hand, because HCN is a weak acid the conjugate CN- will be able to act as a weak base (has some basic properties, more on this later!)
The stronger a base is, the weaker its conjugate acid will be.
Wize Concept
If we have a strong acid that dissociates completely, that means the reaction is only going in the forward direction to produce ions:
HCl → H+ + Cl-
There is no reverse reaction. As a result, it makes sense that Cl- has no basic properties.
If we consider a weak acid instead, there is incomplete dissociation, and a reverse reaction is possible:
CH3COOH + H2O ⇌ CH3COO- + H3O+
Since a reverse reaction is possible, it makes sense that the conjugate base does have some basic properties!
Note: The conjugate base of a weak acid would still be a weak base, but if we took an even weaker acid, that means the conjugate base would be a stronger weak base!
There are 3 things I want you to look at this table and see:
1) See the list of strong acids again (remember to have these memorized!)
2) Notice that the stronger the acid, the weaker the conjugate base.
3) Notice that if we look at a row in this table, we can see an acid, and it's conjugate base. Try to cover up a few bases and guess the conjugate bases. For example, what is the conjugate base of NH3? It is NH2-!
Don't memorize the table!
