Conjugate Acid & Base Pairs
- When acids and bases are mixed with water they dissociate into ions
- From the previous section it is important to remember that: a Brønsted-Lowry acid donates a proton (H+), and a Brønsted-Lowry base receive a proton (H+)
- Reactants are linked to products by the transfer of one proton
- E.g The reaction below show how reactants and products are related between them
CH3COOH (aq) + H2O (l) ⇌ CH3COO- (aq) + H3O+ (aq)
acid base conjugate base conjugate acid
- The reactant CH3COOH and the product CH3COO- have a difference of one proton
- The reactant H2O and the product H3O+ have a difference of one proton
- These pairs reactant-product are called conjugate acid-base pairs
- A conjugate acid-base pair is two substances that are related linked by the transfer of a proton
Exam Tip
If you want to figure out conjugate acid-base pairs you can remember that: a base receives a proton to become a conjugate acid, or an acid donates a proton to become a conjugate base
Relative strengths of conjugate acids and bases
- The relative strength of acids and bases can be predicted by analyzing the stability of their conjugate pair
- A strong acid is a good proton donor
- The conjugate base of a strong acid is a weak base
- The conjugate base of a weak acid is a strong base
- A strong base is a good proton receiver
Exam Tip
If a chemical species is an strong acid/base its conjugate pair must be weak (chemically stable), and vice versa
Worked example
Identify the conjugate acid-base pairs in the following chemical reaction and determine their relative strengths
HBr (aq) + H2O (l) → Br– (aq) + H3O+ (aq)
Answer:
Step 1: Identify the conjugate acid-base pairs
- HBr and Br-
- H2O and H3O+
Step 2: Identify the strong acid and determine the strength of the conjugate base
- By definition, if a compound is strong acid its conjugate base must be weak
- HBr is a well known strong acid. You can tell that because the one way arrow shows fully dissociation in water
- Therefore, Br-, which is its conjugate pair, is weak base
Step 3: Identify the base in the reactants, and determine the strength of the conjugate acid
- By definition, if a compound is weak base its conjugate acid must be strong
- H2O is an amphiprotic substance that can react as a weak acid or a weak base. It has a weak character because of its stability as a neutral molecule and it is not a good proton donor or acceptor
- Since water reacts with an acid, it acts as a weak base
- Therefore, H3O+, which is its conjugate pair, is a strong acid
Worked example
Order the following acids in increasing order of strength: HF, HCl, HBr, HI
Answer:
- Since iodine has the largest atomic radius and fluorine has the smallest, the bond length in increasing order is:
HF < HCl < HBr < HI
- The longer the bond, the weakest it is. Therefore, the bond strength in increasing order is the opposite to bond length
HI < HBr < HCl < HF
- Strong acids are good proton donors. Therefore, the acid with the weakest bond will release its proton easier. Since HI is the acid with the weakest bond, it is the strongest acid. In consequence, HF which is the acid with the strongest bond, is the weakest acid.
- Therefore, the following acids in increasing order of strength are:
HF < HCl < HBr < HI
