Conjugate Acids & Bases (Cambridge (CIE) A Level Chemistry) : Revision Note
Conjugate Acids & Bases
A Brønsted-Lowry acid is a species that can donate a proton
A Brønsted-Lowry base is a species that can accept a proton
In an equilibrium reaction, the products are formed at the same rate as the reactants are used
CH3COOH (aq) + H2O (l) ⇌ CH3COO- (aq) + H3O+ (aq)
acid base conjugate base conjugate acid
The reactant CH3COOH is linked to the product CH3COO- by the transfer of a proton from the acid to the base
Similarly, the H2O molecule is linked to H3O+ ion by the transfer of a proton
These pairs are therefore called conjugate acid-base pairs
A conjugate acid-base pair is two species that are different from each other by a H+ ion
Conjugate here means related
In other words, the acid and base are related to each other by one proton difference
Worked Example
Identifying conjugate acid-base pairs
Identify the conjugate acid-base pairs in the following equilibrium reaction:
NH3 (g) + H2O (l) format('truetype')%3Bfont-weight%3Anormal%3Bfont-style%3Anormal%3B%7D%3C%2Fstyle%3E%3C%2Fdefs%3E%3Ctext%20font-family%3D%22math1aa495e18c7e3a21a4e48923b92%22%20font-size%3D%2216%22%20text-anchor%3D%22middle%22%20x%3D%2210.5%22%20y%3D%2216%22%3E%26%23x21CC%3B%3C%2Ftext%3E%3C%2Fsvg%3E){kind=small}
NH4+ (aq) + OH– (aq)
Answer:
In the forward reaction:
NH4+ is the conjugate acid of the base NH3
OH- is the conjugate base of the acid H2O
In the reverse reaction
NH3 is the conjugate base of the acid NH4+
H2O is the conjugate acid of the base OH–
Worked Example
In the equilibrium reaction shown below, which species are a conjugate acid-base pair?
CH3CH2CH2COOH (aq) + H2O (l) ⇌ CH3CH2CH2COO- (aq) + H3O+ (aq)
A. CH3CH2CH2COOH and H2O
B. H2O and H3O+
C. H2O and CH3CH2CH2COO-
D. CH3CH2CH2COO- and H3O+
Answer:
The correct option in B
A conjugate acid-base pair differ only by an H+ ion
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