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Conjugate Acid-Base Pairs (DP IB Chemistry: SL)

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Conjugate Acid-Base Pairs

  • 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 a reaction at equilibrium, 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

Amphiprotic Species

  • Species that can act both as proton donors and acceptors are called amphiprotic
    • Eg. water as a Brønsted-Lowry acid

Equilibria Water as Brønsted -Lowry Acid, downloadable AS & A Level Chemistry revision notes

The diagram shows water acting as a Brønsted-Lowry acid by donating a proton to ammonia which accepts the proton using its lone pair of electrons

Lewis Diagram of Reaction between Water and Ammonia, downloadable IB Chemistry revision notes

The Lewis diagram for the reaction of water with ammonia to show how water acts as a Brønsted-Lowry acid and ammonia as a Brønsted-Lowry base

  • Eg. water as a Brønsted-Lowry base

Equilibria Water as Brønsted -Lowry Base, downloadable AS & A Level Chemistry revision notes

The diagram shows water acting as a Brønsted-Lowry base by accepting a proton from hydrochloric acid proton using its lone pair of electrons

Lewis Diagram of Reaction between Water and Hydrochloric Acid, downloadable IB Chemistry revision notes

The Lewis diagram for the reaction of water with hydrochloric acid to show how water acts as a Brønsted-Lowry base and ammonia as a Brønsted-Lowry acid

What is the difference between amphiprotic and amphoteric?

  • A compound that is amphoteric means it has both basic and acidic character
    • When the compound reacts with an acid, it shows that it has basic character
    • When it reacts with a base, it shows that it's acidic
    • An example of this is aluminum oxide which reacts with both hydrochloric acid and sodium hydroxide:

Al2O3 (s)    +    6HCl (aq)          2AlCl3  (aq)         +   3H2O (l)       

 Al2O3 (s)       +   2NaOH (aq)     +   3H2O (l)            2NaAl(OH)4 (aq)       

  • When a compound is amphiprotic, it means it can act as a proton donor and as a proton acceptor
  • Aluminium oxide is not amphiprotic, even though it is amphoteric

Amphiprotic versus Amphoteric Table

8-1-2-amphiprotic-versus-amphoteric

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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Stewart

Author: Stewart

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Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.