IB DP Chemistry: SL复习笔记8.1.2 Conjugate Acid-Base Pairs

Conjugate Acid-Base Pairs

  • Brønsted-Lowry acid is a species that can donate a proton
  • Brønsted-Lowry base is a species that can accept a proton
  • In a reaction 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 CH3COOby 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
  • 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

1.7-Equilibria-Water-as-Bronsted-Lowry-Acid

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

8.1.2-Lewis-Diagram-of-Reaction-between-Water-and-Ammonia

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

1.7-Equilibria-Water-as-Bronsted-Lowry-Base

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

8.1.2-Lewis-Diagram-of-Reaction-between-Water-and-Hydrochloric-Acid

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.   CH3CH2CH2COOand H3O+

Answer

The correct option in B

    • A conjugate acid-base pair differ only by an H+ ion

 

 

 

 

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