CIE AS Chemistry (9701) 2019-2021

Revision Notes

1.7.7 Ionic Equilibria

Brønsted–Lowry Theory

  • The Brønsted-Lowry Theory defines acids and bases in terms of proton transfer between chemical compounds
  • A Brønsted-Lowry acid is a species that gives away a proton (H+)
  • A Brønsted-Lowry base is a species that accepts a proton (H+) using its lone pair of electrons

Equilibria Brønsted-Lowry Acid_Base Definition, downloadable AS & A Level Chemistry revision notes

The diagram shows a Brønsted-Lowry acid which donates the proton to the Brønsted-Lowry base that accepts the proton using its lone pair of electrons

  • Species that can act both as acids and bases are called amphoteric
    • 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

Equilibria Dot & Cross Diagram of Reaction between Water and Ammonia, downloadable AS & A Level Chemistry revision notes

The diagram shows a dot & cross 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

Equilibria Dot & Cross Diagram of Reaction between Water and Hydrochloric Acid, downloadable AS & A Level Chemistry revision notes

The diagram shows a dot & cross 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

  • The Brønsted-Lowry Theory is not limited to aqueous solutions only and can also be applied to reactions that occur in the gas phase

 

Equilibria Brønsted -Lowry Theory in Gaseous Reactions, downloadable AS & A Level Chemistry revision notes

Example of a Brønsted-Lowry acid and base reaction in the gas state

Exam Tip

An atom of hydrogen contains 1 proton, 1 electron and 0 neutrons. When hydrogen loses an electron to become H+ only a proton remains, which is why a H+ ion is also called a proton.

Conjugate Acids & Bases

  • A Brønsted acid is a species that can donate a proton
  • A Brønsted 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

Conjugate Acids & Bases equation

  • 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
  • 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
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