IB Chemistry home > Syllabus 2016 > Acids and Bases > Theories of acids and bases

Syllabus ref: 8.2 Syllabus ref: 18.1

Acids are normally defined according to Arrhenius idea of an aqueous solution that contains free hydrogen ions. However, there are other theories which allow the definition of acids to be extended to cover other non-aqueous systems.

Nature of science:

Falsification of theories - HCN altering the theory that oxygen was the element which gave a compound its acidic properties allowed for other acid-base theories to develop.

Theories being superseded-one early theory of acidity derived from the sensation of a sour taste, but this had been proven false.

Public understanding of science-outside of the arena of chemistry, decisions are sometimes referred to as "acid test" or "litmus test".

Theories can be supported, falsified or replaced by new theories: Acid-base theories can be extended to a wider field of applications by considering lone pairs of electrons. Lewis theory doesn't falsify Brønsted-Lowry, but extends it.


Essential idea: Many reactions involve the transfer of a proton from an acid to a base

A Brønsted-Lowry acid is a proton/H+ donor and a Brønsted-Lowry base is a proton/H+ acceptor

Amphiprotic species can act as both Brønsted-Lowry acids and bases.

A pair of species differing by a single proton is called a conjugate acid-base pair.

Understandings - HL

Essential idea: The acid-base concept can be extended to reactions that do not involve proton transfer.

A Lewis acid is a lone pair acceptor and a Lewis base is a lone pair donor.

When a Lewis base reacts with a Lewis acid a coordinate bond is formed.

A nucleophile is a Lewis base and an electrophile is a Lewis acid.

Applications and skills

Deduction of the Brønsted-Lowry acid and base in a chemical reaction.

Deduction of the conjugate acid or conjugate base in a chemical reaction.

Applications and skills - HL

Application of Lewis' acid-base theory to inorganic and organic chemistry to identify the role of the reacting species.

In Chapter 8.2