Theories of acids and bases

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. (2.5)

Theories being superseded—one early theory of acidity derived from the sensation of a sour taste, but this had been proven false. (1.9)

Public understanding of science—outside of the arena of chemistry, decisions are sometimes referred to as "acid test" or "litmus test". (5.5)

• 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.

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.

Guidance:

• Lewis theory is not required here.
• The location of the proton transferred should be clearly indicated. For example,CH₃COOH/CH₃COO^- rather than C₂H₄O₂/C₂H₃O₂^-.
• Students should know the representation of a proton in aqueous solution as both H⁺ (aq) and H₃O⁺ (aq).
• The difference between the terms amphoteric and amphiprotic should be covered.

• Acidus means sour in Latin, while alkali is derived from the Arabic word for calcined ashes. Oxygene means acid-forming in Greek, and reflects the mistaken belief that the element oxygen was responsible for a compound’s acidic properties. Acid–base theory has been developed by scientists from around the world, and its vocabulary has been influenced by their languages.

Theory of knowledge:

• Acid and base behaviour can be explained using different theories. How are the explanations in chemistry different from explanations in other subjects such as history?

Utilization:

• Syllabus and cross-curricular links:
• Topic 3.2—the acid/base character of oxides
• Topic 8.5—non-metal oxides are responsible for acid precipitation Option B.2—amino acids acting as amphiprotic species
• Option D.4—antacids are bases which neutralize excess hydrochloric acid in the stomach

  Aims:

• Aim 9: Each theory has its strengths and limitations. Lavoisier has been called the father of modern chemistry but he was mistaken about oxygen in this context.