18. Acids and bases

18. Acids and bases

18.1 Lewis acids and bases

Nature of science:

  • 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.

Understandings:

  • 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:

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

18.2 Calculations involving acids and bases

Nature of science:

  • Obtaining evidence for scientific theories - application of the equilibrium law allows strengths of acids and bases to be determined and related to their molecular structure.

Understandings:

  • The expression for the dissociation constant of a weak acid (K a) and a weak base (K b).

  • For a conjugate acid base pair, K a × K b = K w .

  • The relationship between K a and pK a is (pK a = -log Ka ), and between Kb and pK b is (pK b = -log K b).

Applications and skills:

  • Solution of problems involving [H+ (aq)], [OH-(aq)], pH, pOH, K a, pK a, K b and pK b.

  • Discussion of the relative strengths of acids and bases using values of K a, pK a, K b and pK b.

18.3 pH curves

Nature of science:

  • Increased power of instrumentation and advances in available techniques - development in pH meter technology has allowed for more reliable and ready measurement of pH.

Understandings:

  • The characteristics of the pH curves produced by the different combinations of strong and weak acids and bases.

  • An acid–base indicator is a weak acid or a weak base where the components of the conjugate acid–base pair have different colours.

  • The relationship between the pH range of an acid–base indicator, which is a weak acid, and its pK a value.

  • The buffer region on the pH curve represents the region where small additions of acid or base result in little or no change in pH.

  • The composition and action of a buffer solution.

Applications and skills:

  • The general shapes of graphs of pH against volume for titrations involving strong and weak acids and bases with an explanation of their important features.

  • Selection of an appropriate indicator for a titration, given the equivalence point of the titration and the end point of the indicator.

  • While the nature of the acid–base buffer always remains the same, buffer solutions can be prepared by either mixing a weak acid/base with a solution of a salt containing its conjugate, or by partial neutralization of a weak acid/base with a strong acid/base.

  • Prediction of the relative pH of aqueous salt solutions formed by the different combinations of strong and weak acid and base.

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