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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