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Topic 8: Acids & Bases


8.1 Theories of Acids & Bases
Essential Idea:
  • Many reactions involve the transfer of a proton from an acid to a base.
Understandings:
  • 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 & 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.
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  • Theories being superseded—one early theory of acidity derived from the sensation of a sour taste, but this had been proven false.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".
PPT: Theories of Acids & Bases
Khan Academy: Bronsted Lowry Theory
Deduce Conjugate Acid or Base Practice with Answer Key
8.2 Properties of Acids & Bases
Essential Idea:
  • The characterization of an acid depends on empirical evidence such as the production of gases in reactions with metals, the colour changes of indicators or the release of heat in reactions with metal oxides and hydroxides.
Understandings:
  • Most acids have observable characteristic chemical reactions with reactive metals, metal oxides, metal hydroxides, hydrogen carbonates and carbonates.
  • Salt and water are produced in exothermic neutralization reactions.
Applications & Skills:
  • Balancing chemical equations for the reaction of acids.
  • Identification of the acid and base needed to make different salts.
  • Candidates should have experience of acid-base titrations with different indicators.
Nature of Science:
  • Obtaining evidence for theories—observable properties of acids and bases have led to the modification of acid-base theories
PPT: Properties of Acids & Bases
Music Video: All About That Base (No Acid)
Properties & Models of Acids & Bases
Chemistry - Acids, Bases and Salts: Reactions of Acids and Bases
8.3 The pH Scale
Essential Idea:
  • The pH scale is an artificial scale used to distinguish between acid, neutral and basic/alkaline solutions.
Understandings:
  • pH = -log[H+(aq)] and [H+] = 10^-pH
  • A change of one pH unit represents a 10-fold change in the hydrogen ion concentration [H +].
  • pH values distinguish between acidic, neutral and alkaline solutions.
  • The ionic product constant, Kw = [H+][OH-] = 10^-14 at 298 K.
Applications & Skills:
  • Solving problems involving pH, [H+] and [OH-].
  • Students should be familiar with the use of a pH meter and universal indicator.
Nature of Science:
  • Occam’s razor—the pH scale is an attempt to scale the relative acidity over a wide range of H+ concentrations into a very simple number.
PPT: The pH Scale
8.4 Strong and Weak Acids & Bases
Essential Idea:
  • The pH depends on the concentration of the solution. The strength of acids or bases depends on the extent to which they dissociate in aqueous solution.
Understandings:
  • Strong and weak acids and bases differ in the extent of ionization.
  • Strong acids and bases of equal concentrations have higher conductivities than weak acids and bases.
  • A strong acid is a good proton donor and has a weak conjugate base.
  • A strong base is a good proton acceptor and has a weak conjugate acid.
Applications & Skills:
  • Distinction between strong and weak acids and bases in terms of the rates of their reactions with metals, metal oxides, metal hydroxides, metal hydrogen carbonates and metal carbonates and their electrical conductivities for solutions of equal concentrations.
Nature of Science:
  • Improved instrumentation—the use of advanced analytical techniques has allowed the relative strength of different acids and bases to be quantified.
  • Looking for trends and discrepancies—patterns and anomalies in relative strengths of acids and bases can be explained at the molecular level.
  • The outcomes of experiments or models may be used as further evidence for a claim—data for a particular type of reaction supports the idea that weak acids exist in equilibrium.
PPT: Strong and Weak Acids and Bases
8.5 Acid Deposition
Essential Idea:
  • Increased industrialization has led to greater production of nitrogen and sulfur oxides leading to acid rain, which is damaging our environment. These problems can be reduced through collaboration with national and intergovernmental organizations.
Understandings:
  • Rain is naturally acidic because of dissolved CO2 and has a pH of 5.6. Acid deposition has a pH below 5.6.
  • Acid deposition is formed when nitrogen or sulfur oxides dissolve in water to form HNO3, HNO2, H2SO4 and H2SO3.
  • Sources of the oxides of sulfur and nitrogen and the effects of acid deposition should be covered.
Applications & Skills:
  • Balancing the equations that describe the combustion of sulfur and nitrogen to their oxides and the subsequent formation of H2SO3, H2SO4, HNO2 and HNO3.
  • Distinction between the pre-combustion and post-combustion methods of reducing sulfur oxides emissions.
  • Deduction of acid deposition equations for acid deposition with reactive metals and carbonates.
Nature of Science:
  • Risks and problems—oxides of metals and non-metals can be characterized by their acid–base properties. Acid deposition is a topic that can be discussed from different perspectives. Chemistry allows us to understand and to reduce the environmental impact of human activities.
PPT: Acid Deposition