8. Acids and bases

8.1 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.
Theories being superseded - one early theory of acidity derived from the sensation of a sour taste, but this had been proven false.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".

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 and skills:

Nature of science:

Obtaining evidence for theories - observable properties of acids and bases have led to the modification of acid-base theories.

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 and 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:

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.

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 and skills:

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.

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 and 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:

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.

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 and 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.

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