9. Redox processes

9. Redox processes

9.1 Oxidation and reduction

Nature of science:

  • How evidence is used - changes in the definition of oxidation and reduction from one involving specific elements (oxygen and hydrogen), to one involving electron transfer, to one invoking oxidation numbers is a good example of the way that scientists broaden similarities to general principles.

Understandings:

  • Oxidation and reduction can be considered in terms of oxygen gain/hydrogen loss, electron transfer or change in oxidation number.

  • An oxidizing agent is reduced and a reducing agent is oxidized.

  • Variable oxidation numbers exist for transition metals and for most main-group non-metals.

  • The activity series ranks metals according to the ease with which they undergo oxidation.

  • The Winkler Method can be used to measure biochemical oxygen demand (BOD), used as a measure of the degree of pollution in a water sample.

Applications and skills:

  • Deduction of the oxidation states of an atom in an ion or a compound.

  • Deduction of the name of a transition metal compound from a given formula, applying oxidation numbers represented by Roman numerals.

  • Identification of the species oxidized and reduced and the oxidizing and reducing agents, in redox reactions.

  • Deduction of redox reactions using half-equations in acidic or neutral solutions.

  • Deduction of the feasibility of a redox reaction from the activity series or reaction data.

  • Solution of a range of redox titration problems.

  • Application of the Winkler Method to calculate BOD.

9.2 Electrochemical cells

Nature of science:

  • Ethical implications of research - the desire to produce energy can be driven by social needs or profit.

Understandings:

Voltaic (Galvanic) cells:

  • Voltaic cells convert energy from spontaneous, exothermic chemical processes to electrical energy.

  • Oxidation occurs at the anode (negative electrode) and reduction occurs at the cathode (positive electrode) in a voltaic cell.

Electrolytic cells:

  • Electrolytic cells convert electrical energy to chemical energy, by bringing about non-spontaneous processes.

  • Oxidation occurs at the anode (positive electrode) and reduction occurs at the cathode (negative electrode) in an electrolytic cell.

Applications and skills:

  • Construction and annotation of both types of electrochemical cells.

  • Explanation of how a redox reaction is used to produce electricity in a voltaic cell and how current is conducted in an electrolytic cell.

  • Distinction between electron and ion flow in both electrochemical cells.

  • Performance of laboratory experiments involving a typical voltaic cell using two metal/metal-ion half-cells.

  • Deduction of the products of the electrolysis of a molten salt.

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