Redox reactions and redox equilibria
The concept of redox reactions is first introduced at GCSE where students will have met the idea of defining oxidation and reduction in terms of electrons. This is extended and refined in the first year of A level by the introduction of the concept of oxidation state (or number). Students need to learn to correctly assign oxidation states to atoms in compounds and ions. This then leads to redefining oxidation as involving an increase in oxidation number and reduction as involving a decrease.
Redox reactions are then revisited in the second year and a much more rigorous approach is taken, leading to the concept of electrochemical cells and standard electrode potentials. Students need to learn a significant amount of terminology and to be able to apply this to a discussion of electrohemical cells, and to use data on standard electode potentials to calculate the overall potential for an electrochemical cell. This is an area that students often find difficult and practice is needed to become comfortable and proficient with the various mathematical concepts and manipulations.
One particular area of difficulty is assigning the electrodes in an electrochemical cell, as the definition of anode and cathode seems to conflict with that learned at GCSE in electrolysis. At GCSE students will have been taught that the cathode is negative, yet in an electrochemical cell the cathode is positive. This problem arises because at GCSE the electrodes have been assigned based of polarity, and this is a fundamental mistake. The electrodes in any electrochemical process are correctly assigned based on process and not polarity. Thus, if we define the anode as the electrode where oxidation takes place, and the cathode as being where reduction takes place, the conflict disappears. The incorrect approach to assigning anode and cathode based on polarity is unfortunately very common in text books and other otherwise reliable resources.
Cell potential is actually a thermodynamic property, and so the link is made between this and feasibility of reaction. In addition, students should appreciate that the reactions occuring in electrochemical cells are equilibria processes. Some syllabuses include the quantitative relationships between Gibbs function and cell potential, and Gibbs function and equilibrium constant, whereas others leave the relationship at a qualitative level.
The application of the concepts developed is then applied to understanding the function of energy cells.
Redox titrations may appear is this section, or in the section on transition element chemistry, depending on the syllabus. They are included in this list, as from a pedagogical point of view, this seems the most sensible.
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