Formula sheet: Redox Reactions
Redox Reactions13 min readLesson 1 of 6
1.1 Two ways of looking at redox
In the classical view, oxidation is gain of oxygen or loss of hydrogen, and reduction is the opposite. In the electron transfer view, oxidation is loss of electrons and reduction is gain of electrons. The two views agree in every reaction, and the electron transfer view is the one that works for ionic and aqueous systems.
Oxidation and reduction always occur together, because electrons lost by one species must be gained by another. The species that loses electrons is oxidised and is the reducing agent; the species that gains electrons is reduced and is the oxidising agent. A redox reaction can be split into two half reactions, one for oxidation and one for reduction, and the electrons cancelled when the half reactions are added.
1.2 Oxidation number
The oxidation number is the charge that an atom would carry if the compound were ionic, assigned by a set of rules. The oxidation number of an element in its free state is zero, of a monoatomic ion equals its charge, of oxygen is usually minus two and of hydrogen is usually plus one. The algebraic sum of the oxidation numbers in a neutral compound is zero and in a polyatomic ion equals the ion's charge.
The oxidation number is useful for identifying redox changes. An increase in oxidation number is oxidation and a decrease is reduction, so the oxidation number change tracks the electron transfer even when no ions are shown. Oxidation numbers are only a bookkeeping device; they need not equal the actual charge on an atom in a covalent compound.
1.3 Electrode potential and feasibility
When a metal is dipped in a solution of its own ions, an equilibrium is set up between the metal and the ions, and the potential difference that develops is the electrode potential. The standard electrode potential is measured against the standard hydrogen electrode, whose potential is taken as zero. A more positive reduction potential means a greater tendency to be reduced.
The difference of the electrode potentials of the two couples in a cell gives the cell potential, and a positive cell potential means the reaction is feasible. In the Daniell cell, zinc is oxidised at the anode and copper ions are reduced at the cathode, and the cell works because zinc has the more negative reduction potential.
Exam
Balance the atoms first, then the oxygen with water, then the hydrogen with protons, and finally the charge with electrons. A half reaction that is not charge balanced will give the wrong number of electrons in the final equation.