Understanding Oxidation Reduction Reactions

Oxidation reduction reactions (redox for short) are the core of energy supply in batteries. In short, when a battery is supplying energy, redox reactions are occurring that are converting chemical energy into electrical energy. Chemical energy refers to energy stored in the bonds between atoms. Some bonds require more energy to form than others. When these high energy bonds break and new lower energy molecules are formed in a redox reaction, the energy difference is released. Batteries operate by harnessing that released energy and using it to drive electrical devices.

Battery Recharging

When a battery is being recharged the reverse of the discharge process is occurring. This is also a pair of oxidation reduction reactions, except that electrical energy is being supplied to the battery to force the chemicals into forms that hold more chemical energy. This is the process that is occurring when the alternator in your car spins and supplies current to the battery.

In old times oxidation was the term given for a substance reacting with oxygen. The current definition is broader: oxidation is an increase in the oxidation number of the element which is associated with loss of electrons. From this we get the abbreviation OIL: Oxidation is Loss (of electrons).

Reduction

Reduction is the opposite of oxidation so it is no surprise that reduction is defined as a decrease in the oxidation number of the element which is associated with gain of electrons. This gives us a second abbreviation RIG: Reduction is Gain (of electrons).

OILRIG

Now we can see what the picture of the little oilrig is doing there; OILRIG is a convenient and easy way to remember the rules for electron loss and gain in oxidation reduction reactions.

Some Examples

The best way to understand the process clearly is to see several examples. There is an excellently laid out breakdown of the methane and oxygen reaction given in the balancing redox reactions page which not only shows the oxidation numbers in the reactants and products, but also gives the step by step process for understanding the electron transfer that occurs in the reaction.

Anyway, let us continue with some simple examples. The reactants and products both have their oxidation numbers stated; the product that has been oxidized in the reaction is shown in red and the one that has been reduced is shown in green.