Introduction

Chemistry deals with the enormous variety of matter and the many ways one kind of matter transforms into another. Redox reactions — reactions in which oxidation and reduction occur together — are one of the most important classes of these transformations. They govern a striking range of phenomena, both physical and biological, and find extensive practical use across pharmaceutical, biological, industrial, metallurgical and agricultural fields.

Some familiar examples make the reach of redox chemistry clear. Burning fuels — coal, petrol, wood and others — to release energy for domestic, transport and other commercial purposes is a redox process, and so is respiration, by which living cells "burn" glucose to release energy. Photosynthesis, the reverse process by which green plants build carbohydrates from carbon dioxide and water while releasing oxygen, is redox chemistry too. On the industrial side, redox reactions drive the electrochemical extraction of highly reactive metals and non-metals that would otherwise be impossible to isolate, the large-scale manufacture of chemicals such as caustic soda, the operation of both dry and wet batteries, and the slow corrosion of metals such as the rusting of iron.

Redox chemistry is not just an industrial-age topic — it sits at the centre of some of today's most pressing environmental questions too, from the Hydrogen Economy (using liquid hydrogen as a clean fuel) to the chemistry behind the depletion of the ozone layer.

This chapter builds the concept of oxidation and reduction from three progressively more powerful points of view — the classical idea (gain or loss of oxygen/hydrogen), the electron-transfer idea, and the oxidation-number idea — and then uses these ideas to classify redox reactions, balance redox equations, and connect redox chemistry to electrode processes and electrochemical cells, a foundation for the deeper electrochemistry you will study in Class XII.