CBSE Class 10 Chemistry Chemical Reactions and Equations Notes Set 21

Oxidation and Reduction

Oxidation and reduction always occur together, so such reactions are called redox reactions. In \( \ce{CuO + H2 -> Cu + H2O} \) copper oxide is reduced and hydrogen is oxidised, so hydrogen is the reducing agent. In \( \ce{ZnO + C -> Zn + CO} \) carbon is the reducing agent.

Effects of Oxidation in Daily Life

1. Corrosion: slow attack on a metal by air and moisture. Iron rusts to hydrated iron(III) oxide, \( \ce{Fe2O3.xH2O} \). Copper gets a green coating and silver turns black. Gold and platinum do not corrode.
2. Rancidity: fats and oils in food are oxidised, giving a bad smell and taste. It is prevented by antioxidants, air-tight containers, refrigeration and flushing the packet with nitrogen.
3. Prevention of rusting: painting, oiling, galvanising (coating with zinc), alloying and electroplating.
4. Combustion is oxidation with release of heat and light, and respiration is slow oxidation of food in the body.

Energy Diagrams

In an exothermic reaction the products have lower energy than the reactants and the energy difference is released. In an endothermic reaction the products have higher energy and energy is absorbed. The hump in the diagram is the activation energy, which is the minimum energy the reactants need to start the reaction. A catalyst lowers it.