Science

Chemistry

Chemistry Other

Chapter 13- Oxidation and Reduction

Oxidation

Gain of oxygen
Loss of hydrogen
Loss of electrons
Increase in oxidation state (e.g. from -1 to 0)
For half equation of reduction, electrons are shown on the left side, e.g.
Pb2+ + 2e- →Pb

Reduction

Loss of oxygen
Gain of hydrogen
Gain of electrons
Decrease in oxidation state (e.g. from 0 to -1)

For half equation of oxidation, electrons are shown on the right side, e.g.

2Br- → Br2 + 2e-

Oxidation state: the charge an atom of an element would have if it existed as an ion in a compound.
Redox reactions: chemical reactions which involve oxidation of one substance and reduction of another.
Oxidising agent: a substance that causes another substance to be oxidised, and is reduced itself in the process.
Reducing agent: a substance that causes another substance to be reduced, and is oxidised in that process.
To check if oxidation or reduction have taken place in an reaction, write a balanced chemical equation with oxidation states, and compare them on both sides.
Test for oxidising agents:

Test	Observation when positive	Explanation
Add aqueous Potassium iodide, KI, to unknown substance.	Brown solution is formed	Iodide ions are oxidised to iodine by oxidising agent.2I- I2 + 2e-
Dip a piece of starch-iodide paper in unknown solution.	Starch-iodide paper turns blue from white	Iodide ions are oxidised to iodine which reacts with starch to give a blue colour.

Test for reducing agents:

Tests	Observation when positive	Explanation
For gases: Place a piece of filter paper soaked with acidified potassium manganate (VII), KMnO4, at the mouth of the test tube	Filter paper turns from purple to colourless	The manganate (VII) ion, MnO4-, is reduced to manganese (II) ion, Mn2+ (colourless)
For solutions: Add acidified potassium manganate (VII) to unknown solution	Colour changes from purple to colourless	The manganate (VII) ion, MnO4-, is reduced to manganese (II) ion, Mn2+ (colourless)