Redox reactions - reduction and oxidisation

Reduction and oxidisation involves chemical reactions where an electron is lost and gained.

The molecule being reduced gains an electron.

The molecule being oxidised loses an electron (usually to oxygen). It is an electron donor or reducing agent.

Mnemonics include OIL RIG, and LEO (the lion) goes GER.

Oxidation state

Using H2O as an example. O is more electronegative that H, so electrons will be around the O atom more than the H atom.

Both hydrogen atoms have been oxidised (lost an electron), the oxygen has been reduced (gained electrons). The oxygen's charge has been reduced by 2. Both H have a 1+ oxidation state, while O's oxidation state is 2-.

Charges should balance, and each atom in a molecule can have an oxidation state.

To calculate the oxidation state for an atom, look at all the other atoms adjacently bound to it, and sum up the charges.

Hydrogen in biology

Hydrogen typically bonds with C, O, P, N. In biological systems, when H binds, it will usually give it's electron away.

So if a C loses a H and gives it to a O, it can also be thought of as C losing an electron and O gaining an electron.

In organic chemistry, the molecule that gains a H is reduced.

Oxidation and reduction review from biological point-of-view at Khan Academy.