AS/A-level Chemistry - Oxidation and Reduction

Do you remember the redox reaction in AS/A-level Chemistry?

• When an oxidation reaction and a reduction reaction occur simultaneously, this is called a Redox Reaction.
• An oxidising agent removes electrons. (reduced)
• A reducing agent gives electrons to something else. (oxidised)

Example

• When we heat a piece of copper in oxygen, the surface of the copper becomes coated with a black solid.
• The copper’s surface is oxidised –it gains oxygen.
• The product, copper ll oxide is an ionic compound:

Cu (s)+½ O (g) → Cu 2+ O 2−(s)

• The reaction is composed of two half-reactions which can be described by two half equations.

Cu → Cu 2+ + 2e-
½ O2 + 2e- → O 2−

• The copper is losing electrons in one half-reaction, and the oxygen is giving them in the other.
• The oxygen is being reduced.
• Reduction and oxidation occur together when copper reacts with oxygen, i.e. A redox reaction.

• When chlorine is bubbled through potassium iodide solution, a displacement reaction takes place.
• The colourless solution turns brown as iodine is produced.
• Chlorine has displaced the Iodine from the solution.
• The overall equation for the reaction is:

Cl2 (g)+2Kl (aq) → 2KCl (aq)+I2 (aq)

• One reactant, potassium iodide solution, contains a mixture of potassium ions and iodine ions that move around independently of each other.
• The same is true for the potassium ions and chloride ions in the product, potassium chloride solution.
• The potassium ions take no part in the chemical reaction and are spectator ions.
• On removing them from the overall equation we obtain the ionic equation:

Cl2 (g)+ 2I-(aq) + 2e- (Oxidation)

• This can be written as two half equations:

2I- (aq) → I2 (aq) + 2e- (Oxidation)
Cl2 (aq) → 2e- + 2Cl- (aq) (Reduction)

The electrons lost by the iodide ions are accepted by the chlorine molecules, which maks the chlorine molecules the oxidising agent in this reaction.The iodide ions musttherefore be the reducing agent, and this is confirmed by the fact that they donate the electrons needed to reduce the chlorine molecules.

• Oxidation states simplify the whole process of working out what is being oxidised and what is being reduced in redox reactions.
• Oxidation state shows the total number of electrons which have been removed from an element–a positive oxidation state or added to an element –a negative oxidation state.
• Each atom in a molecule or ion is assigned and oxidation state to show how much it is oxidised or reduced.

Rules about oxidation state

• Atoms in elements are in oxidation state 0.
• In simple ions the oxidation state is the same as the charge of the ions.
• Elements just bonded to identical atoms like O2and H2, have an oxidation state of 0.
• The oxidation state of a simple monatomic ion, Na+ is the same as its charge.
• In compounds or compound ions the overall oxidation state is just the ion charge. (i.e. the sum of all the oxidation states is equal to the charge of the ion)
• The sum of the oxidation states for a neutral compound is 0.
• Combined oxygen is nearly always -2 except in peroxides where its -1.
• Combined hydrogen is +1 except in metal hydrides where it is -1.

That's the end of the topic!

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