Leaving Certificate Chemistry Oxidation and Reduction Michael O’Leary Patrician Academy, Mallow

Earlier Theories An early theory on oxidation was that when a substance combined with oxygen it is oxidised. e.g. Reduction is the opposite of oxidation.When oxygen is removed from a substance, it is said to be reduced. e.g. The carbon is said to be oxidised The haematite, Fe 2 O 3 is reduced

Place a piece of zinc in a solution of copper sulfate and observe ‘OIL RIG Theory A later theory stated that oxidation was loss of electrons e.g. Copper has left the solution to become plated onto the zinc plate Here, also zinc has lost two electrons Zinc is oxidised

‘OIL RIG Theory This theory also stated that reduction was gain of electrons e.g. Cu 2+ has gained 2 electrons Copper is reduced There is a complete transfer of electrons from zinc to copper

‘OIL RIG Theory Thus we have the theory OXIDATION IS LOSS OF ELECTRONS REDUCTION IS GAIN OF ELECTRONS

Oxidising & Reducing Agents A substance that allows oxidation to take place by gaining electrons itself is called the oxidising agent. The oxidising agent gains electrons The oxidising agent is reduced Similarly, The reducing agent is oxidised

Consider the reaction between carbon and oxygen According to the earlier theory, here the carbon is oxidised. But as CO 2 is a covalent molecule and NO IONS here, there is NO complete transfer of electrons from Carbon to form Carbon Dioxide. i.e. CO 2 is a covalent molecule where the electrons are shared. A new theory of oxidation and reduction is needed…

Oxidation Number In order to overcome this problem, the concept of OXIDATION NUMBER was introduced. OXIDATION NUMBER is defined as - the charge that an atom has or appears to have when electrons are distributed according to certain rules.

Rules for Assigning Oxidation Numbers  The oxidation number (O.N.) for atoms of elements in the uncombined state is zero (0) e.g. O.N. of sodium in Na is 0 (zero), the O.N. of hydrogen in H 2 is 0 (zero)  In neutral molecules such as H 2 0, the sum of the O.N.s is 0 (zero) Here, each Hydrogen is +1 and Oxygen is -2  In complex ions such as CO 3 2-, the sum of the O.N.s of the elements is equal to the charge on the ion

Rules for Assigning Oxidation Numbers - continued  In ions, the O.N. of a monatomic ion equals the charge of the ion e.g. the oxidation number of Na + is +1; the oxidation number of N 3- is -3 The O.N. of F - is -1  The O.N. of oxygen is -2, except in peroxides (H 2 O 2 ) when it is -1, and when in OF 2 when it is +2  The O.N. of hydrogen is +1 except in metal hydrides e.g. NaH, MgH 2 when it is -1  The O.N. of chlorine is -1 except when combined with oxygen or fluorine

Calculating Oxidation Numbers What are the oxidation numbers of the elements in CO 2 ? The O.N. of oxygen is -2 Since there are 2 oxygen atoms, the total contribution of oxygen is -4, therefore the O.N. of the carbon atom is +4.

Question What are the oxidation numbers of the elements in H 2 SO 4 ? Solution: Start with elements whose O.N. can be deduced directly from the rules H 2 SO 4 The O.N. of hydrogen is The O.N. of oxygen is The sum of the O.N.s = 0 X = O.N. of sulfur 2(+1) + X + 4(-2) = 0 X = X = 6 +6

Quiz What is the oxidation number of Hydrogen in H 2 O Hydrogen in H 2 O 2 Oxygen in Cu 2 O Calcium in CaCl 2 -3 Nitrogen in NH 3 -1 Oxygen in H 2 O 2

And Finally What is the oxidation number of Carbon in CO Hydrogen in OH - Hydrogen in MgH Carbon in C 6 H 12 O 6 +7 Manganese in MnO Sulfur in SO 4 2-