1. Where do those electrons go? An introduction to oxidation and Reduction

2. OXIDATION GAIN OF OXYGEN 2Mg + O 2 ——> 2MgO magnesium has been oxidised as it has gained oxygen OXIDATION & REDUCTION - Definitions

3. REDUCTION REMOVAL (LOSS) OF OXYGEN CuO + H 2 ——> Cu + H 2 O copper(II) oxide has been reduced as it has ‘lost’ oxygen However as chemistry became more sophisticated, it was realised that another definition was required

4. ... OXIDATION Removal (loss) of electrons ‘OIL’ species will get less negative or more positive Oxidation is loss REDUCTION Gain of electrons ‘RIG’ species will become more negative or less positive Reduction is Gain REDOX When reduction and oxidation take place – when electrons are transferred form one species to another OXIDATION AND REDUCTION IN TERMS OF ELECTRONS Oxidation and reduction are not only defined as changes in O and H

5. Used to...tell if oxidation or reduction has taken place work out what has been oxidised and/or reduced OXIDATION NUMBERS (STATES) RULES FOR ASSIGNING OXIDATION NUMBERS 1.The oxidation number of a free, uncombined element is zero. 2.The charge on a simple (monatomic) ion is the oxidation number of the element in that ion. 3.In a polyatomic ion, the sum of the oxidation numbers of the atoms in that ion is equal to the charge on the ion. 4.In compounds (whether covalent or ionic), the sum of the oxidation numbers of all atoms in the compound is zero. 5.The oxidation number of oxygen is -2, except in peroxides where it is -1. (The only peroxide you are likely to meet is hydrogen peroxide H 2 O 2 ) 6.The oxidation number of hydrogen is +1, except in the case of hydrides where it is -1.(A hydride is rare and is is when hydrogen combines with a metal e.g LiH)

6. Q. What are the oxidation states of the elements in the following? a) C b) Fe 3+ c) Fe 2+ d) O 2- e) He f) Al 3+ Q. What are the oxidation states of the elements in the following? a) C b) Fe 3+ c) Fe 2+ d) O 2- e) He f) Al 3+ A. a) C 0b) Fe 3+ +3c) Fe 2+ +2 d) O 2- -2e) He 0f) Al 3+ +3 A. a) C 0b) Fe 3+ +3c) Fe 2+ +2 d) O 2- -2e) He 0f) Al 3+ +3

7. because CO 2 is a neutral molecule, the sum of the oxidation numbers must be zero the oxidation number of oxygen is -2, so the oxidation number of carbon must be +4 OXIDATION NUMBERS Explanation MOLECULES The SUM of the oxidation numbers adds up to ZERO ELEMENTSH in H 2 = 0both are the same and must add up to Zero COMPOUNDSC in CO 2 = +4 O in CO 2 = -21 x +4 and 2 x -2 = Zero

8. OXIDATION NUMBERS in SO 4 2- the oxidation number of S = +6there is ONE S O = -2there are FOUR O’s +6 + 4(-2) = -2 so the ion has a 2- charge POLYATOMIC IONS The SUM of the oxidation numbers adds up to THE CHARGE e.g.NO 3 - sum of the oxidation states = - 1 SO 4 2- sum of the oxidation states = - 2 NH 4 + sum of the oxidation states = +1 Examples

9. Q. Give the oxidation state of the element other than O, H in the following ; SO 2 NH 3 NO 2 NH 4 + Cl 2 O 7 NO 3 - NO 2 - SO 3 2- S 2 O 3 2- MnO 4 2- Q. Give the oxidation state of the element other than O, H in the following ; SO 2 NH 3 NO 2 NH 4 + Cl 2 O 7 NO 3 - NO 2 - SO 3 2- S 2 O 3 2- MnO 4 2- A. SO 2 +4NH 3 -3 NO 2 +4 NH 4 + -3 Cl 2 O 7 +7 NO 3 - +5 NO 2 - +3 SO 3 2- +4 S 2 O 3 2- +2 MnO 4 2- +6 A. SO 2 +4NH 3 -3 NO 2 +4 NH 4 + -3 Cl 2 O 7 +7 NO 3 - +5 NO 2 - +3 SO 3 2- +4 S 2 O 3 2- +2 MnO 4 2- +6

10. OXIDATION NUMBERS CALCULATING OXIDATION NUMBERS Q. What is the oxidation state of each element in the following compounds/ions ? CH 4 PCl 3 NCl 3 CS 2 ICl 5 PCl 4 + H 3 PO 4 NH 4 Cl H 2 SO 4 MgCO 3 SOCl 2

11. OXIDATION NUMBERS CALCULATING OXIDATION NUMBERS Q. What is the oxidation state of each element in the following compounds/ions ? CH 4 C = - 4H = +1 PCl 3 P = +3Cl = -1 NCl 3 N = +3Cl = -1 CS 2 C = +4S = -2 ICl 5 I = +5Cl = -1 PCl 4 + P = +4Cl = -1 H 3 PO 4 P = +5H = +1O = -2 NH 4 ClN = -3H = +1Cl = -1 H 2 SO 4 S = +6H = +1O = -2 MgCO 3 Mg = +2H = +4O = -2 SOCl 2 S = +4Cl = -1O = -2