Redox Reactions. Oxidation Reduction

Oxidation and Reduction Oxidation: Gain of oxygen Loss of electrons Reduction: Loss of oxygen Gain of electrons Increase in oxidation number Decrease in oxidation number

4 Experiments: 1.Burning magnesium 2.Copper in silver nitrate solution 3.Chlorine solution and potassium iodide solution 4.Exploding hydrogen Word equation Balanced symbol equation

2Mg(s) + O 2 (g)  2MgO(s) Oxidised – gains oxygen Must be a redox! Mg  Mg 2+ O  O 2- CHARGE OF A - = GAINED ELECTRONS CHARGE OF A += LOST ELECTRONS Put the e - in. +2e - Oxidised – loss of e - Reduced – gain of e -

Cu(s) + 2AgNO 3 (aq)  Cu(NO 3 ) 2 (aq) + 2Ag(s) Ag +  Ag Cu  Cu 2+ Complete the half- equations +e - +2e - Oxidised? Reduced? Oxidised – loss of e - Reduced – gain of e -

H 2 (g) + ½ O 2 (g)  H 2 O(g) Covalent! No H + or OH - Need a new definition. oxidation reduction Reducing agent Oxidising agent

agents An oxidising agent is a substance that brings about oxidation(itself reduced) example- hydrogen peroxide for bleaching hair A reducing agent is a substance that brings about reduction.(itself oxidised)aa sulphur dioxide used to bleach straw

In terms of oxidation number Oxidation: Gain of oxygen Loss of electrons Reduction: Loss of oxygen Gain of electrons Increase in oxidation number Decrease in oxidation number

Oxidation Numbers- the seven rules The oxidation number of an atom in an uncombined element is zero. E.g. Mg in Mg, O in O 2. The oxidation number of an ion of an element is the same as its charge. O.N Br-= -1 O.N Mg in Mg 2+ = +2 ALKALI METALS= +1 ALKALINE EARTH METALS=+2 HALOGENS= -1

Oxidation Numbers The oxidation numbers of atoms in a compound add up to zero. F O-2 H+1 Cl Oxidation state of C in CO 2 ? x – 4 = 0 x = +4 Put the +!

Oxidation Numbers The oxidation numbers of atoms in a compound add up to zero. F O-2 H+1 Cl Oxidation state of Mg in MgCl 2 ? +2

Oxidation Numbers The oxidation numbers of atoms in a compound add up to zero. F O-2 H+1 Cl Oxidation state of N in NH 3 ? -3

Oxidation Numbers The oxidation numbers of atoms in an ion add up to the charge on the ion. F O-2 H+1 Cl Oxidation state of S in SO 4 2- ? x – 8 = -2 x = +6

Oxidation Numbers The oxidation numbers of atoms in an ion add up to the charge on the ion. F O-2 H+1 Cl Oxidation state of S in S 2- ? -2

Oxidation Numbers The oxidation numbers of atoms in an ion add up to the charge on the ion. F O-2 H+1 Cl Oxidation state of N in NH 4 + ? -3

OXYGEN HYDROGEN Oxygen has a charge Oxygen has a charge of –2 EXCEPT in peroxides where the charge is –1 In the compound OF 2 Where it has a value of +2. This is because F has a greater electronegative number than oxygen Hyrogen has a charge of +1 except in Metal hydrides where it has an O.N of –1 Metal Hydrides are ionic compounds NaH (+1)(-1)

Halogens- assign charge of –1 unless bonded to more electronegative element Cl 2 O (+1)2(-2) Cl= +1 when writing formulas the most electronegative is placed second. Halogen

H 2 (g) + ½ O 2 (g)  H 2 O(g) Covalent! No H + or OH - Need a new definition.

Oxidation: Gain of oxygen Loss of electrons Reduction: Loss of oxygen Gain of electrons Increase in oxidation number Decrease in oxidation number

H 2 (g) + ½ O 2 (g)  H 2 O(g) Covalent! No H + or OH - Need a new definition O H

H 2 (g) + ½ O 2 (g)  H 2 O(g) O H Oxidised? H – increase in oxidation number Reduced? O – decrease in oxidation number

Balancing Redox Reactions Using Oxidation Numbers balance the following equation. Solution: 1 assign oxidation number 2. Note element that changes oxidation number. 3.show the number of electrons lost and gained.

4.Work out ratio of oxidising agent to reducing agent. 5.Balance remaining items by inspection method.

Worked Example MnO 4 ֿ+Fe 2+ +H + Mn 2+ +Fe 3+ +H 2 O (+7)4(-2) (+2)(+1) (+2) (+3) 2 (+1)(-2) ֿֿֿֿֿֿֿֿֿֿֿֿֿ MnO 4 ֿ+Fe 2+ +H + Mn 2+ +Fe 3+ +H 2 O (+7)4(-2) (+2)(+1) (+2) (+3) 2 (+1)(-2) GAINS 5 ELECTRONS LOSES 1 ELECTRON

Balance remaining items 1MnO 4 ֿ:5Fe 2+ 1MnO 4 ֿ+5Fe 2+ +H+ 1Mn 2+ +5Fe3+ +H2O 1MnO 4ֿ+5Fe2+ +8H+ 1Mn2+ +5Fe3+ +4H2O

Well done!