Redox Reactions

What is a redox reaction? A Reduction reaction and an Oxidation reaction occurring simultaneously Zn(s) + Cu2(aq)  Zn2+(aq) + Cu(s)

What is oxidation Loss of Electrons Gain of Oxygen Loss of Hydrogen Zn(s)  Zn2+(aq)+ 2e– Gain of Oxygen Fe2O3 (s) + 3CO(g)  2Fe(s) + CO2 (g) Loss of Hydrogen 2C8H18(s) + 25O2(g)  16CO2 (g) + 18H2O(l) Gain in Oxidation Number

What is reduction Gain of Electrons Loss of Oxygen Gain of Hydrogen Cu2(aq) + 2e–  Cu(s) Loss of Oxygen Fe2O3 (s) + 3CO(g)  2Fe(s) + CO2(g) Gain of Hydrogen 2C8H18(s) + 25O2(g)  16CO2 (g) + 18H2O(l) Loss in Oxidation Number

What is an oxidant or oxidising agent They cause OXIDATION to occur Undergo Reduction Be reduced themselves

What is a reductant or reducing agent They cause REDUCTON to occur Undergo Oxidation Are oxidized themselves

Oxidation Numbers     Can use to determine if a reaction is a REDOX reaction They have no physical meaning They treat molecular substances as if they were ionic Can also be referred to as Oxidation States 

Oxidation Number Rules Oxid No. of the atoms of free (uncombined elements is zero Cu in elemental copper is zero O in elemental O2 is zero

Oxidation Number Rules Oxid No of a monoatomic ion is the same as the charge on the ion In CaCl2 Oxid No of Ca is 2+ Oxid No of Cl is 1–

Oxidation Number Rules Oxid No of oxygen in most cases is –2 Eg CaO Ca = +2 , O = –2 Eg H2O H = +1, O = –2 Exception is H2O2 (peroxide) where Oxid No of O is –1

Oxidation Number Rules Oxid No of Hydrogen in most cases is +1 Exception is in Metal Hydrides, where Oxid No = –1 Eg NaH where Na = +1, H = –1

Oxidation Number Rules In a molecule or molecule ion the negative Oxid No. is assigned to the atom of the most electronegative element Eg CF4 C = +4, F = –1 Flourine is the more electronegative so it has the negative Oxid No.

Oxidation Number Rules The sum of the Oxid Nos in a molecule or neutral compound is zero Eg H2O the sum is 2X(+1) + – 2 = 0

Oxidation Number Rules The sum of the Oxid Nos in a molecule ion is equal to the charge on the ion Eg NH4+ N = –3, H = +1 the sum is – 3 + 4(+1) = +1 For SO42– the Oxid No is –2

How Are Oxidation Numbers Used An INCREASE in oxidation number indicates Oxidation A DECREASE in oxidation number indicates Reduction C(s) + O2(g)  CO2(g)  0 0 +4 2(–2) C increases from 0 to +2 Oxidation O decreases from 0 to –2 Reduction

Writing Half Reactions Balance all elements except hydrogen or oxygen Balance the oxygen atoms by adding H2O Balance the hydrogen atoms by adding H+ ions Balance the charge on both sides by adding electrons e– Add states