Author: J R Reid Oxidation and Reduction – Introduction LEO goes GER Examples Balancing simple equations Why gain/lose electrons? Electronegativity

Defining Oxidation and Reduction Chemistry is all about atoms making or breaking bonds with other atoms. Sometimes atoms lose the hold over their electrons, or they may gain another atom’s electrons: Oxidation – when an atom loses it’s electrons Reduction – when an atom gains more electrons than it had before A quick way to remember this is: LEO goes GER Loss of Electrons is Oxidation Gain of Electrons is Reduction

Examples: Oxidation: Al → Al e - Fe 2+ → Fe 3+ + e - 2F - → F 2 + 2e - Reduction: Al e - → Al Fe 3+ + e - → Fe 2+ F 2 + 2e - → 2F -

Exercises – Oxidation or Reduction? Identify these reactions as either oxidation or reduction reactions: O 2 + 4e - → 2O 2- H 2 → 2H + + 2e - Zn → Zn e - MnO H + + 5e - → Mn H 2 O Cr 2 O H + +6e - → 2Cr H 2 O

Balancing Simple Equations We know from previous years that equations need to be balanced, e.g. H 2 + Cl 2 → 2 HCl We also have to balance equations to make all the charges balance, e.g. H → H + The equation above has a total of no charge on the left but one + on the right. We can add electron(s) (i.e. one negative) to any side to make both side’s charges balance, e.g. H → H + + e -

Exercises – Completing and Balancing Equations Balancing Equations: Na + Cl 2 → NaCl Mg + HNO 3 → Mg(NO 3 ) 2 + H 2 Balancing Ionic Equations: Na + + O 2- → Al 3+ + OH - → Balancing Redox Half Equations: Na → Na + F 2 → 2F - O 2- → O 2

Why Gain or Lose Electrons? Atoms hold their electrons in shells (otherwise known as energy levels). As we add more electrons we fill up these levels: The first level can hold 2 electrons The second level can hold 8 electrons The third level can hold 8 electrons… Atoms become stable when their levels (shells) are full. Note: “Electron configuration” is the name we give to the arrangement of the electrons within atoms or ions, e.g. Sodium’s electron configuration is: 2, 8, 1

Exercises – Stable Shells Element Number of Electron Electron Configuration This Atom wants to become… Ion Na112,8,12,8Na + B 17 2,8,3 P 6

Electronegativity Some elements are better at bonding to electrons than others. This feature is called “electronegativity” and is discussed in more detail in the ‘Atoms and Bonding’ topic. Elements with greater electronegativity will hold their electrons more strongly – they are less likely to be oxidised (in fact they are more likely to end up gaining electrons) Elements with weak electronegativity have a weak hold on their electrons, they are more likely to lose them and therefore are more likely to be oxidised

Electronegativity Trends The general rule is that electronegativity increases as you move towards the right and the top of the periodic table This means that fluorine is the best holder and taker of electrons (notice that we’ve ignored group 18 – why?) Therefore the best losers of electrons are at the bottom, left hand corner of the periodic table H1H1 He 2 Li 3 Be 4 B5B5 C6C6 N7N7 O8O8 F9F9 Ne 10 Na 11 Mg 12 Al 13 Si 14 P 15 S 16 Cl 17 Ar 18 K 19 Ca 20 Sc 21 Ti 22 V 23 Cr 24 Mn 25 Fe 26 Co 27 Ni 28 Cu 29 Zn 30 Ga 31 Ge 32 As 33 Se 34 Br 35 Kr 36 Rb 37 Sr 38 Y 39 Zr 40 Nb 41 Mo 42 Tc 43 Ru 44 Rh 45 Pd 46 Ag 47 Cd 48 In 49 Sn 50 Sb 51 Te 52 I 53 Xe 54 Cs 55 Ba 56 La 57 Hf 72 Ta 73 W 74 Re 75 Os 76 Ir 77 Pt 78 Au 79 Hg 80 Tl 81 Pb 82 Bi 83 Po 84 At 85 Rn 86 Fr 87 Ra 88 Ac 89

Applied Electronegativity We can use this trend to predict what will happen when two chemicals are put together. Here are some examples: Cl - and F 2 are put together – Chlorine is a better loser than Fluorine so: 2Cl - + F 2 → 2F - + Cl 2 (Chlorine loses electrons and Fluorine gains) Na and Cl 2 are put together – 2Na + Cl 2 → 2Na + + 2Cl - (Sodium is the loser, chlorine is the gainer)