Author: J R Reid

Oxidation and Reduction –Introduction

LEO goes GERExamplesBalancing simple equationsWhy 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 electronsReduction – when an atom gains more electrons than it had before

A quick way to remember this is:LEO goes GER

Loss of Electrons is OxidationGain of Electrons is Reduction

Examples:

Oxidation:Al → Al3+ + 3e-

Fe2+ → Fe3+ + e-

2F- → F2 + 2e-

Reduction:Al3+ + 3e- → Al Fe3+ + e- → Fe2+

F2 + 2e-

→ 2F-

Exercises – Oxidation or Reduction?

Identify these reactions as either oxidation or reduction reactions:

O2 + 4e- → 2O2-

H2 → 2H+ + 2e-

Zn → Zn2+ + 2e-

MnO4- + 8H+ + 5e- → Mn2+ + 4H2O

Cr2O72-

+ 14H+ +6e- → 2Cr3+ + 7H2O

Balancing Simple Equations

We know from previous years that equations need to be balanced, e.g.

H2 + Cl2 → 2HCl 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 + Cl2 → NaCl

Mg + HNO3 → Mg(NO3)2 + H2

Balancing Ionic Equations: Na+ + O2- → Al3+ + OH- →

Balancing Redox Half Equations: Na → Na+

F2 → 2F-

O2- → O2

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 electronsThe second level can hold 8 electronsThe 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

Na 11 2,8,1 2,8 Na+

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 tableThis 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

H1

He2

Li3

Be4

B5

C6

N7

O8

F9

Ne10

Na11

Mg12

Al13

Si14

P15

S16

Cl17

Ar18

K19

Ca20

Sc21

Ti22

V23

Cr24

Mn25

Fe26

Co27

Ni28

Cu29

Zn30

Ga31

Ge32

As33

Se34

Br35

Kr36

Rb37

Sr38

Y39

Zr40

Nb41

Mo42

Tc43

Ru44

Rh45

Pd46

Ag47

Cd48

In49

Sn50

Sb51

Te52

I53

Xe54

Cs55

Ba56

La57

Hf72

Ta73

W74

Re75

Os76

Ir77

Pt78

Au79

Hg80

Tl81

Pb82

Bi83

Po84

At85

Rn86

Fr87

Ra88

Ac89

Applied Electronegativity

We can use this trend to predict what will happen when two chemicals are put together. Here are some examples:

Cl- and F2 are put together – Chlorine is a better loser than Fluorine so:

2Cl- + F2 → 2F- + Cl2 (Chlorine loses electrons and Fluorine gains)Na and Cl2 are put together –

2Na + Cl2 → 2Na+ + 2Cl-

(Sodium is the loser, chlorine is the gainer)