author: j r reid oxidation and reduction – introduction leo goes ger examples balancing simple...
TRANSCRIPT
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)