REDOX Reactions
Redox means…
 Oxidation – Reduction Reactions
 Redox Rxns involve the…
Complete or partial transfer of electrons
Change in Oxidation Numbers
(note that this change is a “pseudo” change – not real – it just
looks like it’s changing)
OXIDATION
The oxidation of an element is the:
LOSS OF ELECTRONS
- Associated w/ an increase in oxidation number
For example: change in oxidation numbers from (-1) to (0) is the
oxidation of that element
REDUCTION
The reduction of an element is the:
GAINOF ELECTRONS
- Associated w/ a decrease in oxidation numbers (i.e. it
gets more negative)
For example: the oxidation number of an element
decreases from -1 to -3 (i.e. it has gained two electrons)
Remember…
LEO the lion says GER
Lose electrons = oxidation (LEO)
Gain electrons = reduction (GER)
In a redox reaction…
You CANNOT HAVE AN
OXIDATION Rxn without a
REDUCTION Rxn….they are
partners. One does not exist
without the other!!!
Rules for Assigning Oxidation Numbers
1.
2.
3.
4.
5.
6.
Free elements are assigned an oxidation number of 0
(as are diatomic molecules: Br2 I2 N2 Cl2 H2 O2 F2 )
The oxidation state (number) for any simple one-atom
(monatomic) ion is equal to its charge (i.e. Na+ is +1)
Usually an atom’s oxid state is the charge it would have if it were
an ion (i.e. Group 1 = +1 ; Group 17 = -1, etc.)
H is always +1 unless bound to a metal (then its -1)
O is always -2 (unless it’s a peroxide, and then its -1)
The sum of all of the oxidation numbers within a compound
must equal the overall charge of that compound
Lets Assign Some Ox Numbers!
NaCl
H2O
H2SO4
Assigning Oxidation Numbers
NaCrO2
H2O2
OH-1
HNO3
NO
Cl2
Ca(ClO3)2
NaOH
Na2CrO4
Ag
AgNO3
Ca(OH)2
CaCl2
KNO3
Redox Reactions
 Again…one CANNOT happen without the other.
 They are pairs!!
 LEO  Lose e- = oxidation
 GER  Gain e- = reduction
Lose Electrons = OXIDATION
An oxidation reaction looks like this:
Fe+2  Fe+3 + e-
In this reaction:
1. Oxidation numbers increases (more positive)
from +2  +3
Gain Electrons = REDUCTION
A reduction reaction looks like this:
Fe+3 + e-  Fe+2
In this reaction:
1. The oxidation number Decreases - is changing from +3
 +2 (it is getting more negative)
Agents – things that cause redox rxns
 Oxidizing Agent
 Substance that is being reduced
 Hence, causing the oxidation
 Reducing Agent
 Substance that is being oxidized
 Causing the reduction
So…they HAVE to happen together!!
But remember…they happen together
in a chemical reaction…
S(s) + HNO3 (aq)  SO2 (g) + NO(g) + H2O (l)
In the above reaction, both oxidation and reduction are
occurring. Can you tell where the oxidation is? What about
the reduction?
But, the above rxn is unbalanced – Redox rxns are oftentimes
super complex, so we have a special way to balance them…
How?? Use HALF REACTIONS!!
What is a half reaction you say??
The idea behind using half reactions is simple:
They are reactions that show oxidation and reduction
separately. Then, they can be combined (or added together)
to give the overall, BALANCED, redox reaction
But…prepare yourself, there are a lot of rules & it can be
confusing….you need to be very methodical about it.
Buckle in…here we go…HALF RXNS
BAHHH…What does this mean??
Let’s look @ an example and try to make sense of this…
S(s) + HNO3 (aq)  SO2 (g) + NO(g) + H2O (l)
Ok, let’s break it down to help us see what’s being oxidized and
what’s being reduced:
S(s) + H+1 (aq) + NO3-1(aq)  SO2(g) + NO(g) + H2O(l)
Step 1: Assign oxidation numbers to figure out what was reduced &
oxidized
Oxidation Half Reaction
 What was oxidized: S (went from 0  +4)
So, let’s look at the species involving sulfur
S (s)  SO2(g)
S (s)  SO2(g)
Reduction Half Reaction
 What was reduced? N (+5  2)
Half reaction:
NO3-1 (aq)  NO (g)
Let’s balance the reaction using our rules…
NO3-1 (aq)  NO (g)
Now that I have my half reactions, let’s
continue…
Also…you must add the spectator ions back in….and then
balance to ensure everything is equal!!
Did we have any spectator ions here?? No.
KMnO4 (aq) + HCl (aq)  MnCl2 (aq) + Cl2(g)
+ H2O (l) + KCl (aq)
With a partner & a white board – find space @ lab tables,
desks or the floor to try to figure this out
Remember my
addition to
Step 5: Add the
spectator ions
back in &
balance
them…
GOOD LUCK
Br2(l) + SO2(g)  Br-(aq) + SO42-(aq)
Small Scale Half-Rxns Lab (from book)