Rules for Assigning
Oxidation #’s

Elements by themselves are neutral
Ox # = 0


Ex: Na0, O20, Mg0
Monoatomic Ions show their oxidation # as
their written charge

Ex: H+1, Cl-1, Al+3

Group #1 Elements in compounds:
Ox # = +1
Ex: LiBr, NaCl

Group #2 Elements in compounds:
Ox # = +2
Ex: MgBr2, CaCl2

Aluminum in compounds:
Ox # = +3
Ex: AlCl3

Fluorine in compounds
Ox # = -1
Ex: LiF, MgF2

Hydrogen:
Ox # = +1 (most common)
 when combined with nonmetals


Ex: HCl, H2SO4, H2S
Ox # = -1
 When combined with metals (metal hydrides)


Ex: LiH, MgH2

Oxygen: Oxidation # = -2 (usually)


Ex: CO2, H2O
Special Cases:
Peroxides: Oxygen is a diatomic ion (O2)-2 with
total charge of -2, each oxygen has -1 charge
 Ex: H2O2, BaO2

When bonded with Fluorine: Ox # = +2
 Ex: OF2

How to Find Charges If No “Rule”?

Sum of Individual Ox # of all the elements
in a compound = 0

All compounds are NEUTRAL
Ex: NaClO3
 What is the ox # for each element?


Draw your Lines….
How to Find Charges If No “Rule”?

Sum of Ox # of all atoms in a polyatomic
ion is equal to the charge of that ion.
Ex: PO4-3
 What is the charge of each element?

Let’s Practice

What is the oxidation number of sulfur in
Na2S2O3?
(1) -1
(2) +6
(3) +2
(4) +4

What is the oxidation number of chromium
in the chromate ion, CrO4 2– ?
(1) +6
(2) +3
(3) +2
(4) +8

In which compound does chlorine have the
highest oxidation number?
(1) NaClO
(2) NaClO3
(3) NaClO2
(4) NaClO4

What is the oxidation number assigned to
manganese in KMnO4?
(1) +7
(2) +3
(3) +2
(4) +4
Mahjong Oxidation # Game:
 http://www2.stetson.edu/mahjongchem/

Oxidation and Reduction
Reactions
Oxidation (Read only)
Original definition:
When substances combined with oxygen.
Ex:
All combustion (burning) reactions
CH4(g) + 2O2(g)
CO2(g) + 2H2O(l)
All “rusting” reactions
4Fe(s) + 3O2(g)
2Fe2O3(s)
Reduction (Read Only)
Original Definition:
Reaction where a substance “gave up” oxygen.
Called “reductions” because they produced
products that were “reduced” in mass because
gas escaped.
Ex:
2Fe2O3(l) + 3C(s)
4Fe(l) + 3CO2(g)
Oxidation/Reduction Reactions
Oxidation: LOSS of one or more electrons.
Reduction: GAIN of one or more electrons
Oxidation & reduction always occur together.
•
Deal with a movement of ELECTRONS
between atoms during a reaction.
•
Electrons travel from what is oxidized
towards what is reduced.
One atom loses e-,
the other gains eThis is called
“electron transfer”
Remember!!
Or…Remember
Redox Reactions:
ALWAYS involve changes in charge
If charges don’t change it’s NOT a Redox rxn.
Conservation of “Charge”
Total electrons lost = Total electrons gained
Redox Reactions must
balance electrons as
well as atoms!
Let’s Practice
Which changes occur when Pt2+ is reduced?
(1) Pt2+ gains electrons and its oxidation number increases.
(2) Pt2+ gains electrons and its oxidation number decreases.
(3) Pt2+ loses electrons and its oxidation number increases.
(4) Pt2+ loses electrons and its oxidation number decreases.

Answer: 1
Given the balanced equation representing a redox
reaction:
2Al + 3Cu2+ → 2Al3+ + 3Cu
Which statement is true about this reaction?
(1) Each Al loses 2e- and each Cu2+ gains 3e(2) Each Al loses 3e- and each Cu2+ gains 2e(3) Each Al3+ gains 2e- and each Cu loses 3e(4) Each Al3+ gains 3e- and each Cu loses 2e-

Answer: 2

In an oxidation-reduction reaction, reduction
is defined as the
(1) loss of protons
(2) loss of electrons
(3) gain of protons
(4) gain of electrons

Which change in oxidation number indicates
oxidation?
(1) –1 to +2
(2) +2 to –3
(3) –1 to –2
(4) +3 to +2

When a neutral atom undergoes oxidation,
the atom’s oxidation state
(1) decreases as it gains electrons
(2) decreases as it loses electrons
(3) increases as it gains electrons
(4) increases as it loses electrons
Oxidizing/Reducing Agents
Oxidizing Agent:
substance reduced
 Gains electrons
Reducing Agent:
substance oxidized
 Loses electrons
The “Agent” is the “opposite”
Identify What is Changing in
Charge
What is oxidized and reduced?
What are the oxidizing and reducing agents?
Ex:
3Br2 + 2AlI3
2AlBr3 + 3I2
0
+3 -1
3Br2 + 2AlI3
+3 -1
0
2AlBr3 + 3I2
Br2 is reduced and is the oxidizing agent
I-1 is oxidized and is the reducing agent
What is oxidized and reduced?
What are the oxidizing and reducing agents?
Mg + CuSO4
2K + Br2
Cu + 2AgNO3
MgSO4 + Cu
2KBr
Cu(NO3)2 + 2Ag
NOTE:
Atoms in a polyatomic ion DO NOT change in charge!
0
+2
+2
Mg + CuSO4
0
MgSO4 + Cu
Mg oxidized (reducing agent)
Cu+2 reduced (oxidizing agent)
0
0
+1 -1
2K + Br2
2KBr
K oxidized (reducing agent)
Br2 reduced (oxidizing agent)
0
+1
Cu + 2AgNO3
Cu oxidized (reducing agent)
Ag+1 reduced (oxidizing agent)
+2
0
Cu(NO3)2 + 2Ag
Redox or Not Redox
(that is the question…)
Redox Reactions: must have atoms changing in
charge.
Not all reactions are redox.
Easy way to spot a redox reaction!!!
Look for elements entering and leaving
compounds.
Is it Redox?
Look for Changes in Charge!
Are elements entering and leaving compounds?
Synthesis:
Ex:
2H2 + O2
2H2O
Decomposition:
Ex:
2KClO3
2KCl + 3O2
Is it Redox?
Synthesis: YES
0
0
+1 -2
Ex: 2H2 + O2
2H2O
Decomposition: YES
+1 +5 -2
Ex: 2KClO3
+1 -1
0
2KCl + 3O2
Is it Redox?
Combustion:
CH4 + 2O2
CO2 + 2H20
Single Replacement:
Zn + CuCl2
ZnCl2 + Cu
Is it Redox?
Combustion: YES
-4 +1
0
CH4 + 2O2
+4 -2
CO2 + 2H20
Single Replacement:
0
+2 -1
Zn + CuCl2
+1 -2
+2 -1
YES
0
ZnCl2 + Cu
Is it Redox?
Double Replacement:
AgNO3 + LiCl
AgCl + LiNO3
Is it Redox?
Double Replacement: NO!!!!
Ions switch partners, but don’t change in charge
+1 +5 -2
+1 -1
AgNO3 + LiCl
+1 -1
+1 +5 -2
AgCl + LiNO3
Remember charges of atoms inside polyatomic
Ions do not change!
Let’s Practice
Which equation represents an oxidation
reduction reaction?
(1) CH4 + 2O2 → CO2 + 2H2O
(2) H2SO4 + Ca(OH)2 → CaSO4 + 2H2O
(3) MgCrO4 + BaCl2 → MgCl2 + BaCrO4
(4) Zn(NO3)2 + Na2CO3 → 2NaNO3 + ZnCO3
Which reaction is an example of an
oxidation reduction reaction?
(1) AgNO3 + KI →AgI + KNO3
(2) Cu + 2 AgNO3 → Cu(NO3)2 + 2 Ag
(3) 2 KOH + H2SO4 → K2SO4 + 2H2O
(4) Ba(OH)2 + 2HCl → BaCl2 + 2H2O
Writing Half Reactions
Redox Reactions are composed of two parts
or half reactions.
Half Reactions Show:
Element being oxidized or reduced.
Change in charge
# of moles of e- being lost or gained
http://www.kentchemistry.com/links/Redox/flash/RedoxOxNumbers.swf
Writing Half Reactions
0
0
+1 -1
2Na + F2
Oxidation:
or
2NaF
Na
2Na
Na+1 + 1e2Na+1 + 2e-
Note: e- are “lost” (on the right of arrow)
Reduction:
or
F + 1eF2 + 2e-
F-1
2F-1
Note: e- are “gained” (on the left of arrow)
Ox’s Have Tails!!

Oxidation Half reactions always have
“tails” of electrons
Na
Na+1 + 1e-
0
+2 -1
+2
Zn + CuCl2
-1
0
ZnCl2 + Cu
Zn+2 + 2e-
Ox:
Zn
Red:
Cu+2 + 2e-
Cu
https://www.youtube.com/watch?v=-wvnX1f7yRY
Tutorial on Writing Half Reactions
Balancing Simple Redox Rxns
Must be:
Balanced for Mass
ATOMS balance
Balanced for Charge
Total e- Lost = Total e- Gained
Applications of Redox Reactions
Corrosion of Metals
Metals gets oxidized by
oxygen in the air forming
metal oxides (rust)
4Fe(s) + 3O2(g) → 2Fe2O3(s)
Prevention: Use paint, oil,
“Plating” or attach to negative
terminal of a battery.
Gold doesn’t
rust…Why?

Ships or pipes made of Fe will “rust
away” into Fe+2 ions when oxidized.

Blocks of more reactive metals (see
Table J) like Mg or Zn are attached to
ships or underground pipes.

Electrons travel from more reactive
metal to less reactive iron preventing it
from oxidizing.

Aluminum forms an oxide coating that
protects the metal from further corrosion.
Photograph Development involves oxidation and
reduction of silver atoms and ions
Bleach acts on
stains by oxidizing
them, getting reduced
in the process
Explosives form gases like
N2 from nitrogen compounds!
Car Batteries
Lead/Acid Battery
 Lead is both oxidized and reduced

Pb(s) + PbO2(s) + 2H2SO4(aq) → 2PbSO4(s) +
2H2O(l)
http://www.youtube.com/watch?v=j
W_KkEhCy68&safe=active
Reactivity of Metals
Reference Table J
Metals Higher on Table J
are more ‘active”
“Active” metals are
easily oxidized (lose electrons),
and good “reducing agents”
Single Replacement Reactions:
Elements higher on Table J “replace” elements that are lower.
Cu0(s) + AgNO3(aq)
Ag0(s) + CuNO3(aq)
Ag0(s) + CuNO3(aq)
wouldn’t happen!!!
Copper replaces
silver!
Reactivity of Nonmetals
Reference Table J
Nonmetals higher on Table J
are more “active”
“Active” nonmetals
are more easily reduced
(gain electrons), and
good “oxidizing agents”
Balancing Harder
Redox Reactions
(Honors)
Oxidation Number Method
(Balancing in Acid Solution)
•
•
•
•
•
•
•
•
Find ox #’s and use brackets to connect elements
changing in charge.
Balance atoms changing in charge
Find total e- involved in each change
If necessary balance e- by multiplication
Balance all other atoms except H and O
Balance oxygen by adding H2O to side deficient
Balance hydrogen by adding H+1 to side deficient
Check for balance with respect to atoms and charge.
Half Reaction Method
(Ion/Electron Method)
(In acid solution)








Separate equation into two “basic” half reactions
Balance all atoms except H and O
Balance oxygen by adding H2O
Balance hydrogen by adding H+1
Balance charge by adding electrons to more
positive side
If necessary balance e- by multiplication
Add together half reactions and simplify
Check for balance of atoms and charge
Crash Course Chemistry: Redox
 https://www.youtube.com/watch?v=lQ6FB
A1HM3s&index=11&list=PL8dPuuaLjXtPH
zzYuWy6fYEaX9mQQ8oGr
