REDOX reactions
Oxidation and Reduction
Redox chemistry
• The study of oxidation and reduction reactions
• Oxidation and reduction reactions involve the
loss or gain of electrons.
• When one species is oxidized another is
reduced.
• Neither oxidation, nor reduction, can occur
alone.
Reduction
• Refers to reactions in which a species gains
electrons.
Oxidation
• Refers to reactions in which a species loses
electrons
Question….
• When NaCl is formed from it’s elements,
describe what is happening in terms of
electrons.
• There is a transfer of an electron from Na  Cl
• Remember an electron has a charge of
negative 1
• Na + Cl  Na+ + Cl-
Na + Cl  Na+ + Cl• Sodium, Na, gives up or loses an electron to
become Na+
• Na is oxidixed
• Chlorine, Cl, gains an electron to become Cl• Cl is reduced
• More than one electron can be lost or gained
in a redox reaction
• In the reaction 6Li(s) + N2(g)  2 Li3N(s) three
electrons are gained by each nitrogen atom.
Which one was which again…?
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OIL RIG
Oxidation is loss of electrons
Reduction is gain of electrons
OR
LEO the lion says GER
Loss of Electrons is Oxidation
Gain of Electrons is Reduction
A word about oxidation
• Oxidation often involves a reaction with
oxygen, for example when iron reacts with
oxygen in the air an oxidation reaction occurs
to form rust.
• REMEMBER, the consumption of oxygen
should be treated as a clue that redox
chemistry might be taking place
• NOT ALL OXIDATION REACTIONS INVOLVE
OXYGEN.
Reducing Agent
• Remember, oxidation and reduction occur
together
• When one species is oxidized, another is
reduced.
• A reducing agent is one which causes another
species to be reduced while itself is being
oxidized
• Reducing agents lose electrons
Oxidizing Agent
• An oxidizing agent causes another species to
be oxidized.
• An oxidizing agent is reduced in a redox
reaction
Half reactions
• We can write half reactions for the oxidation
or reduction that is taking place in a redox
reaction
For example:
• 2Na(s) + Br2(l)  2NaBr(s)
• Each Na gives up an electron, so the Na is
oxidized and is itself a reducing agent
• We can write a half reaction for this part
• 2Na  2Na+ + 2e-
2Na(s) + Br2(l)  2NaBr(s)
• The reduction half reaction is:
• Br2 + 2e-  2Br• Each bromine atom gains an electron so
bromine is reduced, while itself being the
oxidizing agent.
Summary
• 2Na  2Na+ + 2e• Br2 + 2e-  2Br-
oxidation
reduction
• Sodium is a reducing agent – it caused the
bromine to be reduced
• Bromine is an oxidizing agent – it caused the
sodium to be oxidized.
Redox Terminology
• OXIDATION—loss of electron(s) by a species;
increase in oxidation number.
• REDUCTION—gain of electron(s); decrease in
oxidation number.
• OXIDIZING AGENT—electron acceptor;
species is reduced.
• REDUCING AGENT—electron donor; species
is oxidized.
Oxidation numbers
• To identify whether atoms are oxidized or
reduced
• Tracking how oxidation numbers change as a
reaction takes place allows us to see if an
atom is oxidized or reduced
Oxidation numbers
• Decrease in oxidation number = the atom has
been reduced
• Increase in oxidation number = the atom has
been oxidized.
Rules for Assigning
Oxidation Numbers
• The oxidation number of any uncombined element is
0.
• The oxidation number of a monatomic ion equals the
charge on the ion.
• The more-electronegative element in a binary
compound is assigned the number equal to the
charge it would have if it were an ion.
• The oxidation number of fluorine in a compound is
always –1.
Rules for Assigning
Oxidation Numbers
• Oxygen has an oxidation number of –2 unless it is
combined with F, when it is +2, or it is in a peroxide,
such as H2O2, when it is -1
• The oxidation state of hydrogen in most of its
compounds is +1 unless it is combined with a metal,
in which case it is -1
• In compounds, the elements of groups 1 and 2 as
well as aluminum have oxidation numbers of +1, +2
and +3 respectively.
Rules for Assigning
Oxidation Numbers
• The sum of the oxidation numbers of all atoms
in a neutral compound is 0.
• The sum of the oxidation numbers of all atoms
in a polyatomic ion equals the charge of the
ion.
The oxidation number of any
uncombined element is 0
• What is the oxidation number of H in H2?
• What is the oxidation number of Ne in Ne?
• What is the oxidation number of O in O2?
•Zero
The oxidation number of a monatomic ion
equals the charge on the ion
• What is the oxidation number of Cl in Cl-?
• -1
• What is the oxidation number of Na in Na+?
• +1
• What is the oxidation number of Al in Al3+?
• +3
The more-electronegative element in a binary
compound is assigned the number equal to the charge
it would have if it were an ion.
Electronegativity tends to
decreas
e
down a
e across a period
group and increas
So the elements located in the top right hand
side of the periodic table (excluding the noble
gases) are the most electronegative.
The more-electronegative element in a binary
compound is assigned the number equal to the charge
it would have if it were an ion.
• What is the oxidation number of Cl in HCl?
• -1
• What is the oxidation number of S in H2S?
• -2
The oxidation number of fluorine in a
compound is always –1.
• What is the oxidation number of F in CF4?
• What is the oxidation number of F in FCl?
• What is the oxidation number of F in CH2F2?
•-1
Oxygen has an oxidation number of –2 unless it
is combined with F, when it is +2, or it is in a
peroxide, such as H2O2, when it is –1.
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What is the oxidation number of O in H2O?
-2
What is the oxidation number of O in H2O2?
-1
What is the oxidation number of O in CaO?
-2
What is the oxidation number of O in Al2O3?
-2
What is the oxidation number of O in OF2?
+2
The oxidation state of hydrogen in most of its
compounds is +1 unless it is combined with a
metal, in which case it is –1
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What is the oxidation number of H in H2O?
+1
What is the oxidation number of H in HCl?
+1
What is the oxidation number of H in H3P?
+1
What is the oxidation number of H in CaH2?
-1
In compounds, the elements of groups 1 and 2
as well as aluminum have oxidation numbers of
+1, +2 and +3 respectively.
• What is the oxidation number of Li in Li2O?
• +1
• What is the oxidation number of Ca in CaO?
• +2
• What is the oxidation number of Al in Al2O3?
• +3
The sum of the oxidation numbers of all atoms
in a neutral compound is 0.
• What is the sum of the oxidation numbers in
H2O?
• What is the sum of the oxidation numbers
H2SO4?
• What is the sum of the oxidation numbers in
CaH2?
•ZERO!
The sum of the oxidation numbers of all atoms
in a polyatomic ion equals the charge of the ion.
• What is the sum of the oxidation numbers in PO43-?
• -3
• What is the sum of the oxidation numbers SO42-?
• -2
• What is the sum of the oxidation numbers in OH-?
• -1
Determine the oxidation numbers
for each atom:
• Class worksheet
Balancing Redox Reactions
• 2 methods
• Half reaction method
• Oxidation number change method
Half reaction method for Balancing
Redox Reactions
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1. Assign oxidation numbers to all species
2. Write the oxidation half reaction
3. Write the reduction half-reaction
4. Balance for charge
5. Re-write equation with factors from step #4
6. Balance for mass.
Balancing Redox Reactions
• H3PO2 + H+ + Cr → Cr3++ P + H2O
• Step 1: Assign Oxidation numbers to
everything.
• +1 +1 -2
+1
0
+3
0
+1 -2
• H3PO2 + H+ + Cr → Cr3++ P + H2O
+1 +1 -2
+1
0
+3
0
+1 -2
H3PO2 + H+ + Cr → Cr3++ P + H2O
• Steps 2 and 3: Write oxidation and reduction
half reactions
• Cr → Cr3++ 3 eOxidation half-reaction
• P+ + e- → P
Reduction half-reaction
• Step 4: Balance for Charge.
• Cr → Cr3++ 3 eOxidation half-reaction
• 3(P+ + e- → P)
Reduction half-reaction
+1 +1 -2
+1
0
+3
0
+1 -2
H3PO2 + H+ + Cr → Cr3++ P + H2O
• Step 5: Re-write the original equation with the
factor from balancing for charge.
• Cr → Cr3++ 3 eOxidation half-reaction
• 3(P+ + e- → P)
Reduction half-reaction
• 3H3PO2 + H+ + Cr → Cr3++ 3P + H2O
Balancing redox reactions
• Step 6: Balance for mass.
• 3H3PO2 + H+ + Cr → Cr3++ 3P + H2O
• 3H3PO2 + 3 H+ + Cr → Cr+3 + 3P + 6H2O
Oxidation number change method
• Step 1: Assign oxidation numbers to all species
• Step 2: Identify which species are oxidized and
which are reduced
• Step 3: track the change in oxidation number
for a species on either side of the equation
• Step 4: Use coefficients to make the increase
in oxidation number equal to the decrease
• Step 5: balance for atoms and charge
Mg + HCl  MgCl2 + H2
• Step 1: assign oxidation numbers to all species
• 0
+1 -1
+2 -1
0
• Mg + HCl  MgCl2 + H2
• Step 2: Identify which species are oxidized and which
are reduced
• H decreases in oxidation number so it is reduced
• Mg increases in oxidation number so it is oxidized
Mg + HCl  MgCl2 + H2
• Step 3: track the change in oxidation number
for a species on either side of the equation
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0-----------(+2)------>+2 (Oxidation)
• Mg + HCl  MgCl2 + H2
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+1--------------(-1)-------->0 (reduction)
Mg + HCl  MgCI2 + H2
• Step 4: Use coefficients to make the increase
in oxidation number equal to the decrease
• 0---------(+2)------>+2 (Oxidation)
• Mg + HCl  MgCl2 + H2
•
+1-------------2x(-1)-------->0 (reduction)
• Mg + 2HCl  MgCl2 + H2
For extra help
• Try the worked problems in the text book,
p648, 649 and 651, 652