Unit 5 – Electrochemistry
5.1 Redox Reactions
Lesson Goals:
• define and identify:
• – oxidation – reduction – oxidizing agent
• – reducing agent – half-reaction – redox reaction
A) Introduction to Electrochemistry
• Free-write: in 2 minutes, write down any ideas or words that you
know about the physics or chemistry of a Battery.
• Ideas: What is it made of? How does it work? Any vocabulary
related to batteries? What do the numbers on the battery mean?
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A) Introduction to Electrochemistry
• Free-Write:
Made of metal and chemicals inside
Portable energy source that produces electricity..
Which is???
+/- ends
• Cathode / Anodes
Volts – V (voltage)
Electric Potential Difference
Voltage Causes Electrons to flow in a current through a
circuit
Amps –A (Current)
A) Introduction to Electrochemistry
Definition: Electrochemistry
a branch of chemistry that studies the conversion of chemical energy into
electrical energy and vice versa.
Applications of electrochemistry include: batteries, corrosion, electroplating,
metal purification.
A) Introduction to Electrochemistry
Example 1: When a copper wire is placed into silver nitrate solution:
Solution turns from colourless → blue
Silver forms on the copper
We can represent this chemical equation as follows:
2 Ag+(aq) + Cu(s) → 2 Ag(s) + Cu2+(aq)
(colourless)
(blue)
We can split up the reaction into two parts:
2Ag+(aq) + 2e-  2Ag(s)
Cu(s)  Cu2+(aq) + 2eThese are called Half-Reactions
B) Oxidation and Reduction Reactions
Definition: Half-Reaction
A chemical reaction where a chemical species gains or loses electrons.
e.g. X+(aq) + e- → X(s)
A(s) → A2+(aq) + 2e-
Oxidation reaction
Half-reaction where the species loses electrons
e.g. Cu(s) → Cu2+(aq) + 2eNote: The name of oxidation comes the reaction of oxygen with certain
substances like some metals.
(Fe + O2  Fe2O3)
B) Oxidation and Reduction Reactions
Reduction reaction
 Half-reaction where the species gains electrons
 e.g. Zn2+(aq) + 2 e- → Zn(s)
 Note: The name of reduction refers to the reduction of charge
B) Oxidation and Reduction Reactions
To help remember
Practice:
1. Identify the following half-reactions as oxidation or
reduction
(Hint: If electrons are not shown, look at change in
charge)
Reduction
a) Fe2+ + 2e-  Fe ; _____________________
Oxidation
b) 2O2-  O2 + 4e- ; ____________________
Oxidation
c) Mn2+  Mn3+ + e- ; _____________________
Reduction
d) 2H+ + 2e-  H2 ; ____________________
Oxidation
e) K  K+
; _____________________
Oxidation
f) 2Cl-  Cl2; _____________________
Reduction
g) Al3+  Al ; _____________________
Reduction
h) Cu2+  Cu+; _____________________
C) Redox Reactions
Definition:
Reduction Oxidation Reaction (REDOX)
A reaction between two chemical species involving a transfer of electrons
(oxidation reduction reactions)
½ Reactions:
Redox Reaction:
2H+ + 2e-  H2
Zn  Zn2+ + 2e-
2H+ + Zn  H2 + Zn2+
C) Redox Reactions
• Example 2:
a) What is happening to Zn? Losing
eletron…..oxidation
b) What is happening to Cu2+? Gain
electron , reducing
C) Redox Reactions
Definition: Oxidizing Agent (OA)
The species that causes the other species to be oxidized (causes them to
lose electrons).
The species is actually being reduced
In Example 2: Cu2+ is the OA because it causes the oxidation (losing e) of Zn
and in that process Cu2+ is reduced (gains 2e).
C) Redox Reactions
Definition: Reducing Agent (RA)
 The species that causes the other species to be reduced (causes them to
gain electrons).
 The species is actually being oxidized
 In Example 2: Zn is the RA because it causes the reduction (gaining e) of
Cu2+ and in that process Zn is oxidized (loses 2 e).
C) Redox Reactions
Example 3:
Electrons:
Species is:
Agent:
Fe3+ (aq)
+
Cu (s)
 Fe2+(aq) + Cu+(aq)
 Loses 1
 Gains 1
 Oxidation
 Reduction
 Oxidizing Agent  Reducing Agent
Practice:
2. In the reaction: 2Fe2+ + Cl2  2Fe3+ + 2Cl- …..
Identify: OA: Cl2 RA: ____Fe2+__________
The species being oxidized:___Fe2+__________
Species being reduced:___Cl2__________
The species gaining electrons:_____Cl2_________
Species losing electrons:______Fe2+_________
The product of oxidation________Fe3+___________
Product of reduction________Cl-___________
C) Redox Reactions
• Practice:
• 2. In the reaction: 2Fe2+ + Cl2  2Fe3+ + 2Cl- ….. Identify:
2+
a) OA: __________________
RA: Fe
______________
Cl2
b) The species being oxidized:_____________
Fe2+
Species being reduced:_____________
Cl2
c) The species gaining electrons:______________
Cl2
Species losing electrons:_______________
Fe2+
d) The product of oxidation___________________
Fe3+
Product of reduction___________________
Cl-
D) Converting Redox  Half Reactions
• Redox reactions can be broken up into oxidation & reduction half
reactions.
Redox Reaction
Half-Reactions
• Half-reactions always have e-‘s BUT REDOX reactions never show e-‘s!
D) Converting Redox  Half Reactions
Example 4: What ½ reactions are going on?
Pb2+(aq) + Zn(s) → Pb(s) + Zn2+(aq)
• Write the ½ REACTIONS:
Reduction
Oxidation
Pb2+(aq) + 2e- → Pb(s)
Zn(s)
→ Zn2+(aq) + 2e-
SUMMARY: Converting Redox Half Reactions
Look at charges to determine the oxidation and reduction half
reactions;
Oxidation: Loses electrons (charge more positive)
Reduction: Gains electrons (charge more negative)
D) Converting Redox  Half Reactions
Example 5: Given the redox reaction:
F2(g) + Sn2+(aq) → 2F-(aq) + Sn4+(aq)
• Write the oxidation & reduction half reactions.
Reduction
F2(g) + 2e- → 2F-(aq)
Oxidation
Sn2+(aq)
→ Sn4+(aq) + 2e-
D) Converting Redox  Half Reactions
Converting Half Reactions  Redox
• Remember NO ELECTRONS CAN BE SHOWN in a redox reaction.
– As electrons are produced in the oxidation reaction, they’re used up ion
the reduction reaction.
Example 6:
• Reduction half-reaction: O2 + 4e-2O2Zn → Zn2+ + 2e- ) x 2
• Oxidation half-reaction: (
• Add reactions together:
O2 + 2Zn + 4e-  2O2- + 2Zn2+ + 4e• Overall Redox reaction:
O2 + 2Zn  2O2- + 2Zn2+
D) Converting Redox  Half Reactions
Example 7:
Reduction half-reaction:(MnO - + 8H+ + 5e-Mn2+ + 4H O) x 2
4
2
Oxidation half-reaction: (Pb → Pb2+ + 2e- ) x 5
• Add reactions together:
2MnO4- + 16H+ + 10e- + 5 Pb 2Mn2+ + 8H2O + 5Pb2+ + 10e• Overall Redox reaction:
2MnO4- + 16H+ + 5 Pb 2Mn2+ + 8H2O + 5Pb2+
Practice:
3. Write the half-reactions for following reactions.
i) oxidation half-reaction
ii) reduction half-reaction
2Cl- + Sn2+  Cl2 + Sn
2Cl-  Cl2 + 2ei) ______________________________
Sn2+ + 2e-  Sn
ii) ______________________________
Fe2+ + Cu+  Fe3+ + Cu
Fe2+  Fe3+ + ei) ______________________________
Cu+ + e-  Cu
ii) ______________________________
Mg + Pb2+  Mg2+ + Pb
Mg  Mg2+ + 2ei) ______________________________
Pb2+ + 2e-  Pb
ii) ______________________________
Co + Ni2+  Co2+ + Ni
Co  Co2+ + 2ei) ______________________________
Ni2+ +2e-  Ni
ii) ______________________________
Practice:
4. Write the redox reactions for following half-reactions.
i)
Fe3+ + 3e-  Fe
ii) Cu2+ + e-  Cu+
iii) 2Cl-  Cl2 + 2eiv) S + 3H2O  H2SO3 +4H+ +4ea) i) and iii)
3+ + 6Cl-  2Fe + 3Cl
2Fe
2
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b) i) and iv)
4Fe3+ + 3S + 9H2O  4Fe + 3H2SO3 + 12H+
_______________________________________________________________
2Cu2+ + 2Cl-  2Cu+ + Cl2
c) ii) and iii)
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4Cu2+ + S + 3H2O  4Cu+ + H2SO3 + 4H+
d) ii) and iv)
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Homework
• Take out your Student Handbook and write in
your homework
Homework /Next Class(es)?
5.1 Questions
Date
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