REDOX reaction

1. (1) Review the concept of “Oxidation”
and “Reduction.”
(2)Define the term “cell potential.”
(3)Illustrate the Standard Hydrogen
Electrode (SHE) and Standard
Electrode (reduction) potentials.
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2. What is Electrochemistry?
 Electrochemistry is the study of chemical reactions
and their associated electrical changes.
 It is also looks at chemical changes that an electrical
current can bring about.
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3. Transfer of electrons
 Electrochemistry involves the transfer of electrons.
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4. Cell Potential
 If a strip of metal (electrode) is placed in a solution of its
ions, the metal loses electrons to form its cation.
M(s) → Mn+
(aq) + n e-
 The cations in solution may accept some of the lost
electrons and get reduced to the metal atoms.
Mn+
(aq) + n e- → M(s)
 At equilibrium, the charge difference that develops
between the metal strip and the solution is called a
potential difference.
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5. Half Cell
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6. Relative Cell Potential
 More reactive metals lose electrons with greater ease
and so the equilibrium position of the redox reaction
below lies more to the left.
Mn+
(aq) + n e- ⇌ M(s)
 More reactive metals therefore possess more negative
reduction potentials and less reactive metals will
possess more positive reduction potential differences.
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7. Cell-Potential
 Cell potential (measured in Volts and sometimes
known as the electromotive force, emf) is the
tendency of species to lose or gain electrons.
 E° represents the standard electrode potential,
which is the potential of a species compared with
the potential of a Standard Hydrogen Electrode.
2H+ + 2e- ↔ H2 E°
cell = 0.00V 7

8. Standard Hydrogen Electrode
The Standard Hydrogen Electrode (S.H.E.) consists of
Hydrogen gas at 298K and 1 atm bubbling over a platinum
electrode immersed in a solution of H+ ions with
concentration 1 moldm-3.
2H+
(aq) + 2e- ⇌ H2 (g, 1 atm) Eᶿ = 0.00 V 8

9. Standard cell potential of Zn
half cell
 Zn (aq) ⇌ Zn2+
(aq) + 2e- Eᶿ = + 0.76 V
 Zn2+
(aq) + 2e- ⇌ Zn (s) Eᶿ = - 0.76 V
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10. Cell-Potential
 For species with a positive value, they have
a tendency to be reduced.
 They are strong oxidizing agents.
 They are usually non-metals.
 For species with a negative value, they have
a tendency to be oxidized.
 They are strong reducing agents.
 They are usually metals.
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11. Table of standard reduction potentials
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12. Cell-Potential
 The total cell-potential can be expressed as follows:
E°
cell = E°
ox. + E°
red.
 E.g.:
Zn(s) + Cu2+
(aq) ↔ Zn2+
(aq) + Cu(s)
 E°
Zn2+(aq)+ 2e-⇌Zn (s) = -0.76V
 E°
Cu2+(aq) + 2e-⇌ Cu (s) = +0.34V
 Thus:
E°
cell = (0.76 + 0.34)V
= 1.10V
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