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Reduction- Oxidation Reactions (1)

213 PHC10th lecture

(1 )Gary D. Christian, Analytical Chemistry, 6th edition.

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By the end of the lecture the student should be able to:

Define oxidation, reduction, oxidizing agent, and reducing agent.

Understand the principals of electrochemical cell and electrode potential

Calculate the electrode potential by Nernest equation.

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WHAT IS A REDOX REACTION?

It is a reaction occurs between an oxidizing agent and a reducing agent.

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OXIDATION REDUCTION

A loss of electrons to give a higher oxidation state

Fe2+ Fe3+ + e-

A gain of electrons to give a lower oxidation state

Ce4+ + e- Ce3+

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Oxidizing Agent Reducing Agent

Take on electronsGets reduced

Ce4+ + e- Ce3+

Give up electronsGets oxidized

Fe2+ Fe3+ + e-

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Fe2+ + Ce4+ Fe3+ + Ce3+

reducing oxidizing oxidized

reduced agent agent form

form(Reduced form) (oxidized form)

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The reducing or oxidizing tendency of a substances will depend on its reduction potential.

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Electrode Potential (Eo)Each half-reaction will generate a

potential that adopted by an inert electrode dipped in the solution.

Individual electrode potential can’t be measured.

The difference between 2 electrode potentials can be measured.

The standard hydrogen electrode is used to measure the potential of any half reaction because it’s potential is zero.

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The more +ve Eo = (oxidation).

The more -ve Eo = (reduction).

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Fe3+ + e- Fe2+ Eo = 0.771 V

Sn4+ + 2e- Sn2+ Eo = 0.154 V

2Fe3+ + Sn2+ 2Fe2+ + Sn4+

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Questions?

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Example Fe3+ + e- = Fe2+ Eo = 0.771 V I3- + 2e- = 3I- Eo = 0.535 V

2Fe3+ + 3I- = 2Fe2+ + I3-

Eocell = 0.771 – 0.535 = 0.235 V

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Quiz

1. What is the overall cell reaction and the cell potential for the two half-reactions?

A) Cu2+ + 2e = Cu Eo = 0.34 V Zn2+ + 2e = Zn Eo = -0.76

V

B) Fe3+ + e = Fe2+ Eo = 0.77 V

Ti4+ + e = Ti3+ Eo = 0.15 V

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The Nernst equation

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The Nernst equation

Describes the dependence of potential on concentration.

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E = Eo – 2.3026 RT / n F log [Red] / [Ox]

E = reduction potential at specific conc.Eo = standard potential.n = no. of electronsR = gas const. (8.3143)T = absolute temp.F = Faraday const. (96.487)

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At 25oC

E = Eo – (0.05916 / n) log [Red] / [Ox]

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Questions?

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Summary:Definition of oxidation-reduction

reactions.Electrochemical cells.Electrode potential.Nernast equation

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Thank you