Prepared by:Mrs Faraziehan Senusi

PA-A11-7C

Electrochemical Cells

Corrosion & Prevention

Chapter 3Oxidation and Reduction

Oxidation-Reduction Concepts

Voltaic Cell

Electrolytic Cell

Reference: Chemistry: the Molecular Nature of Matter and Change, 6th ed, 2011, Martin S. Silberberg, McGraw-Hill

• ‘Oxidation’ : combination of a substance with oxygen results in an increase in the oxidation number

of an element in that substance Example:

formation of rust, Fe2O3, iron(III) oxide

4Fe (s) +3O2 (g) 2Fe2O3 (s)

Combustion reactions

C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)

‘loss of electron by an element increase in oxidation number’

Oxidation-Reduction

• ‘Reduction’ : removal of oxygen from a compound results in an decrease in the oxidation number

of an element in that substance Example:

tungsten used in light bulb filaments can be prepared by reduction of tungsten(VI) oxide with hydrogen at 1200°C

WO3 (s) +3H2 (g) W(s) +3H2O (g)

‘gain of electron by an element decrease in oxidation number’

Oxidation-Reduction

• Oxidation-Reduction Concepts Oxidation – Loss of Electrons (LEO) Reduction – Gain of Electrons (GER)

Oxidizing Agent – Species that causes another species to be oxidized (lose electrons)

Oxidizing agent is reduced (gains e-) Reducing Agent – Species that cause another species to be

reduced (gain electrons)Reducing agent is oxidized (loses e-)

• Oxidation (e- loss) always accompanies Reduction (e- gain)

• Total number of electrons gained by the atoms/ions of the oxidizing agent always equals the total number of electrons lost by the reducing agent

Oxidation-Reduction

Oxidation-Reduction

A summary of redox terminology :

Oxidation-Reduction

Oxidation Number

– Also called ‘oxidation state’, signifies the number of charges the atom would have in a molecule (or an ionic compound) if electrons were transferred completely.

– The oxidation number in a binary ionic compound equals the ionic charge

– The oxidation number for each element in a covalent compound (or polyatomic ion) are assigned according to the relative attraction of an atom for electrons

Oxidation-Reduction

(ions composed of only one atom)

Oxidation-Reduction

Some guideline to assign oxidation numbers :

ON is -1 when it combines with elements that are less electronegative that itself)

Lets try!!

Assign oxidation numbers to all the elements in the following compounds and ion:

(a) Li2O

(b) HNO3

(c) Cr2O7 2–

Answer:

(a) By rule 2, Li has ON = +1 (Li+) and Oxygen has ON = -2 (O2-)

(b) Formula for nitric acid, yields a H+ ion and NO3– ion in solution. By rule 4, H has ON =

+1. So, nitrate ion must have ON = -1. Oxygen has ON = -2,

[N(x) O3 (2-)]–

So, x + 3(-2) = -1

and x = +5

(c) By rule 6, sum of ON in dichromate ion, Cr2O7 2– = -2. Oxygen ON = -2,

[Cr2(y) O7

(2-)]2–

So, 2(y) + 7(-2) = -2

and y = +6

Oxidation-Reduction

ProblemAssign oxidation numbers to all the elements in the following compounds and ion:

a) Sulfur trioxide, SO3

b) BrO2-

c) Manganate ion, MnO42-

d) 4Fe + 3O2 2Fe2O3

e) WO3 (s) +3H2 (g) W(s) +3H2O (g)

David P. White Prentice Hall ©

2003

Oxidation-Reduction

• Balancing Redox Reactions

Oxidation Number Method

Half-Reaction Method

• The balancing process must insure that:

The number of electrons lost

by the reducing agent equals

the number of electrons

gained by the oxidizing agent

Redox Reactions

Oxidation Number Method

Assign oxidation numbers to all elements in the reaction

From changes in oxidation number of given elements, identify oxidized and reduced species

For each element that undergoes a change of oxidation number, compute the number of electrons lost in the oxidation and gained in the reduction from the oxidation number change (Draw tie-lines between these atoms)

Multiply one or both these number by appropriate factors to make the electrons lost equal to the electrons gained

Use factors as coefficients in reaction equation

Redox Reactions

Practice Problem

Balance equation with Oxidation Number method: 3 3 2 2 2Cu(s) + HNO (aq) Cu(NO ) + NO (g) + H O(l)

3 3 2 2 2Cu(s) + 4HNO (aq) Cu(NO ) + 2NO (g) + 2H O(l)

Balance the equation

Redox Reactions

• Half-Reaction Method– Applicable to Acid or Base solutions

– Separate the oxidation & reduction steps, reflects their actual physical separation in electrochemical cells

– Does not usually require Oxidation Numbers (ON)

• Procedure Divide the overall reaction into:

Oxidation Half-Reaction Reduction Half-Reaction

Balance each half-reaction for atoms & charge Atoms balanced in order: atoms other than O and H, then O, then H Charge balanced by adding e-

Multiply one or both reactions by some integer to make electrons gained equal to electrons lost

Recombine to given balanced redox equation

• Redox Half-Reaction Method – Example

Divide steps into Half-Reactions

2- - +32 7 2Cr O (aq) + I (aq) Cr (aq) + I (s)

Half-Reaction Method in a “Acidic” solution

# When occur in acidic solution, H2O molecules and H+ are available for balancing

Balance Atoms & Charges for Cr2O72- / Cr3+

Balance Atoms & Charges for I-/ I2

* Add 7 Water molecules to balance Oxygen

* Add 14 H+ ions on left to balance 14 H on right

Add 6 electrons (e-) on leftto balance reaction charges

No need to add H2O or H+

Add 2 electrons (e-) on rightto balance reaction charges

Half-Reaction Method in a “Acidic” solution

* Balance atoms other than O and H

Multiply each half-reaction, if necessary, by an integer to balance electrons lost/gained

2 e- lost in oxidation reaction and 6 e- gained in reduction Multiply oxidation half-reaction by 3

Add 2 half-reactions together

Half-Reaction Method in a “Acidic” solution

# check the atoms and charges balances

Half-Reactions

Multiply each reaction by appropriate integer

2-7 2 3 2 4 2 4

4 2 4 2 3

2--Mn O (aq) + C O Mn O (s) + C O (aq)

Half-Reaction Method in a “Basic” solution# When occur in basic solution, add OH– ion to both sides of the equation for every H+ ion present

Add reactions

Add OH- to neutralize H+ , balance H2O, and form “basic” solution

- 2- - 2-4 2 4 2 3 22MnO + 3 C O + 4OH 2MnO + 6CO + 2H O(aq) (aq) (aq) (s) (aq) (l)

# check the atoms and charges balances

Half-Reaction Method in a “Basic” solution