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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-)
• 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:
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)