Principles of Chemical Science_Oxidation-Reduction and Electrochemical Cells - Lec25

5.111 Lecture Summary #25 Friday, N ovember 7, 2014

Reading for Today: Sections 13.1-13.12 in 5th ( 4th ed : 12.1-12.12)
Reading For Lecture #26: 13.6-13.12 in 5th (4th ed : 12.6-12.12)
Topic: I. Introd uction to Oxid ation -Red uction (Red ox) Reactions
II. Balancing Red ox Reactions
III. Electrochem ical Cells

I. IN TROD UCTION TO OXID ATION -RED UCTION (RED OX) REACTION S
Red ox reactions are a m ajor class of chem ical reactions in w hich there is an exchange of
electrons from one species to another.

For exam ple, 2Mg (s) + O 2 (g)  2MgO

D efinitions

Oxid ation:

Red uction:

Oxid izing agent:

Red ucing agent:


Guidelines for Assigning Oxidation N umbers

1) In free elem ents, each atom has an oxid ation num ber of zero. Exam ple H 2

2) For ions com posed of only one atom the oxid ation num ber is equal to the charge on the
ion. Thus Li+1 has an oxid ation num ber of +1. Group 1 and group 2 m etals have oxid ation
num bers of +1 and +2, respectively. Alum inum has an oxid ation nu m ber of +3 in all its
com pound s.

3) The oxid ation num ber of oxygen in m ost com pound s is -2. H ow ever, in peroxid es such as
H 2O 2 and O 2 , oxygen has an oxid ation state of -1.
-2




4) The oxid ation num ber of hyd rogen is +1, except w hen it is bond ed to m etals in binary
com pound s, such as LiH , N aH , CaH 2. In these cases, its oxid ation num ber is -1.

5) F has an oxid ation num ber of -1 in all its com pound s. Other halogens (Cl, Br, and I) have
negative oxid ation num bers w hen they occu r as halid e ions in com pound s (Ex. N aCl).
H ow ever, w hen com bined w ith oxygen (oxoacid s), they have positive oxid ation num bers
(Ex. ClO -).



1

, 6) In a neutral m olecule, the sum of the oxid ation num bers of all the atom s m ust be zero. In a
polyatom ic ion, the sum of oxid ation num bers of all the elem ents in the ion m ust be equal to
the net charge of the ion.




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