Page 118 - Basic physical chemistry for the atmospheric sciences

6

Oxidation-reduction reactio s
n

6. 1 Some definitions
In C h apter 5 we saw that, in terms of the Br�nsted-Lowry theory,
acid-base reactions involve proton transfer. Another large and im­
n
portant group of chemical reactions, particularly in aqueous solutio s ,
t
involves electron transfer; these are referred o a s oxidation-reduction
(
(or redox) reactions. Redox reactions are involved 1 ) in photosynthe­
sis, which releases oxygen into the Earth' s atmosphere; (2) in the
combustion of fuel , which is responsible for rising concentrations of
s
atmospheric carbon dioxide ; (3) in the formation of acid precipitation ;
and (4) in many chemical reactions in Earth sediments .
Oxidation refers to a loss of electron , and reduction to a gain of
s
s
electron . For example, an oxidation reaction is
2
Cu(s)� Cu + (aq) + 2e - (6. 1 )
where the symbol e - indicates one free electron (which carries one
unit of negative charge). A reduction reaction is
2Ag + (aq) + 2e - � 2Ag(s) (6. 2 )

Since electrons cannot be lost or gained overall, oxidation must al­
ways be accompanied by reduction. Thus , Eqs. (6. 1 ) and (6.2) together
form a redox reaction
2
Cu(s) + 2Ag + (aq)� Cu + ( aq) + 2Ag(s) (6. 3 )
Equation (6. 1 ) is called the oxidation half-reaction and Eq . (6.2) the
reduction half-reaction for the overall reaction Eq. (6. 3 ) .
I f substance A causes the oxidation of substance B , substance A i s
.
called the oxidizing agent r oxidant. Thus, i n Eq. (6 3 ) , Ag + (aq) i s
o
t h e oxidant, because i t causes Cu(s) t o lose electrons (note that the
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