Page 135 - Basic physical chemistry for the atmospheric sciences

Oxidation-reduction reactions 1 2 1

cathode (where electrons are taken up by the reduction reaction ) .
i
Therefore, n this case, the electrons flow from the zinc electrode o the
t
copper electrode. (This is consistent with the fact that negatively
charged particles always flow up a potential gradient, that is, from the
electrode with the lower potential, zinc in this case, to the electrode
with the higher potential, copper in this case. )
The maximum electric potential difference that the cell can generate,
assuming no inefficiencies, is �e ll· The half-reactions and their electric
potentials are
2
Zn(s)� Z n + ( aq) + 2e ­ �en = - ( - 0 . 7 6 V) = 0 . 76 V
Cu 2 + (aq) + 2e - � Cu(s) � ed = 0 . 34 V
2
2
Net: Zn(s) + C u + ( aq)� zn + (aq) + Cu(s) �e1 = 1 1 . 1 0 V
Since �ell is positive the net reaction is spontaneous from left to right.
z
Therefore, Zn(s) reduces Cu + (aq).
Shown in Table 6.2 are standard electrode potentials for some half­
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cell reactions . Note that in Table . 2 :
n
• All o f the reactio s are listed in the form o f reduction half­
reaction . (The corresponding oxidation half-reactions are i n the
s
opposite directions to those shown in Table . 2 . )
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A positive value o f E1 indicates that the spontaneous reaction pro­
ceeds from left to right (i.e. , reduction is the spontaneous reaction).
A negative value of E1 indicates that the spontaneous reaction pro­
ceeds from right to left (i.e . , oxidation is the spontaneous reaction).
A redox reaction must involve an oxidation half-reaction combined
with a reduction half-reaction. Therefore, one reactant in an overall
redox reaction must come from the left side of a table of reduction
half-reactions (i. e . , the reduced form in the half-equation) and one
reactant must come from the right side of the table (i.e . , the oxidized
form in the half-equation). Such combinations could be of two types:
(a) those for which the oxidized form lies below the reduced form
(e.g. , 03(g) and Ag(s) - see Table 6 . 2 ) , and (b) those for which the
z
oxidized form lies above the reduced form (e.g. , Fe + (aq) and Hz(g) -
see Table . 2 ) . Under standard conditions, type (a) combinations pro­
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duce significant redox reactions in the forward direction (because
t.�ell = �x + �ed is positive). 7 For type (b) combinations the redox
reaction is not significant in the forward direction under standard
conditions (because �ell is negative) . Stated in another way: Under
.\'/andard conditions the reduced o rm o f any couple [e.g. , Li(s) f o r the
f