Equilíbrio Químico e Células Eletroquímicas

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190 6 CHEMICAL EQUILIBRIUM
CO: ∆solH−○ = RT2 ln 10 × d
dT
(−5.98 − 980
T/K
) = RT2 ln 10 × 980 K
T2
= R ln 10 × (980 K) = (8.3145 JK−1mol−1) × ln 10 × (980 K)
= +18.8 kJmol−1
P6B.12 �e relationship between K and Kc is given by [6A.18b–209]
K = Kc × ( c
−○RT
p−○
)
∆ν
where ∆ν = ν(products) − ν(reactants)
�is expression for K is substituted into the van ’t Ho� equation [6B.2–214],
d lnK/dT = ∆rH−○/RT2
d
dT
ln
⎡⎢⎢⎢⎢⎣
Kc × ( c
−○RT
p−○
)
∆ν⎤⎥⎥⎥⎥⎦
= ∆rH
−○
RT2
�e le� hand side of this expression is evaluated to give
d
dT
ln
⎡⎢⎢⎢⎢⎣
Kc × ( c
−○RT
p−○
)
∆ν⎤⎥⎥⎥⎥⎦
= d
dT
[lnKc + ∆ν ln( c
−○R
p−○
) + ∆ν lnT]
= d lnKc
dT
+ ∆ν
T
so that
d lnKc
dT
+ ∆ν
T
= ∆rH
−○
RT2
hence d lnKc
dT
= ∆rH
−○
RT2
− ∆ν
T
If ∆rH−○ is assumed not to vary with temperature between T1 and T2, this ex-
pression may be integrated between T1 and T2:
∫
T2
T1
d lnKc = ∫
T2
T1
(∆rH
−○
RT2
− ∆ν
T
) dT
to give
lnKc(T2) − lnKc(T1) = −
∆rH−○
R
( 1
T2
− 1
T1
) − ∆ν ln(T2
T1
)
6C Electrochemical cells
Answers to discussion questions
D6C.2 A galvanic cell is an electrochemical cell that produces electricity as a result
of the spontaneous reaction occuring inside it. An electrolytic cell is an elec-
trochemical cell in which a non-spontaneous reaction is driven by an external
source of current.