PHYSICAL CHEMISTRY IN BRIEF

CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 375

Solution

By integrating equation (11.50) we obtain

lnKw(T) = lnKw(298.15) +

∆rH◦

R

( 1

298. 15

−

1

T

)

= ln

(

1. 005 × 10 −^14

)

+

56500

8. 314

(

1

298. 15

−

1

T

)

ForT = 273.15 K we obtain

Kw(273.15) = 1. 25 × 10 −^15 ,

and forT= 373.15 K

Kw(273.15) = 9. 81 × 10 −^13.

The results show that the ionic product of water largely depends on temperature. In the studied

temperature range it increased 785-times!

The pressure dependence of the ionic product is low; it exhibits itself only at extremely high
pressures.

11.5.3 Dissociation of a week monobasic acid

A weak monobasic acid HA dissociates according to the equation

HA = H++ A−. (11.52)

The equilibrium constant of this reaction is called theacid dissociation constant. We write

K=

aH+aA−

aHA

=

cH+cA−γ^2 ±

cHAγHA

1

cst

. (11.53)

If we do not consider any other reaction in the system (e.g. dissociation of water), we may
express the composition of an equilibrium mixture using the degree of dissociation:

cHA= (1−α)c, cH+=α c, cA−=α c.