PHYSICAL CHEMISTRY IN BRIEF

CHAP. 8: CHEMICAL EQUILIBRIUM [CONTENTS] 237

S Symbols: For this standard state, the superscriptstis replaced with the superscript[c].

• Standard state: the state a given component would have in a hypothetical ideal mixture
  whose properties would be extrapolated from infinite dilution at the temperature and
  pressure of the system to the concentration 1 mol/kg of solvent; mst= 1 mol/kg of
  solvent.
  S Symbols:For this standard state, the superscriptstis replaced with the superscript[m].

Example

What is the activity of nitrogen, whose fugacity at temperatureTand pressure 1 MPa isfN 2 =

0.505 MPa, if we choose the standard state as follows:

a) nitrogen in the state of an ideal gas at the temperature of the system and the standard pressure

101 kPa,

b) nitrogen in the state of an ideal gas at the temperature and pressure of the system

Solution

a)

aN 2 =

fN 2

fst

=

fN 2

pst

=

0. 505

0. 101

= 5,

b)

aN 2 =

fN 2

fst

=

fN 2

p•

=

0. 505

1

= 0. 505.

Note that the activity of nitrogen changes with the standard state.

8.2.6 Equilibrium constant

The equilibrium constant of reaction (8.2.3) is

K≡

∏k

i=1

aνi,ieq. (8.15)

U Main unit:dimensionless quantity.