Fugacity is a concept introduced by Gilbert Lewis to represent the escaping tendency of real gases and their behavior in solutions. Fugacity (f) is defined such that the change in Gibbs free energy (ΔG) for any process involving real gases can be written as ΔG = RTln(f2/f1), similar to the ideal gas equation but using fugacity rather than pressure. For ideal gases, fugacity is equal to pressure, but for real gases f/p is not constant and decreases with decreasing pressure. The gas is considered ideal at very low pressures where f/p approaches 1, allowing fugacities to be determined at other pressures. Fugacities have the same units as pressure.
3. INTRODUCTION
The concept of Fugacity was introduce by Gilbert
newton Lewis.
Fugacity is derived from Latin word fleetness or
the Escaping Tendency.
Fugacity is widely used in solution thermodynamic
to represent the behaviour of real gases.
4. CONCEPTS OF FUGACITY
The concept of fugacity to represent the actual
behaviour of real gases.
for an infinitesimal reversible change occuring in a
system ,
we have from dG = VdP- SdT
dT =0
so,
dG = VdP …..(1)
5. let the system be under isothermal condition,
For 1mole of an ideal gas, PV= RT
V = RT/P
So, from eq.(1),
We have,
dG = RT(dP/P)
dG = RTd(ln P) …….(2)
6. For a real gas, a function f known as fugacity is
introduced and is defined in such a manner that the
realation ,
dG = RTd log f .........(3)
always hold good , whether the gas is ideal or real.
On integration of eq.(3)
we get,
G = RTd log f + c .........(4)
7. Where , c = integration constant which is dependent
on the temprature and nature of the gas. On
integrating equation (3) between proper limits ,
we get ,
𝑮𝟏
𝑮𝟐
𝒅𝑮 = 𝒇𝟏
𝒇𝟐
𝑹𝑻𝒅 𝐥𝐨𝐠 𝒇
∆𝑮 = 𝑮𝟐 − 𝑮𝟏
∆𝑮 = 𝑹𝑻 𝒍𝒐𝒈
𝒇𝟐
𝒇𝟏
………(5)
8. Where , the subscripts 1 and 2 represent the initial
and final states , respectively.
for an ideal gas , eq.(5) becomes ,
∆𝑮 = 𝑹𝑻 𝒍𝒐𝒈
𝒑𝟐
𝒑𝟏
………(6)
comparing eq. (5) and(6) , we see that ,
Fugacity∝ 𝐏𝐫𝐞𝐬𝐬𝐮𝐫𝐞
if we take proportionality constant as unity ,
then
Fugacity = Pressure
9. For a real gas , the values of f and p are not
proportional to one another and f/p is not constant.
As the pressure is lowered, the behaviour
approaches that of ideal behaviour.
So, the gas at very low pressure is taken to be the
refrence state and it is postulated that:
lim
𝑝→0
𝑓
𝑝
= 1
10. this postulate makes the evalution of actual
fugacities at various pressure possible.
Since pressure is expressed in atmosphere;
fugacities are also expressed in atmosphere.
