In this session we will

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In this session we will

• Find out what is fugacity
• See how this takes into account non-ideality
• Also determine fugacity of real gases.

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Fugacity of Pure Species
Fugacity is used to represent behavior of real
gases.
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This is not true for real gases. In order to make
this equation valid, let us choose f such that
Here f is called fugacity and the above equation
defines it partially.
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Thus in case of ideal gases,
Proportionality constant is taken as unity.
For ideal gases, f p.
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As pressure is decreased, we know real gases will
increasingly behave like ideal gases.
Standard state of a real gas is a hypothetical
state in which the gas behaves as an ideal gas.
This happens at low pressure, let us say at p
p0.
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Fugacity coefficient ? is defined as the ratio of
fugacity to the pressure.
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Effect of temperature on fugacity
Partial differential w. r. t. T at constant
pressure,
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Substituting Gibbs Helmholtz equation
Here f0 is fugacity at low pressures. The
variation of fugacity will be negligibly small.
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This equation gives the effect of temperature on
fugacity
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Effect of pressure on fugacity
Under isothermal conditions,
Writing it as partial differential equation with
respect to pressure at constant T,
This equation represents effect of pressure on
fugacity.
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Fugacity determination
We need to integrate this between p 0 to the
pressure at which we want to determine fugacity.
As p ? 0, f ? p
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Using residual volumes
Residual volume is defined by
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Using equation of state
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Fugacity of van der Waals gas
Equation of state is
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Approximate value of fugacity
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Approximate value of fugacity
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At 200K, the compressibility factor of oxygen
varies with pressure as given below. Evaluate the
fugacity at 200K and 100 bar.
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In This session we

• Defined fugacity and its relevance to
  non-ideality
• Formulated the method to determine fugacity of
  real gases

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In the next session we will

• Determine changes due to phase transformation
• Derive Clausius Clapeyron Equation
• Work out numerical problems