Prof'G M Madhu

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Prof.G M Madhu Department of Chemical
Engineering R V College of Engineering, Bangalore
Session-18
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Property Changes of Mixing For ideal solution
For any thermodynamic property For non ideal
solutions, we apply a correction term
known as property change of mixing. For non ideal
solutions
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is the difference in the property of the
solution M and sum of the properties of the pure
components that make it up all at the same
temperature and pressure as the solution, thus in
terms of volume For ideal solutions
More general definition of Properties change in
terms Partial molar
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-change in property of component i
when one mole of pure i in its in its standard
state is brought to the solution of given
composition at same T and P.  In terms of Volume
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Activity and property change of mixing  Free
energy change of mixing (?G) Using the
definition of fugacity the change of free energy
of substance when it is brought from the standard
state to the solution, it can be written as
We know that
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Volume change of Mixing ?V The partial molar
free energy(chemical potential) varies with
pressure as
Similarly for pure substance, we have  
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Result of the above equation
The above equation can be rewritten in the form
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Enthalpy change of mixing ?H The Gibbs Helmoltz
equation is given as
in terms of partial molar properties
The enthalpy of mixing
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Using the above equations
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Problem Calculate the mean heat capacity of a
20mol solution of alcohol in water at 298K given
the following Heat capacity of water 4.18 x
103J/kg K Heat capacity of ethanol 2.18 x103
J/kg K Heat capacity of mixing for 20mole
percent ethanol-water at 298K -758J/mol Heat of
mixing for 20mol ethanol-water at 323K-415
J/mol. Assume the heat capacities of pure
liquids are constant between 298 and 323K.
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Solution 0.8mole water and 0.2 mol ethanol
mixed at 323K
 
 
0.8 mol water cooled from 32K to 298K
               
0.2 mol ethanol cooled from 32K to 298K
Heat of mixing ( 0. 8molwater and 0.2 mol ethanol
at 298K)
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20 percent ethanol water heated to 323 k the
enthalpy of heating
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Phase Equilibrium
Criteria for phase equilibrium If a system says
to be in thermodynamic equilibrium, Temperature,
pressure must be constant and there should not be
any mass transfer.
The different criteria for phase equilibrium are
At Constant U and V At Constant T and V At
Constant P and T
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At Constant U and V An isolated system do not
exchange mass or heat or work with surroundings.
Therefore dQ0, dW0 hence dU0. A perfectly
insulated vessel at constant volume dU0 and
dV0.