Chemical Reaction Equilibria

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Chemical Reaction Equilibria

• Part IV

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Liquid-phase reactions
For a liquid phase, fio is the fugacity of pure
component i at T of the system and Po 1 bar
fi is the fugacity of i at T and P, and fi0 is
the fugacity of i at T and Po
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K for liquids
This expression holds at high pressures, i.e.,
When P gtgt Po
at low to moderate pressures
if the equilibrium mixture is an ideal solution
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Homogeneous reactions

• only one phase
• Read and work examples 13.5, 13.6, 13.7 13.8
  13.9

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Heterogeneous reactions

• more than one phase
• two criteria phase equilibrium and chemical
  equilibrium
• Question where the reaction takes place?
• example A(g) B (l) ? C (l)

liquid
rxn takes place in the gas phase rxn takes place
in the liquid phase rxn takes place in both
phases
gas
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If the rxn takes place in both phases
The value of K depends on the chosen standard
states however, all methods lead to the same
equilibrium composition
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1st example

• Ethylene reacts with water to form ethanol at
  200oC and 34.5 bar there are two phases liquid
  and vapor. The pressure is kept constant.
  Estimate the compositions of the liquid and vapor
  phases.
• F 2-p N 2 -2 2 2
• If we specify T and P the intensive variables are
  fixed.

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assume the rxn takes place in the gas phase

• C2H4(g)H2O(g) ? C2H5OH (g)

standard pressure P0 is 1 bar
calculate K at 200oC yields K 0.031
Next we need to add the VLE equations
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where the liquid-phase fugacities can be written
as
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Use this eqn
Because ethylene much more volatile than H2O
and C2H5OH
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2nd example

• Limestone (CaCo3) decomposes upon heating to
  yield quicklime (CaO) and carbon dioxide. At what
  temperature is the decomposition pressure of
  limestone 1 atm?

CaCO3 (s) ? CaO (s) CO2 (g) Each species
exists pure as an individual phase For the solid
phases, the ratio f/fo 1 (pure) For CO2, f/fo
P/Po 1.033 So, K P 1.0133, we need to find T
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From the data in appendix C

• DGo (298K) 130401 J/mol
• DHo (298K) 178321 J/mol
• Calculate DA, DB, DC, DD
• And get the integral to calculate DG(T), such
  that K 1.0133 exp (- DG(T)/RT)
• Solve for T,
• T1151.83 K

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3rd example

• Ammonium chloride NH4Cl(s) decomposes upon
  heating to yield a mixture of ammonia and
  hydrochloric acid. At what temperature does
  ammonium chloride exert a decomposition pressure
  of 1.5 bar?
• For NH4Cl(s), DHf(298 K)-314,430 J and
• DGf(298 K)-202,870 J.
• NH4Cl(s) pure phase, f/fo 1 (pure)
• The gas mixture NH3 and HCl at 1.5 bar, ideal gas
  mixture, for each gas the fugacity is the partial
  pressure yiP

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yiP 0.5x1.5 0.75 for each gas K 0.75x0.75
0.5625, we must find T that gives that value of
K From Appendix C DGo (298K) 91121 J/mol DHo
(298K) 176013 J/mol Calculate DA, DB, DC,
DD And get the integral to calculate DG(T), such
that K 0.5625 exp (- DG(T)/RT) Solve for T,
T623.97 K
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example 4

• The following isomerization reaction occurs in
  the liquid phase A?B, where A and B are miscible
  liquids for which GE/RT 0.1xAxB. If DGo (298
  K)-1,000 J, what is the equilibrium composition
  of the mixture at 25oC? How much error is
  introduced if one assumes that A and B form and
  ideal solution?

Solve for xA 0.3955
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How much error is introduced if one assumes that
A and B form and ideal solution?

• For the ideal solution

Solve for xA 0.4005