Definition of Thermodynamics

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Thermodynamics Chemical Equilibrium

• Definition of Thermodynamics
• A chemical accounting system that establishes
  a quantitative relationship between chemical
  change and energy.
• For us it is a tool!
• Use of Thermodynamics for Water Chemistry
• Quantify effects of environmental parameters
  T,P,V,C on equilibrium
• Quantify effects of chemical and electrochemical
  gradients upon reaction systems
• Can be used to predict spontaneity or equilibrium
• Can be used to predict composition at equilibrium

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What Thermodynamics Cannot Do!

• Cannot predict rates!
• CH3COOH(aq) 2 H2O 2C graphite K1013
• ?G -74.9 kj/mole Yet very slow reaction!
• Cannot determine reaction mechanisms
• Cannot be applied to changing systems!
• May be approximated by a steady state if reaction
  time frame is small relative to steady state time
  frame
• Rigorously only true for equilibrium systems and
  no real system is ever at equlibrium-- it is only
  approached in the limit as T ? 8

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Thermodyanmic Laws

• Conservation of Energy (Excludes Nuclear)
• Condition for Spontaniety
• Entropy Requirement
• Pure Crystalline Entropy ) at absolute Zero
  Kelvin
• State Entropy is Positive at Other Temps
• Zeroth Law
• Principle of Thermal Equiibrium
• All other Laws Based on This Assumption

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Water Chemistrys First Law

• Relates change in free energy to real system
  changes that we experience
• Temp
• Pressure
• Change in composition
• chemical reaction

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Contains Important Partial Differentials

• Requirement for Spontaneity Equilibrium!

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Chemical Potential of a Gas
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Integrate from P 1atm to P P
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Chemical Potential of an Ideal Solute
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What is µi?? Chemical Escaping Tendency
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Henrys Law Relates Partial Pressure of a Gas to
Its concentration in Solution...
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Thermodynamic Approach to Equilibrium

• Consider the Reaction
• aA bB cC dD
• For this reaction it can be shown

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continuing...
therefore...
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Kinetic Approach to Equilibrium
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Using Your Thermodynamic Tools!
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Example!
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This is a BIG LITTLE number!
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More Examples!
Reaction Proceeds in Direction Opposite as
Written !
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More Examples!
Alternatively, if the system is buffered at pH 6,
what equilibrium concentration of Fe (III) will
the solid support?
if pH 4 OH- 1 x 10-10 x 1.7 x 10-14
Even at very acidic pH values is any dissolution
of mineral notable.This is a very stable solid!
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More Examples! Solubility of a Solid
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THERMODYNAMICS IN ELECTROCHEMICAL SYSTEMS
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RELATION BETWEEN FORMULAS IN TWO SYSTEMS
Note this accepts the convention that oxidized
species e- reduced species
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MORE ELECTRO-THERMO
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Many Thermo Constants were Determined
Electrochemically

• Even Though these may not be true electrochemical
  reactions!

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Some Examples!
Text
Reaction goes virtually to completion as written
with very little Fe present in solution
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Cadmium Complexation
Fair amount of complexing of Cd by NH3
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Copper Hydroxide Solubility
Text
K 1.1 x 10-19 This is a small number - little
solubility
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H2O O2

• This represents the Upper Stability Limit of
  Water
• Balance this reaction

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Now Apply Nernst Eq.
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Lower Stability of H2O

• The Equation You are Writing Relates to your
  Project this Year!
• What Does This Line Look Like?
• What Does it Depend Upon?

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Temperature Effects
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Temperature Effects - Bottom Line!

• If ?H is NEGATIVE - Increasing T Causes a
  DECREASE in Keq!

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Calcium Carbonate Dissolution

• What Happens if We Increase Temperature from 250C
  to 300C?

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The Equil Constant is Bigger? Is This Correct?
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Reactions Acts To Relieve Stress
?H Positive 178.5 kJ Meaning You Must Put Heat
INTO this Reaction to Make It Go!

• 178.5 kJ CaCO3 CaO (s) CO2 (g)
• 178.5 kJ CaCO3 CaO (s) CO2 (g)

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Pressure Effects

• ?G0 -RT lnKeq
• At Constant T, ?j
• Using Our Own Thermodynamic Eq....

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integrating..
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Pressure Effects on Calcium Carbonate Dissoluton
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Activities in Solution - The Third Variable
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Activities in Solution - The Third Variable

• We are concerned with activities in solutions
  because they are the true measure of the chemical
  potential of an ion to react in solution
• µi µi0 RT ln ai
• Now Keq Will Become a Function of I (Ionic
  Strength)

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Activities in Solution - The Third Variable

• Secondly, equilibrium constants are extrapolated
  to zero ionic strength where a concentration.
  We need to be able to calculate equilibrium
  constants at other ionic strengths to be able to
  predict whether we have for example a constant
  Ksp for BaSO4

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Deviations From Ideality

• Note
• 1. When ?i 1, ai Ci
• 2. Ci is the MEASURED concentration!
• 3. ai is the REACTIVE concentration!

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Activity Coefficients

• Note
• Cannot Measure Singe Ion Activity Coefficients!
• Can only measure Mean Free Activity
  Coefficients
• ?i verus ?
• We will return to this subject later

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Deviations From Ideal Behavior

• 5 for NaCl at 0.002M
• 5 for Non Electrolytes at 1M
• OBVIOUSLY ELECTROLYTES ARE MORE IMPORTANT WITH
  RESPECT TO DEVIATIONS FROM IDEAL BEHAVOR - AND WE
  WILL HAVE TO HAVE A MEANS FOR DEALING WITH THIS!

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3 Categories of Electrolytes

• Completely Dissociated
• NaCl Na Cl-
• Weakly Dissociated
• HOAc H OAc-
• Ion Pairs
• MgSO4 Mg2 SO42-

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Debye Hückel Limiting Law

• Where it Came From!

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• Debye reasoned that the extra term had to be due
  to the stabilizing effect of the interaction of
  charged ions in solution
• The theoretical treatment can be found elsewhere,
  but consists of solving the Poison equation in
  the presence of electrolyte concentrations of C
  and C- ions.

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Debye Hückel Limiting Law
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Using the Debye Hückel Law
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Using the Debye Hückel Law an Example
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Effect of Ionic Strength on Dissociation
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Conclusion if an indifferent electrolyte is
added, the dissociation of solute is enhanced!
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What about Single Ion Activity Coefficients Such
as ?Na and ? Cl- ?
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Other Electrolytes ?
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Uneven Electrolytes ?
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Bottom Line

• We Will Typically Use SINGLE ION Activity
  Coeffic1ent Corrections in Writing Reactions
• These activity coefficient corrections can be
  found in tables or calculated using the same
  Debye Hückel or alternative forms of activity
  corrections
• This Last Section Was To Remind You That
  Experimentally We CANNOT Measure Single Ion
  Activity Coefficients

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Calculating Single Ion Activity Coefficient
Corrections !
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Deviations From Ideality

• Deviations From Ideal Behavior Depend on the
  Nature and Charges of the Electrolytes !

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Why Does Debye-Hückel Theory Break Down ?

• Assumes point charges
• Assumes constant dielectric for H2O even at high
  concentrations of electrolyte
• Corrections to theory
• Coulombic
• Chemical
• Statistical Mechanical

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Practical Corrections !
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Ion Size Parameters for Extended Debye Hückel Eq.
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What About Real-World Activity Considerations
When You Do Not Have a Clue as to Ionic Strength ?
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Estimating Ionic Strength !
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You Now Have The Required Set of Thermodynamic
Tools !

• You are about to use them in solving practical
  chemical problems where water is the principle
  solvent !