Electrochemistry

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Department of Chemistry
Electrochemistry Solutions 1. Solutions and
Mixtures Year 1 Module 3 8 Lectures Dr
Adam Lee afl2_at_york.ac.uk
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Aims

• To
• Understand physical chemistry of solutions and
  their thermodynamic properties
• ? predict/control physical behaviour
• ? improve chemical reactions
• Link electrochemical properties to chemical
  thermodynamics
• ? rationalise reactivity.

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Synopsis

• Phase rule
• Clapeyron Clausius-Clapeyron Equations
• Chemical potential
• Phase diagrams
• Raoults law (Henrys law)
• Lever rule
• Distillation and Azeotropes
• Osmosis
• Structure of liquids
• Interactions in ionic solutions
• Ion-ion interactions
• Debye-Huckel theory
• Electrodes
• Electrochemical cells
• Electrode potentials
• Nernst Equation
• Electrode types

Recommended Reading R.G. Compton and G.H.W.
Sanders, Electrode Potentials Oxford Chemistry
Primers No 41. P. W. Atkins, The Elements of
Physical Chemistry, OUP, 3rd Edition, Chapters
5, 6 9. P. W. Atkins, Physical Chemistry,
OUP, 7th Edition, Chapters 7, 8 10 OR 8th
Edition, Chapters 4, 5, 6 7.
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Phase Diagrams
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Gibbs Free Energy
American mathematical physicist developed theory
of chemical thermodynamics. First US engineering
PhDlater Professor at Yale.
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Parisian engineer and mathematician. Derived
differential equation for determining heat of
melting of a solid
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Consider ideal gas at constant temperature dG
Vdp SdT Vdp Since pV nRT,
If initial state 1 STP (1 atm)
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yA
yA
yB
yB
Ginitial nA?A nB?B nA??
RTlnp nB?B? RTlnp Gfinal nA??
RTlnyAp nB?B? RTlnyBp ?G Gfinal -
Ginitial RTnAlnyAp nAlnp
nBlnyBp - nBlnp
nA
nB
A
B
Initial
Final
?Gmixing
ya
yb
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yA
yA
yB
yB
yA
yA
yB
yB
?Gmixing
?Smixing
ya
ya
0
1
yb
yb
0
1
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Chemical Potential (in English!)
Molecules acquire more spare energy
Gibbs Free Energy
?
G ? ln(pressure)
Pressure
Low Pressure
High Pressure




Constant Temperature
Gmolar G? molar RT lnp
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Why do we use Chemical Potential?
Gibbs Free Energy (G) is total energy in entire
system available to do stuff - includes all
molecules, of all substances, in all phases
G nA?A nB?B
No real need to use ?
Free energy from 2 sources
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Why do we different molecules have different
Chemical Potentials?
Involatile
Volatile
Ethanol can soak up much more energy in extra
vibrational modes and chemical bonds - will
respond differently to pressure/temperature
increases
Chemical Potential ? 1. A measure of "escaping
tendency" of components in a
solution 2. A measure of the
reactivity of a component in a solution
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xA runs between 0 (none present) to 1 (pure
solution) Mixing (diluting a substance) always
lowers xA ? this means mixing ALWAYS gives a
negative lnxA
Mixing always lowers ?
Dilution
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b.pt. gt ideal
b.pt. lt ideal
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Summary Raoults Law for Solvents
pA xA . pAT
Volatile high vapour pressure
Involatile low vapour pressure
pBT
pB xB . pBT
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High p0?(A)
Low p0?(A)
High order low S
Less order higher S
A
A
A
A
Strong desire to ? S
Less need to ? S
Boiling of A favoured
A happier in liquid
p0? vapour pressure tendancy of system
to increase S
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Dissolution is EXOTHERMIC
For dissolution of oxygen in water, O2(g)
O2(aq), enthalpy change under standard conditions
is -11.7 kJ/mole.
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Solvent H2O
Solute O2
O2
H2O
Henry's law accurate for gases dissolving in
liquids when concentrations and partial pressures
are low. As conc. and partial pressures
increase, deviations from Henry's law become
noticeable
Consider O2 dissolution in water Important in
Green Chemistry for selective oxidation
Cinnamyl Alcohol
Similar to behavior of gases - deviate from the
ideal gas law at high P and low T. Solutions
obeying Henry's law are therefore often called
ideal dilute solutions.
Aspects of Allylic Alcohol Oxidation Adam F. Lee
et al, Green Chemistry 2000, 6, 279
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Volatile
Involatile
B
A
Tie-line
A-B Composition
Liquid-Gas Distribution
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Example Problem
The following temperature/composition data were
obtained for a mixture of octane (O) and toluene
(T) at 760 Torr, where x is the mol fraction in
the liquid and y the mol fraction in the vapour
at equilibrium The boiling points are
110.6 ?C for toluene and 125.6 ?C for octane.
Plot the temperature/composition diagram of the
mixture. What is the composition of vapour in
equilibrium with the liquid of composition 1.
x(T) 0.25 2. x(O) 0.25
Liquid
Vapour
x(T) 1, T 110.6 ?C x(O) 1, x(T) 0, T
125.6 ?C
P.W. Atkins, Elements of Phys.Chem. page 141
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b.pt. gt ideal
b.pt. lt ideal
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A
B
B
A
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Topics Covered (lectures 2-4) ? Chemical
Potential - ?A(l) ??A(l) RTlnxA -
?A(g) ??A(g) RTlnpA ? Mol fractions - A
nA / nAnB ? Raoults Law - pA poA xA
pB poB xB - ideal solutions -
ve/-ve deviations ? Vapour-pressure diagrams
- Tie-lines - Lever Rule
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Solutions Equations
Phase Rule F c - p 2 c
components degrees of freedom p no. of
phases Clapeyron Equation ?H enthalpy of
phase change ?V volume change associated
with phase change Clausius-Clapeyron
Equation or Mol fractions xA nA /
nAnB ni mols of i yA pA / pA pB pi
partial pressure of i Raoults Law pA poA
xA and pB poB xB Lever Rule (for tie-line
joining phases via point a) nl no. moles
in liquid phase nv no. moles in liquid phase
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A
WHY?!!! Solvent A ?A(l) ??A(l) RTlnxA
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