Physical Chemistry II

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Physical Chemistry II
Chapter VII Electrolytic solution
7.5 The activity of electrolytic solution
7.6 Theories for strong electrolyte
Feb. 26, 2003
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7.5 The activity of electrolytic solution
7.5.1 concepts
For ideal solution or dilute solution of
non-electrolytes
For nonideal solution of non-electrolytes
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For electrolytic solution such as dilute HCl
solution
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Therefore
Because solution only containing single ion
does not exist, the activity of individual ion is
unmeasurable.
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mean activity
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Definition
Mean activity
Mean activity coefficient
Mean molality
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For general electrolyte M?A ?-
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For certain electrolyte M?A ?-
If ?? can be measured, a ? can be then
calculated.
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Example
0.005 mol kg-1 K2SO4, ?? 0.781, please
calculate a?
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7.5.2 mean activity coefficient

• measurement
• (1) vapor pressure
• (2) depression of melting point
• (3) increase of boiling point
• (4) osmotic pressure
• (5) electrochemical method

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2) Influential factors
1) concentration

• m ?0, ? ?1
• m ?, ? has minimal value

Equilibrium between attraction and repulsion
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2) temperature
Table dependence of ? on temperature for 11
type electrolytes
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3)valence types
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conclusion
1) Electrolytes of same valence type have similar
activity coefficient at same concentration. The
coefficient does not depend on the nature of
electrolyte.
2) Activity coefficient strongly depends on
valence types
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7.5.3 ionic strength
1921, Lewis
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Figure ln ? vs. I1/2 for KCl
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Empirical equation
Valid when concentration lt 0.01 m
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7.6 Theories for strong electrolyte
Kinetic property
Static property
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For ideal solution
For nonideal solution
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Theoretical evaluation of Wr 1894 1918-1920
Ghosh 1923Debye-Hückel
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7.6.1 The Debye-Hückel theory
1) Ionic atmosphere
Radius 10-7 10-9 m
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Basic assumption
1) Point charge
2) Coulombic attraction
3) Dielectric constant
4) Boltzmann distribution, Poisson equation
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Shielded Coulombic potential
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Debye length
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Lewiss empirical equation
At 298 K and of aqueous solution A 1.172
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Debye-Hückel limiting law Valid when I lt 0.01 m
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Valid for C lt 0.1 m
Valid for C lt 1 m
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7.6.2 Debye-Hückel-Onsage theory
1927 Lars Onsager

• Relaxation effect

2)Electrophoretic effect
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Kohlrauschs empirical equation. Valid 3?10-5
10-3 m
Fuoss 0.1 m Falkenhagen 5 m, for LiCl 9 m
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1936 Noble Prize Germany, Netherlands 1884/03/24
1966/11/02 Studies on dipole moments and the
diffraction of X rays and electron beams by gases
Peter J. W. Debye
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1968 Noble Prize USA, Norway 1903/11/27 1976/10
/05 Studies on the thermodynamics of
irreversible processes
Lars Onsager
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Progress of the theory for electrolyte
1791 Galvani frog test
1800 Volta Voltas pile
1800 Nicholson and Carlisle electrolysis of
water
1805 Grotthuss orientation of molecules
1857 Clausius embryo of dissociation
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1886 vat Horff colligative property
1887 Arrhenius dissociation and ionization
1918 Ghosh crystalline structure
1923 Debye-Hückel theory
1926 Bjerrum conjugation theory
1927 Onsager Debye- -Hückel-Onsager
1948 Robinson and Stokes solvation theory
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Problems
1) Meaning of infinite dilution
2) Why Debye-Hückels limiting law only valid for
dilution solution
3) explain the salt effect on slight dissoluble
substances