Experiment 3:
Conductance
- Conductance of solutions
Conductance of Solutions
Electrical conduction is a property of ionic solutions.
From a macroscopic point of view, ionic conduction
of solutions is similar to electron or hole conduction
through solid objects.
Although water itself is a poor conductor of
electricity, the presence of ionic species in solution
increases the conductance considerably.
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Conductance of Solutions
Conductivity is the ability of solution to pass an
electric current.
In solutions, cations and anions carry the current.
While for solids (i.e. metals), it is carried by
electrons.
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Conductance of Solutions
Conductivity of a solution depends on:
Concentration
Mobility of ions
Valence of ions
Temperature
In aqueous solutions, the ionic strength varies from
the low conductivity of ultra pure water to the high
conductivity of concentrated chemical samples.
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Measuring Conductivity
Conductivity of solutions is measured by how well
cations and anions migrate to the negative and
positive electrodes, respectively.
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Conductive solution
Conductivity is typically measured in aqueous
solutions of electrolytes.
Electrolytes are substances containing ions. It could
be acids, bases, or salts and it can be either strong
or weak electrolytes.
Conductance behavior as a function of
concentration differs for strong and weak
electrolytes, respectively.
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Electrolyte solutions
Electrolyte solutions obey Ohm’s law just as
metallic conductors do.
Conductance is the reciprocal of resistance (R). And
it is proportional to the cross-sectional area (A) and
inversely proportional to the length (l).
1 𝜅∙𝐴
=
𝑅 𝑙
K is the specific conductance with units Ω-1m-1. The
reciprocal ohm (Ω) is equivalent to siemens (S).
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Molar conductance
Cell constant defines the specific geometric
parameters, and should be calibrated to any
solution with specific conductance.
𝑙𝑒𝑛𝑔𝑡h −1
𝑐 𝑒𝑙𝑙 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 = (𝑢𝑛𝑖𝑡 : 𝑚 )
𝑎𝑟𝑒𝑎
The specific conductance Κ (Greek letter kappa)
increases as the concentration increases.
A more fundamental unit of electrolytic
conductance is the equivalent conductance Λ.
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Molar conductance
The equivalent conductance (Λ) has a unit of
cm2∙equiv-1∙Ω-1. And c is the concentration having a
unit of normality (equiv/L).
𝜅 ∙1000
Λ=
𝑐
For simple 1-1 electrolytes A+B-, the equivalent
conductance is the same as molar conductance (Λm).
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Strong electrolytes
A strong electrolyte is a solute that completely, or
almost completely, ionizes or dissociates in a
solution.
0.5
Λ = Λ − 𝐴 𝑐
0
At concentrations below 0.1M, a plot of Λ against
c½ gives a straight line. The intercept of which
equals Λ0, or the equivalent conductance at infinite
dilution, and A is an empirical constant.
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Strong electrolytes
At infinite dilution the ions act completely
independently, and it is then possible to express
Λ0 as the sum of the limiting conductance of the
separate ions. Hence for a 1-1 electrolyte A+B-:
−
+¿ +𝜆 0 ¿
Λ =𝜆
0 0
The ionic equivalent conductance ( and ) are
equivalent to the ion mobility (U) multiplied by the
Faraday constant.
𝜆
+¿ 𝐹¿
+¿ =𝑈 0 ¿ 𝜆−
0 =𝑈 −
0 𝐹
0
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Ion mobility
Ion mobility (U) is defined as the speed of the ion (v)
under the influence of an electric field strength (E).
±
± 𝑧 ∙𝜈
𝑈0=
𝐸
Note: represents the valences of positive and
negative ions.
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Equivalent conductances
Table of conductivities of selected strong electrolytes:
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Weak electrolytes
For a weakly ionized substance, Λ varies much more
markedly with concentration because the degree of
ionization, α, varies strongly with concentration.
Λ ( 𝐻𝐴𝑐 )= Λ 0 ( 𝐻𝑋 ) + Λ0 ( 𝑀𝐴𝑐 ) − Λ0 ( 𝑀 𝑋 )
0
For sufficiently weak electrolytes, the ionic
concentration is small and the effect of ion
attraction on the mobilities is small.
Λ
𝛼=
Λ0
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Equilibrium constant
Knowing the concentration c of the weak
electrolyte, and its degree of ionization α at that
concentration, the concentrations of H+ and Ac- ions
and of undissociated HAc can be calculated.
The equilibrium constant in terms of concentrations
Kc can be calculated from:
𝐾 𝑐= ¿ ¿
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Equilibrium constant
Once Kc is known, the concentration dependence of
the equivalent conductance can be predicted.
Ostwald’s Law of Dilution:
1 1 Λ∙𝑐
= +
Λ Λ0 𝐾 𝑐 ∙ Λ20
If 1/Λ is plotted against Λ·c, the values of Λ0 and
Kc can be obtained from the intercept and the
slope, respectively.
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