Molar conductivity
La molar conductivity .... m{displaystyle Lambda _{rm {m}}} is the electrical conductivity of an electrolyte based on the concentration or molarity of ions. Because some ions lead electricity better than others, depending on their chemical nature, molar conductivity in aqueous solutions is characteristic of each type of ion and directly proportional to ion migration rates during electrolysis.
Definition
The account of the electric current transport capacity of an electrolyte in solution and is defined as:
.... =κ κ c{displaystyle {mathit {Lambda }}={frac {kappa }{c}}}}
Being:
- κ κ {displaystyle kappa } the conductivity of the dissolution
- c{displaystyle c} the molar concentration of the electrolyte. The magnitude that depends on the electrolyte and the solvent.
Units
The units of molar conductivity in the SI are S{displaystyle mathrm {S} } ⋅ ⋅ {displaystyle cdot } m2{displaystyle mathrm {m} ^{2}} ⋅ ⋅ {displaystyle cdot } morl− − 1{displaystyle mathrm {mol} ^{-1}. Expressed in terms of the SI base units: kg− − 1{displaystyle mathrm {kg} ^{-1} ⋅ ⋅ {displaystyle cdot } s3{displaystyle mathrm {s} ^{3} ⋅ ⋅ {displaystyle cdot } A2{displaystyle mathrm {A} ^{2}} ⋅ ⋅ {displaystyle cdot } morl− − 1{displaystyle mathrm {mol} ^{-1}.
Example
Conductivity, κ κ {displaystyle kappa }of an aqueous KCl dissolution of molar concentration equal to 1.00 morl{displaystyle mathrm {mol} } ⋅ ⋅ {displaystyle cdot } dm− − 3{displaystyle mathrm {dm} ^{-3} at 25 °C and 1 atm is 0.112 S{displaystyle mathrm {S} } ⋅ ⋅ {displaystyle cdot } cm− − 1{displaystyle mathrm {cm} ^{-1}. Calculate the conductivity grind of the KCl in this dissolution.
Numeric values
Ion molar limit conductivity 25 °C in distilled water.
| Cation | ..0+(S·cm2mol)−1) | Anion | ..0−(S·cm2mol)−1) |
|---|---|---|---|
| H+ | 349.8 | OH− | 198,6 |
| Li+ | 38.7 | F− | 55.4 |
| Na+ | 50.1 | Cl− | 76.4 |
| K+ | 73.5 | Br− | 78.1 |
| Rb+ | 77.8 | I− | 76.8 |
| Cs+ | 77.3 | NO3− | 71.5 |
| Ag+ | 61.9 | ClO3− | 64.6 |
| NH4+ | 73.4 | ClO4− | 67.4 |
| N(C)2H5)4+ | 32.4 | HCO3− | 44,5 |
| 1/2 Mg2+ | 53.1 | HCOO− | 54.6 |
| 1/2 Ca2+ | 59.5 | CH3COO− | 40.9 |
| 1/2 Ba2+ | 63.6 | 1/2 SO42− | 80.0 |
| 1/2 Cu2+ | 53.6 | 1/2 CO32− | 69.3 |
| 1/3 La3+ | 69.7 | 1/3 Fe(CN)63− | 100.9 |
| 1/3 Ce3+ | 69.8 | 1/2 (C2O4)2− | 74.2 |
Molar conductivity at infinite dilution
It is the value of molar conductivity when c→ → 0{displaystyle cto 0}. For strong electrolytes (totally dissociated) it is obtained by extrapolation to zero of the molar conductivity when it is represented against the square root of the concentration (Kohlrausch law).
.... =.... ∞ ∞ +Kc{displaystyle {mathit {Lambda }}={mathit {Lambda }}}{infty }+K{sqrt {c}}}}}}}
Being .... ∞ ∞ {displaystyle {mathit {Lambda }{infty }}{infty }} the conductivity molar to infinite dilution, K{displaystyle K} an empirical constant c{displaystyle c} the electrolyte concentration.
If it is a weak electrolyte, Ostwald's law of dilution applies:
1.... =1.... ∞ ∞ +cKc.... (.... ∞ ∞ )2{displaystyle {frac {1}{mathit {Lambda }}}}{{frac {1}{{{mathit {Lambda }{infty }}}}{frac {c}{K_{c}}}{frac {mathit {Lambda }{{ft}{
where Kc{displaystyle K_{c}} is the constant balance in concentrations.
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