Strong electrolytes
Strong electrolytes are substances that are virtually fully ionized in solution, and include ionic solids and strong acids. As a result of their complete ionization, the con centration of ions in solution is proportional to the concentration of strong electrolyte added. In an extensive series of measurements during the nineteenth century, Friedrich Kohlrausch showed that at low concentrations the molar conductivities of strong electrolytes vary linearly with the square root of the concentration:
Λm=Λ°m−Kc1/2
This variation is called Kohlrausch’s law. The constant Λ°m is the limiting molar conductivity, the molar conductivity in the limit of zero concentration (when the ions are effectively infinitely far apart and do not interact with one another). The constant K is found to depend more on the stoichiometry of the electrolyte (that is, whether it is of the form MA, or M2A, etc.) than on its specific identity. In due course we shall see that the c1/2 dependence arises from interactions between ions: when charge is conducted ionically, ions of one charge are moving past the ions of interest and retard its progress.
Kohlrausch was also able to establish experimentally that Λ°m can be expressed as the sum of contributions from its individual ions. If the limiting molar conductivity of the cations is denoted λ+ and that of the anions λ−, then his law of the independent migration of ions states that
Λ°m=ν+λ+ + ν−λ−
Where ν+ and ν−are the numbers of cations and anions per formula unit of electrolyte (for example, ν+=ν−=1 for HCl, NaCl, and CuSO4, but ν+=1,ν−=2 for MgCl2). This simple result, which can be understood on the grounds that the ions migrate independently in the limit of zero concentration, lets us predict the limiting molar conductivity of any strong electrolyte from the data in Table 21.5
Illustration 21.2Calculating a limiting molar conductivity The limiting molar conductivity of BaCl2in water at 298 K is
Λm°= (12.72+2×7.63) mS m2mol−1=27.98 mS m2mol−1.
