Strong electrolytes, conductivity plots

In aqueous electrolyte solutions the molar conductivities of the electrolyte. A, and of individual ions, Xj, always increase with decreasing solute concentration [cf. Eq. (7.11) for solutions of weak electrolytes, and Eq. (7.14) for solutions of strong electrolytes]. In nonaqueous solutions even this rule fails, and in some cases maxima and minima appear in the plots of A vs. c (Eig. 8.1). This tendency becomes stronger in solvents with low permittivity. This anomalons behavior of the nonaqueous solutions can be explained in terms of the various equilibria for ionic association (ion pairs or triplets) and complex formation. It is for the same reason that concentration changes often cause a drastic change in transport numbers of individual ions, which in some cases even assume values less than zero or more than unity. 

Arrhenius postulated in 1887 that an appreciable fraction of electrolyte in water dissociates to free ions, which are responsible for the electrical conductance of its aqueous solution. Later Kohlrausch plotted the equivalent conductivities of an electrolyte at a constant temperature against the square root of its concentration he found a slow linear increase of A with increasing dilution for so-called strong electrolytes (salts), but a tangential increase for weak electrolytes (weak acids and bases). Hence the equivalent conductivity of an electrolyte reaches a limiting value at infinite dilution, defined as... 

Debye-Huckel-Onsager theory — (- Onsager equation) Plotting the equivalent conductivity Aeq of solutions of strong electrolytes as a function of the square root of concentration (c1/2) gives straight lines according to the - Kohlrausch law... 

Generally, strong acids in hydrogen peroxide remain strong. For example, plots of equivalence conductance versus the half-power of concentration yield straight lines which are characteristic of completely dissociated electrolytes. 

Eor the KAc solutions and any other strong electrolytes studied as a function of concentration, plot A versus yfc and extrapolate linearly to c = 0 in order to obtain Aq. In making these extrapolations, beware of increasingly large experimental uncertainties at the lowest concentrations and also of systematic errors due to conducting impurities or dilution errors at low concentrations. If you are making a least-squares fit with Eq. (5) rather... 

How does the cell constant k compare with the geometric value HA obtained from an approximate measurement of the dimensions of your ceU Why is the equivalent conductance Aq so large for an HCl solution How do the slopes of your A versus -Jc plots for strong electrolytes compare with literature values and the values expected from Onsager s theory Find a literature (or textbook) value for the equilibrium constant for HAc ionization. Using this value and Eq. (13), draw a dashed literature/theory line on your plot of log versus -Jm. Are the deviations of your data points from this line reasonable in view of the experimental errors expected in this work What is the limiting factor in the accuracy of your measurements ... 

It is seen that at the higher concentrations the equivalent conductance is very low, which is the characteristic of a weak electrolyte, but in the more dilute solutions the values rise with great rapidity the limiting equivalent conductance of acetic acid is known from other sources to be 390.7 ohms cm. at 25 , and so there must be an increase from 131.6 to this value as the solution is made more dilute than 10 equiv. per liter. The plot of the results for acetic acid, shown in Fig. 20, may bo regarded as characteristic of a weak electrolyte. As mentioned in Chap. I, it is not possible to make a sharp distinction between electrolytes of different classes, and the variation of the equivalent conductance of an intermediate electrolyte, siu h as trichloroacetic, cyanoacetic and mandelic acids, lies between that for a weak electrolyte, e.g., acetic acid, and a moderately strong electrolyte, e.g., nickel sulfate (cf. Fig. 20). 

Fig. 6.7 Plots of the molar conductivity against the square root of the electrolyte concentration for three strong electrolytes ( ) and one weak electrolyte ( ). The data for the strong electrolytes were fitted to a straight line at lower concentrations in order to obtain the limiting molar conductivity Aq.

The Conductance of Strong Electrolytes in Methyl and Ethyl Alcohol. Careful studies of the conductances of electrolytes in methyl and ethyl alcohol have been carried out by Hartley and associates.3 A plot of the equivalent conductance, A, values for a series of sulplio-cyanates in methyl alcohol as functions of the square root of the concentration are given in Pig. 1 It will be seen that the plots are all straight lines as required by Onsager s equation for uni-univalent electrolytes, equation (18), Chapter 18. Since methyl alcohol at 25° has a dielectric constant of 31.5 4 and a viscosity of 0.00545 poise, that equation takes the form ... 

Arrhenius originally believed that his theory and the formulation of Ostwald would explain the conductivity behavior of both strong and weak electrolytes. However, it soon became apparent that strong electrolytes required another explanation. Several lines of evidence indicated this. One was that the plots of A against c for strong electrolytes, unlike those for weak ones, could not be fitted to Ostwald s equations. Another was that the heats of neutralization of solutions of strong acids and bases (e.g., the neutralization of HCl and NaOH) could be explained only if it was assumed that these electrolytes are completely dissociated over a considerable concentration range. 

The molar conductivities on the right-hand side can all be obtained by the extrapolation of a A versus plot, since the substances involved are all strong electrolytes. 

Fig. 12.5. Equivalent conductances (A) of several electrolytes at 25°C. plotted against the square root of the concentration. In dilute sr utions of strong electrolytes, the experimental points are well fitted by straight lines. 

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