I understand why KCl has a higher specific conductivity than NaCl.. but I dont understand why the curvature is more significant for NaCl (KCl is a straight line). Ive provided a picture of this here, and this is also what I get from my experiment. Does anyone mind explaining why?enter image description here

  • $\begingroup$ Could you provide units for the abscissa? $\endgroup$
    – Buck Thorn
    Feb 19 '21 at 19:19
  • $\begingroup$ Also, mS is not a unit of specific conductance (conductivity). $\endgroup$
    – Buck Thorn
    Feb 19 '21 at 19:20
  • $\begingroup$ @BuckThorn this is a diagram i found online, i think it is mass %, im not sure. is specific conductance (conductivity) different than specific conductivity? i also got similar curves in my own experiment.. KCl was a straight line and NaCl was curving, where my y axis was micro siemen/cm and the x axis was mol/dm-3 $\endgroup$
    – Riqueza
    Feb 20 '21 at 7:31
  • 2
    $\begingroup$ Why don't they all start from the same data point at 0%? $\endgroup$
    – ManRow
    Feb 20 '21 at 8:07
  • $\begingroup$ @ManRow hmmm good point, i have no idea? any theories? do you think the greater curvature of NaCl is plausible? tbh im kinda unsure about the unit of the. y axis here and if it is the same as specific conductivity (which is siemen/m) $\endgroup$
    – Riqueza
    Feb 20 '21 at 16:17

For equally charged ions a smaller radius implies a higher charge density, and therefore stronger Coulombic interactions with other charges in solution, including stronger ion-dipole interactions with water molecules, resulting in a more stable hydration sphere, and stronger interactions with counterions (here chloride). Such interactions reduce the mobility of the ion by opposing the effect of the applied external electric field, by either increasing solvent drag or generating an opposing electric field (see for instance this chem libretext for an explanation). This slowing down can be interpreted as an increase in the hydrodynamic or Stokes's radius, and also results in a reduced diffusion coefficient, as shown in the following table (source CRC, $\pu{ 25^\circ C}$):

Ion $\Lambda_+ \pu{(10^-4 m2Smol^-1)}$ $D \pu{(10^-5 cm^2s^-1)}$
$\ce{Cs+}$ 77 .2 2 .056
$\ce{Rb+}$ 77 .8 2 .072
$\ce{K+ }$ 73 .48 1 .957
$\ce{Na+}$ 50 .08 1 .334
$\ce{Li+}$ 38 .66 1 .029
$\frac12$ $\ce{Sr^{2+}}$ 59 .4 0 .791
$\frac12$ $\ce{Ca^{2+}}$ 59 .47 0 .792
$\frac12$ $\ce{Mg^{2+}}$ 53 .0 0 .706
$\frac12$ $\ce{Be^{2+}}$ 45 0 .599

Therefore the smaller monovalent cation, $\ce{Na^+}$, and divalent cation, $\ce{Mg^{2+}}$, have lower molar conductivity than the species lower in the periodic table ($\ce{K^+}$ and $\ce{Ca^{2+}}$).

Additional comments:

(1) Instead of discussing NaCl or KCl, start discussing individual cations and anions, recognizing that these salts completely dissociate into ions in solution. Notice that the different salts share a common anion (chloride, Cl-). Therefore any differences are due to the choice of cation. Sounds obvious, but then you can look at everything from the perspective of the cation's interactions (even if those of the anion will also change between salts, and affect conductivity).

(2) Distinguish between the size of an ion before and after hydration. Bare (unhydrated) $\ce{Na^+}$ is smaller than $\ce{K^+}$, but hydrated $\ce{Na^+}$ is bigger than $\ce{K^+}$.

(3) Bigger bare (unhydrated) cation means weaker interactions and therefore faster mobility.

(4) What drives the formation of a larger hydrated ionic radius is deep down the same thing that drives interactions with other ions. The hydration shell is not that tight. Interaction with other ions will readily override them.

(5) Stronger interactions with other ions means more deviation from ideality. Ideality here means that individual ions behave the same (their mobility is the same) at all concentrations.

  • $\begingroup$ I thought KCl has a smaller hydrated ionic radii than NaCl though? I am using the molar conductivity values but Im just not sure why the slowing down affect is greater for KCl given it has a smaller hydrated ionic radii $\endgroup$
    – Riqueza
    Feb 20 '21 at 16:02
  • $\begingroup$ @Riqueza $\ce{Na^+}$ interacts more strongly with other ions and this causes it to respond more strongly to an increase in concentration. $\endgroup$
    – Buck Thorn
    Feb 20 '21 at 16:40
  • $\begingroup$ why does Na+ interact more strongly than K+ when it has a greater hydrated ionic radii? if I were to plot specific conductivity against concentration, should I expect a greater curvature for NaCl or KCl? $\endgroup$
    – Riqueza
    Feb 20 '21 at 16:45
  • $\begingroup$ @Riqueza $\ce{Na+}$ interacts more strongly with other charges period. This includes charges on other molecules or charges on other ions such as chloride ion. As you increase concentration of chloride this means $\ce{Na+}$ has more ions to slow it down. $\endgroup$
    – Buck Thorn
    Feb 20 '21 at 17:53
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    $\begingroup$ @buckthorn While I agree 2+2- salts have higher association tendency, compare also Q concentrations versus Mg^2+/SO4^2- seawater concentration. Sure effects due ionic neighborhood comes first, but when concentration is high enough, solution gets quite crowded. $\endgroup$
    – Poutnik
    Feb 21 '21 at 12:39

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