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Consider conductometric titration of Strong Acid with strong base say HCl + NaOH.

My question is why initially the conductance decreases. If H+ of HCl is consumed by OH- of NaOH, forming water and hence conductance decreases. I agree to these. But On other hand NaCl is also forming in same amount which can give rise to Conductance. So in my point of view Conductance must remain same or differ slightly. Instead of decreases considerably.

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You correctly identify that the number of cations do not change in solution, so from a charge perspective it appears that there should be no significant change in the conductivity.

However, consider two things:

  1. Sodium ions are much heavier and larger than free protons and so should travel through water more slowly. Charge moving more slowly means lower conductivity.
  2. In addition to their smaller size and lightness, Protons can also "hop" from water molecule to water molecule and so can move through the solution much more quickly and easily. Other water molecules do not act as obstacles

Point 2 is called the Grotthuss Mechanism.

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  • $\begingroup$ H+ -1091 Na+ -406 $\endgroup$
    – Jay Patel
    Apr 28, 2016 at 13:33
  • $\begingroup$ These are values of Hydration enthalpy of H+ and Na+. For H+ it is more means it is highly solvated by water molecules compared to Na+ hence it can roam freely in water compare to Na+. So By applying this concept Na+ must have higher conductance. $\endgroup$
    – Jay Patel
    Apr 28, 2016 at 13:36
  • $\begingroup$ Is my concept is true? $\endgroup$
    – Jay Patel
    Apr 28, 2016 at 13:37
  • $\begingroup$ @JayPatel It's very wrong as you're using thermodynamics instead of kinetics. The answer is ok +1 $\endgroup$
    – Mithoron
    Apr 28, 2016 at 20:02
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simple..proton is light than sodium so the mobility of proton is greater than sodium ion thats why conductance decrease

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