Why does degree of ionization decrease with concentration?

[esha]

Degree of ionization is the number of particles which dissociate by the total number of particles. if that's the case then higher number of particles should mean higher number of particles which get dissociated. this should also mean higher the concentration the degree of ionization will also be higher. but the reverse is true. why?

 

[Borek]

Number of dissociated particles can be calculated from the equilibrium constant and concentration of the substance and doesn't necessarily follow simplified intuition expressed in your assumption:

higher number of particles should mean higher number of particles which get dissociated

But even when this assumption is correct (which it typically is), your conclusion

this should also mean higher the concentration the degree of ionization will also be higher

is also wrong. Say, you have a solution where degree of dissociation is 10%, you double the concentration, number of dissociated particles gets 50% higher (assumption fulfilled) yet the degree of dissociation goes down to 7.5% (conclusion is false).

 

[esha]

it might seem dumb but i can't understand why the ratio won't remain the same? why would it decrease

 

[Borek]

Because it is governed by other relationship. Have you learned about chemical equilibria?

 

[esha]

ok so is le chattelier's principle working over here? let's say that we have a normal reaction where concentration was C initially
AB <=> A+ + B-
K= C(alpha) ^2 / 1-alpha (alpha = degree of ionization)
in the second case i change the concentration
but k value has to remain constant. hence if we need to regain back the same k value i wud have to multiply something smaller. so alpha value decreases. am i correct in this deduction ??

 

[Borek]

Yes, that's more or less what it is about. Note, that Le Chatelier's principle is only a proxy for the effects equilibrium has on the system. Instead of trying to understand the system in terms of dissociation degree and Le Chatelier's principle, work on understanding the equilibrium and you will always get to the correct result.

 

[esha]

ok... so what i deduced was completely from Mathematical point of view. can u please explain why does it happen so in the real world? and one more stuff. in case of strong acids and bases we know that no such equilibrium exists when they ionize. so why does increase in concentration inhibit conductivity of the electrolyte?

 

[Borek]

using math (together with a correct model) is a perfectly valid approach to finding the answer.

so why does increase in concentration inhibit conductivity of the electrolyte?

In general it doesn't, unless we are talking about very high concentrations, where we are no longer dealing with water solutions.

We are losing linearity much earlier, but that's because the linear model (that is, one in which conductivity is directly proportional to the concentration) is only an approximation and works well only for diluted solutions.