Expert: Dr. Henry Boyter, Jr. - 6/29/2005

Question
I am not a student and this is not homework(I am 63y old) I asked this yesterday.  How does the pH of a solution influence the rate that it will dissolve a chemical compound? I work part time for a soft water company and one of the tech people told me this but could'nt explain it.

Thanks

JIm

Answer




In any posts, per the instructions, add why you are asking the question.  Helps tell the students for the working people.











You could be a 63 year old student, but ok.

There are several ways this can happen.  It is usually not a change in rate, but a change in the reaction taking place.  Let's look at something like calcium hydroxide (Ca(OH)2).  When it dissolves the equation is

Ca(OH)2 = Ca+2  +  2OH-

If the solution is made very basic, lots of OH- already present, the dissolving step is slowed, because the reaction above is out of balance.  The dissolving is governed by physical law (and thus math) which says that if one of the components is already present, you can't dissolve as much.  Note that this says not as much CAN dissolve.  The "rate" of the dissolving that does happen is the same no matter the pH.

That is a basic concept that should be remembered and is mixed up alot.  Because it takes longer to turn all the solid into dissolved matter, we say it is slower.  But the actual step when it does dissolve on a molecular level may actually not be that different.  It is the difference in the rate of dissolving versus the rate of the reaction that lead to dissolving.

Let's go back to the first equation.  If we are in acid conditions, the OH- is eaten up by the acid.  Now the Ca(OH)2 sees a shortage on the other side and continues to react "dissolve" with the acid.  Note it is really a reaction that leads to more dissolved calcium, not just a change in the "rate" of dissolving.  Note also, you lose the hydroxide until the acid conditions are gone.

Most other increases in rate are like this in that they involve a reaction.  Take a penny.  Put it in water, it dissolves very, very, very slowly.  Add some acid and it dissolves fast.  What is really happening is that the penny is reacting with the acid to produce a more soluble species.

Summary - what we call faster or slower dissolving is really changing the conditions of the solution by either retarding a reaction by introducing an imbalance or changing the "dissolving" reaction itself.  You change the perceived "rate" by changing the situation, not the actual rate for that species you started with.