These reactions have reached equilibrium in a solution at pH 9.33 that contains
ID: 486611 • Letter: T
Question
These reactions have reached equilibrium in a solution at pH 9.33 that contains 10-2 mol/L CaCO3(s) and 10-3.3 mol/L Ca2+ .
(b) Would you expect the Ca2+ concentration to increase, decrease, or not change if the system changed in the following ways? (For this part of the problem, do not assume that activity coefficients are 1.0.) In each case, explain your reasoning briefly. (i) The concentration of CaCO3(s) is doubled. (ii) The ionic strength of the solution increases, e.g., by addition of NaCl. For this part of the question, consider only the effect on the first reaction shown above. (iii) The pH is lowered slightly, without significantly altering the ionic strength.
CaCO3(s) 2+ CO32- HCO3- HT CO32- Keg 10 8.3 Keg 10 10.33Explanation / Answer
Answer:
This question is based on Le Chatelier's principle stated as,
" Any system at an equilibrium when imposed by an external constrain react in such way that the constrain imposed is nulified. The system work out this by shifting an equilibrium in appropriate direction whichnulifies the constrain imposed."
The given equilibrium system is,
CaCO3 (s) <---------> Ca2+ (aq.) + CO32- (aq.)
HCO3- (aq.) <---------> CO32- (aq.) + H+ (aq.)
a) Concentrration of CaCO3 is doubled.
On increase in concentrration of CaCO3 more and more CaCO3 will be ionized in an aqueous phase to lower the
CaCO3 concntration to equilibrium stage.
As ionization of CaCO3 increases concentration of [Ca2+] will also increase.
Answer: on doublin concentration of CaCO3 concentration of Ca2+ will also increase.
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b) Addition of a strong electrolyte increases the solubility i.e. ionization of sparingly soluble salt this phenomenon is known as "salting in" .
Increase in ionic strength of solution will increase ionization of CaCO3 which leads to increase in concentration ofCa2+ions.
Answer : Increasing ionic strength of CaCO3 solution will increase [Ca2+].
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(c) Lowering of pH means increase in acidity i.e. increase in # of protons.
From second equilibrium reaction it's clear that there is an ionization of bicarbonate ion HCO3- into CO32- and H+ protons.
On decreasing pH there will be more number of protons in the soluton at equilibrium and hence system utilizes these excess protons in protonting caronate ions CO32- back to HCO3- and this lowers the [CO32-].
Now this lowering of [CO32-] ions disturb the first equilibrium and hence to compansate loss of CO32- ions more and more CaCO3 ionizes in solution and this increases [Ca2+].
Answer : Lowering pH f solution will increase [Ca2+]
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