Le Chatelier\'s Principle 1. Methyl red, HMR is a common acid-base indicator. In
ID: 1028244 • Letter: L
Question
Le Chatelier's Principle 1. Methyl red, HMR is a common acid-base indicator. In solution it ionizes according to the equation: HMR (aq)H. (aq) + MR' (aq) red yellow If methyl red is added to distilled water, the solution turns yellow. If a drop or two of 6 M HCI is added to the yellow solution, it turns red. If to that solution one adds a few drops of 6 M NaOH the color reverts to yellow. Why does adding 6 M HCI to the yellow solution of methyl red tend to cause the color to change to red? (note that in solution HCI exists as H+ and Cl- ions.) a. Why does adding 6 M NaOH to the red solution tend to make it turn back to yellow? (Note that in solution NaOH exists as Na* and OH ions. How does increasing [OH ] shift Reaction 3 in the dis- cussion section? How would the resulting change in [H'] affect the dissociation reaction of HMR?) b. 2. Copper hydroxide is only very slightly soluble in water. The reaction by which it goes into solution is: Cu (OH)2(s) Cu2+(aq) + 2 OH(aq) a. Formulate the expression for the equilibrium constant, Ksp for the above reaction. Would you expect the solubility of Cu (OH)2 in HCl to be more, less or the same compared to the solubility of Cu(OH) in water? b. c. Explain why Cu(OH) might have very appreciable solubility in 1 M HCI. (Consider the effect of Reaction 3 on the Cu(OH)2 solution reaction.)Explanation / Answer
1) HMR <-------------> H+ + MR-
red yellow
a) when a drop of 6MHCl is added more H+ are added to the yellow solution of methyl red.
then
according to Lechtlier principle , the concentration of product H+ is increased and the equilibrium shifts to left side that is the reactant side.
Thus the solution shows the color RED of the reactant.
b)Now to the red solution of methyl red a few drops of 6M NaOh are added.
Thus more oH- ions are added to solution.
These OH- ions react with the H+ ions (product) thus decreasing the concentration of product H+.
H+ + OH- -----------> H2O(l)
Thus the equilibrium shifts towards right side according to Lechtlier principle.
Hence the yellow color is restored.
Q2)
Cu(OH)2 (s) ------------> Cu+2 (aq) + 2OH-(aq)
a) Ksp = [Cu+2] [OH-]2
b) When Hcl is added /salt taken in HCl, the solution has more H= ions which can react with H+ as
H+ + OH- ------> H2O (l)
thus forcing the solubility equilibrium towards right side.
Thus the solubility of Cu(OH)2 is more in HCl than in pure water.
c) As the OH- ions of Cu(OH)2 react with H+ of HCl, Cu(OH)2 has appreciable solubility in Hcl than in water.
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