Question

Section Name Experiment 2 Advance Study Assignment: Properties of Systems in Chemical Equilibrium 1. Methyl orange, HMO, is a common acid-base indicator. In solution it ionizes according to the equation: HMO(aq) H(aq) + MO (aq) red yellow If methyl orange is added to distilled water, the solution turns yellow. If a drop or two of 6 M HCl 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. a. Why does adding 6 M HCl to the yellow solution of methyl orange tend to cause the color to change to red? (Note that in solution HCl exists as Hand Clions.) b. 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 Nat 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 HMO?) 2. Magnesium hydroxide is only very slightly soluble in water. The reaction by which it goes into solution is: Mg(OH),(s) = Mg" (aq) + 2 OH" (aq) for the above reaction. a. Formulate the expression for the equilibrium constant, K b. It is possible to dissolve significant amounts of Mg(OH), in solutions in which the concentration of either Mget or OH is very, very small. Explain, using why this is the case. c. Explain why Mg(OH), might have very appreciable solubility in 1 M HCl. (Consider the effect of Reaction 3 on the Mg(OH), solution reaction.)

Answer 1

1) According to the Lechatelier's principle, any change in concentration of reactant or product will shift the equilibrium in that direction where it's effect reduces.

a) Adding HCl, increase the concentration of H+ ions thus equilibrium shift backward and solution turn red from yellow.

b) Adding NaOH, decrease the concentration of H+ because OH- produced from NaOH react with H+ and thus shift the equilibrium towards right side and solution become yellow.

H⁺ + OH^- $\rightarrow$ H ₂O