2+ Year Member
Jun 20, 2016
Cheese State

upload_2016-8-11_21-51-4.png upload_2016-8-11_21-51-12.png

Here is my thought process:
Tube 6 had the addition of CuSO4, according to Le Chat's principle, this will allow rxn 1 to shift toward the left side (because there is an increase of SO4-2 ions from CuSO4), causing an increase in I- ions. Relating this to rxn 2, an increased amount of I- will cause a shift to the left causing S4O6 to be used up.

I definitely overthought about this because it was simply, overtime, S4O6 gets used up, so B is the correct answer.

Maybe I didn't understand the passage? Why would D be incorrect?


5+ Year Member
Mar 23, 2014
Medical Student
question #57 asks about the function of CuSO4 and the answer is that it's a catalyst. Common-ion effect is valid, but comparing tubes 1 and 6 show that 6 completes the reaction in a faster amount of time holding the solution volumes and temperature constant. This is the indicator that CuSO4 is acting as a catalyst.

With that said, focus on what that time column means. It's the length of time for the combined solution to turn dark blue, which is an indicator that all the S2O3 (reaction 2) is used up. So after 19 sec, the concentration of S4O6 (reaction 2 product) will no longer increase.