Chemistry 3131: Analytical Chemistry II (Instrumental Analysis)

Winter 2006: Quiz 3

Question 1
 Q = It = 0.005 C/s * 127s = 0.635 C  (1 point)
# of e- = 0.635C / 96485 C mol-1 = 6.58 x 10-6 mol
mol Cr3+ = 6.58 x 10-6 mol / 3 = 2.19 x 10-6 mol  (1 point)
[Cr3+] = 2.19 x 10-6 mol / 0.010 L = 2.19 x 10-4 M  (1 point)
 
Question 2
 using E = Eo - (0.0592/n) * log (1/[Cr3+]), remember that n = 3, Eo = -0.740
high end E  = -0.760V
low end E = -0.858V
 
Question 3
 pHunk = pHstandard - (Eunk - Estandard)/0.0592 = 3.00 - (0.483-0.572)/0.0592 = 4.50
  
Question 4
 (a)  Choose any three steps:
  • movement from bulk solution to double layer (by stirring/convection, concentration gradient, or potential gradient)
  • phase change when attaching to the electrode (adsorption or crystallisation)
  • electron transfer between electrode and adsorbed species
  • desorption or redissolution of the new species
  • movement back to the bulk solution (by same processes as the first step)

You could also say that one of the steps is a pre-reaction.
 


(b) In any case, rate limitation will give a voltage decrease (less than Eo) for a galvanic cell.  For an electrolytic cell, more voltage would be required to force the reaction (greater than Eo)


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Last updated, April 5, 2006
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