MathDB

42

Part of 2013 National Chemistry Olympiad

Problems(1)

2013 USNCO Problem 42: Electrochemistry

Source:

4/21/2013
Use the standard reduction potentials to determine what is observed at the cathode during the electrolysis of a 1.0M1.0 \text{M} solution of \ce{KBr} that contains phenolphthalein. What observation(s) is(are) made?
\ce{O2 (g)} + \ce{4H^+ (aq)} + 4e^- \rightarrow \ce{2H2O (l) }; \text{ E}^\circ = \text{1.23 V} \ce{Br2 (l)} + 2e^- \rightarrow \ce{2Br^- (aq)} ; \text{ E}^\circ = \text{1.07 V} \ce{2H2O (l)} + 2e^- \rightarrow \ce{H2 (g)} + \ce{2OH^-} ; \text{ E}^\circ = \text{-0.80 V} \ce{K^+ (aq)} + e^- \rightarrow \ce{K (s)} ; \text{ E}^\circ = \text{-2.92 V}
<spanclass=′latex−bold′>(A)</span>Solid metal forms <span class='latex-bold'>(A) </span>\text{Solid metal forms}\qquad
<spanclass=′latex−bold′>(B)</span>Bubbles form and a pink color appears<span class='latex-bold'>(B) </span>\text{Bubbles form and a pink color appears}\qquad
(C) \text{Dark red } \ce{ Br2} \text{ forms}\qquad
<spanclass=′latex−bold′>(D)</span>Bubbles form and the solution remains colorless<span class='latex-bold'>(D) </span>\text{Bubbles form and the solution remains colorless}\qquad