Clicker questions exam 2 - updated.pptx

B the indicator will change color from pale blue to

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B. The indicator will change color from pale blue to red as the end-point is reached. C. The indicator will change from red to pale blue as the end-point is reached. D. The indicator will remain red throughout the titration. E. None of the above. Explanation next slide

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Remember, during a redox titration (figure at right is for iron and cerium, but applies qualitatively to this question): At 1/2Ve, E = E 0 analyte – E reference At 2Ve, E = E 0 titrant – E reference The voltage at the equivalence point must be between these numbers. In this case (the question above), the indicator reduction potential is always higher than the electrode potential, so it will always be predominantly in its reduced form. To see this mathematically, suppose that, at the equivalence point, the voltage is 0.5V and suppose the standard reduction potential of the indicator is 1V. We can write a Nernst equation for the indicator: Plugging in E = 0.5V, E 0 for the indicator = 1V, and n=1 (for example), we get that the ratio of the reduced to the oxidized form is nearly 3 x 10 8 .
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