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For the electrochemical cell represented as A | A+(x M) || B+(y M) | B the measured emf is +0.20 V. Which statement about the overall cell reaction is correct?

1. A+ + e- → A; B+ → B
2. The overall cell reaction cannot be determined from the given information
3. A + B+ → A+ + B
4. A+ + B → A + B+

Correct answer: A + B+ → A+ + B

Solution

A positive cell emf (+0.20 V) means the reaction proceeds spontaneously as written: A is oxidized at the anode (A -> A+ + e-) and B+ is reduced at the cathode (B+ + e- -> B). Overall: A + B+ -> A+ + B.

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