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MCAT Chemistry Question — Nernst Equation

The voltage of a galvanic cell, E, depends on the concentration of reactants and products. The Nernst equation,

E = Eo – (0.0592/n)log(Q)

can be used to calculate the cell potential. Eo is the standard cell potential, n is the number of moles of electrons exchanged in the reaction, and Q is the reaction quotient. Which of the following is true regarding the value of E?


A. E is always less than Eo

B. E is always greater than Eo

C. E is always equal to Eo

D. E can be less than, greater than, or equal to Eo

Click for Explanation

According to the Nernst equation, the cell potential E will vary based on the logarithm of the reactant quotient Q. The reactant quotient follows the same equation as the equilibrium constant K, but measures the ratio of the actual concentration of products to concentration of reactants at any time during the reaction. Therefore, when Q > 1, log(Q) is positive and E < Eo. If Q < 1, log(Q) is negative, then E > Eo. Lastly, E is equal to Eo when Q = 1 because the log of one is equal to zero. Thus, E can be less than, greater than, or equal to Eo making D the correct answer.

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