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MCAT Chemistry Question — Standard Free Energy

  • by John
  • Mar 26, 2014
  • Chemistry, MCAT Blog, MCAT Chemistry, MCAT Question of the Day

Which of the following statements most accurately describes the cell potential (Eo), change in Gibb’s free energy (ΔGo), and equilibrium constant (K) for a non-spontaneous chemical reaction?

 

  1. Eo < 0, ΔGo > 0, K < 1
  2. Eo < 0, ΔGo < 0, K < 1
  3. Eo > 0, ΔGo < 0, K > 1
  4. Eo > 0, ΔGo > 0, K > 1

 

Explanation

 

This question is testing your understanding of the relationship between the change in standard free energy  (ΔGo), electrochemical cell potential (Eo), and the equilibrium constant (K) for chemical reactions. To answer this question, you must apply the chemical principles relating to the equation, ΔG = -nFEcell, and a derivative of the Nernst Equation, ΔG = ΔGo + RTln(Q).

 

The question stem indicates that the chemical reaction is non-spontaneous, which corresponds to a positive change in standard free energy. According to the relationship between ΔGo and Eo based on the equation ΔGo = -nFEo, a positive ΔGo corresponds to a negative Eo. Thus, answer choices B and C can be eliminated. To determine the equilibrium constant, it is necessary to understand that the change in free energy of a chemical reaction (ΔG) is equal to zero and the reactant quotient (Q) is equal to the equilibrium constant (K). Thus, ΔGo = -RTln(K). Since ΔGo > 0, K < 1 making A the correct answer.

 

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