MCAT Biochemistry Question 167: Answer and Explanation

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Question: 167

2. At 25°C the ΔG° for a certain reaction A → B + 2 C is 0. If the concentration of A, B, and C in the cell at 25°C are all 10 mM, how does the ΔG compare to the measurement taken with 1 M concentrations?

  • A. ΔG is greater than ΔG°, thus the reaction is spontaneous.
  • B. ΔG is less than ΔG°, thus the reaction is spontaneous.
  • C. ΔG is greater than ΔG°, thus the reaction is nonspontaneous.
  • D. ΔG is less than ΔG°, thus the reaction is nonspontaneous.

Correct Answer: B

Explanation:

To solve this question, we can use the equation ΔG = ΔG° + RT ln Q. Q, the reaction quotient, is for this reaction. Plugging in the variables, we get ΔG = 0 + RT

Because both R and T are positive, we know that ΔG must be negative and therefore lower than the original value. A negative ΔG corresponds to a spontaneous reaction.

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