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MCAT Chemistry Question — Hypothetical Reaction

Consider the following hypothetical reaction:

Q2 (g) + AX4 (g) → 2 QX2 (g) + A (g) ΔH = 23.4 kJ / mol

Which of the following is true regarding this reaction?


A) Spontaneous at all temperatures
B) Nonspontaneous at all temperatures
C) Spontaneous at low temperatures
D) Spontaneous at high temperatures



Remember that to be spontaneous, a reaction must have a ΔG that is negative. To answer this question, start with the Gibbs Free Energy equation:


The reaction starts with two moles of gas and ends with three moles of gas, so the ΔS is positive. ΔH is given as a positive number in the question. The equation now looks like this:

ΔG = (+) – T(+)

To make ΔG negative and the reaction spontaneous, we want a large T so that we’re subtracting a large positive number. Thus the answer is (D).

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