# PCAT Chemical Processes – Hypothetical Reaction

• by
• Mar 20, 2018
• PCAT Question of the Day

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

##### Click for Explanation

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:

ΔG = ΔH – TΔS

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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