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PCAT Chemical Processes – Hypothetical Reaction

The Keq for the following hypothetical reaction is 5.16 x 10-6.


A2B (g) + C2 (g) → ABC (s) + AC (g)


Which of the following is true concerning this reaction?


A) The Keq value indicates that reactants are highly favored.

B) The Keq value indicates that products are highly favored.

C) The formation of a solid indicates a decrease in ΔS.

D) The formation of a solid indicates a negative value for ΔG.

Click for Explanation

The Keq equation for the above reaction is:


Keq = [AC] / [A2B][C2]


Remember that solids and liquids do not appear in Keq equations. Since the value of Keq is given as 10-6, we can conclude that the reaction vastly favors the reactants in the denominator. Thus (A) is the correct answer.


B: This would be true if the Keq were much more than 1, rather than less than 1 as given in the question.

C: Going from gas to solid indicates a decrease in ΔS.

D: The phase changes present in a reaction do not determine the ΔG. They have an effect, but are not the sole factor. You must also account for ΔH.

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