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Describe the reaction below.

PCl3(g) + Cl2(g) -> PCl5(g); Delta(Hf) = -87.9kJ/mol

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Correct Answer: **C.** *Delta(Grxn) < 0 at only low temperatures*

**On the MCAT, numbers can be intimidating, but remember that if the answer choices have no numbers in them, we should think conceptually instead of empirically.** Gibbs equation helps us determine if a reaction is spontaneous or non-spontaneous. It is given as:

**G = H – TS**

In this case, the enthalpy change H is a negative number (exothermic). We know that the entropy change (S) is also negative (the products are more ordered than the reactants) because the reactants are 2 moles of gas and the products are only 1 mole of gas (a synthesis of phosphorus pentachloride). Our equation now be read differently as:

G = (negative number) – temperature * (negative number)

Temperature is in Kelvin and thus always a positive number, but we must consider two possibilities regarding temperature: (1) If this positive temperature is relatively low, the second part of the equation is a small positive number. Thus, if you add a small positive number to a negative number, it will probably still be negative (G<0). (2) If the positive temperature is relatively high, the second part of the equation is a large positive number. If you add a large positive number to a negative number, it will probably become positive (G>0).

In both these cases, ambiguity is important because we don’t know the temperature or S. The only answer that appropriately describes the case we have here is C.

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