Thermodynamic Quantities

Correct Answer: C. Only G would be positive.

G = H – TS

“The sublimation of carbon dioxide occurs quickly at room temperature” means that it is spontaneous and so G must be negative (by definition). Sublimation means:

Solid CO2 + heat -> vapor

Entropy (randomness) is clearly increasing (thus positive S) because we are moving from a structured, ordered solid to randomly moving gas particles. Heat is required so it is endothermic meaning H is positive. The question is asking about the reverse reaction so all 3 signs are reversed: G is now positive; H is now negative; S is now negative.

Going deeper: Notice that a negative S multiplied by a –T (see the Gibbs free energy equation) creates a positive term which overshadows the effect of the negative H and thus G is still positive. Also, please keep in mind that sometimes the real MCAT will provide the Gibbs free energy equation but sometimes they won’t.

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