MCAT General Chemistry Question 222: Answer and Explanation

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Question: 222

7. When table sugar is exposed to air it undergoes the following reaction:

C₁₂H₂₂O₁₁ + 12 O₂ → 12 CO₂ + 11 H₂O

(∆G = -5693 kJ/mol)

When this reaction is observed at the macroscopic level, it appears as though nothing is happening, yet one can detect trace amounts of CO₂ and H₂O being formed. These observations are best explained by the fact that the reaction is:

A. thermodynamically favorable but not kinetically favorable.
B. kinetically favorable but not thermodynamically favorable.
C. neither kinetically nor thermodynamically favorable.
D. both kinetically and thermodynamically favorable.

Correct Answer: A

Explanation:

A Given the very negative ∆G value, this is a very thermodynamically favorable, spontaneous chemical reaction (eliminate choices B and C). It is important to make the distinction in this case between kinetics and thermodynamics. The reason only trace amounts of products are formed is that the reaction proceeds at an incredibly slow rate (therefore NOT kinetically favorable) due to a high activation energy.

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