Return to MCAT® Blog Homepage

MCAT Chemistry Question — Spontaneous Reaction

Consider the following reaction:

C6H12O6 (s) + 6 O2 (g) → 6 CO2 (g) + 6 H2O (l) ΔH = – 2808 kJ / mol

Under what conditions will this reaction proceed spontaneously?

 

A. Low temperature, low pressure

B. Low temperature, any pressure

C. High temperature, any pressure

D. Any temperature, any pressure

Click for Explanation

Spontaneity is determined by a negative ΔG value in Gibbs free energy equation:

ΔG = ΔH – TΔS

A reaction with a negative ΔH and a positive ΔS will thus be spontaneous under any conditions. The reaction given has a negative ΔH. It also involves converting 1 mole of solid and 6 moles of gas into 6 moles of liquid and 6 moles of gas. That phase change means entropy is increasing (solid to liquid). Thus, with a negative ΔH and a positive ΔS this reaction is spontaneous under any conditions and (D) is the right answer.

A, B: Reactions with a negative ΔH and a negative ΔS are spontaneous at low temperatures (e.g. the freezing of water from liquid to solid).

C: Reactions with a positive ΔH and a positive ΔS are spontaneous at high temperatures (e.g. the boiling of liquid water into steam)

Want more MCAT practice?

We’ve got options for every schedule and learning style!

From the best online MCAT course created by top instructors with 524+ MCAT scores to the most representative full-length practice exams and private tutoring, we can custom tailor your MCAT prep to your goals!

Not sure which option is right for you? Schedule a free MCAT consultation with an MCAT expert using the form below. No obligation, just expert advice.

Create your Free Account to access our MCAT Flashcards SIGN UP NOW
MCAT is a registered trademark of the Association of American Medical Colleges (AAMC), which is not affiliated with Blueprint.