Return to MCAT® Blog Homepage

MCAT Chemistry Question – Combustion of Octane

by Allison Chae
Jan 07, 2017
MCAT Question of the Day
Reviewed By: Liz Flagge

The combustion of octane is a common reaction in automobile engines.

1) 2 C₈H₁₈(l) + 25 O₂ (g) → 16 CO₂ (g) + 18 H₂O (g) ΔH₁ = -250 kJ/mole

2) 18 H₂O (l) → 18 H₂O (g) ΔH₂ = 88 kJ/mole

Net Reaction:

3) 2 C₈H₁₈(l) + 25 O₂ (g) → 16 CO₂ (g) + 18 H₂O (l) ΔH₃ = ?

Given ΔH₁and ΔH2, what is ΔH₃ for the net reaction above?

A. 162 kJ/mole

B. -162 kJ/mole

C. 338 kJ/mole

D. -338 kJ/mole

Click for Explanation

This question asks you to determine the change in enthalpy for a net reaction given the enthalpy change for two other reactions. Hess’s Law tells us that the sum of two reactions gives us the enthalpy change for the net reaction: ΔH₁ + ΔH₂= ΔH₃. Since the net reaction has H₂O (l) as a product, reaction 2 must be reversed and the sign of the enthalpy change negated (-ΔH₂). The sum of the reaction enthalpies for the net reaction is as follows:

ΔH₃= ΔH₁ + (-ΔH₂)

ΔH₃= -250 kJ/mole + (-88 kJ/mole)

ΔH₃= -338 kJ/mole

Therefore, the change in enthalpy (ΔH₃) for the net reaction above is -338 kJ/mole, or answer choice D. Furthermore, the combustion of a fuel is a highly exothermic process, making answer choices A and C incorrect.

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.

Schedule My Free Consultation!

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.

Search the Blog

Free Consultation

Interested in our Online MCAT Course, One-on-One MCAT Tutoring or Med admissions packages? Set up a free consultation with one of our experienced Senior Student Advisors.

Schedule Now

Free MCAT Practice Account

Need great MCAT practice?

Get the most representative MCAT practice possible when you sign up for our free MCAT Account, which includes a half-length diagnostic exam and one of our full-length MCAT practice exams.

Learn More