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MCAT Chemistry Question – Combustion of Octane

by John
Jan 07, 2017
MCAT Question of the Day

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.

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