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Consider the following reaction occurring in an automobile: 2C8H18(g) + 25O2(g) → 16CO2 + 18H2O(g) The sign of ΔH, ΔS and ΔG would be:

1. +, −, +
2. −, +, −
3. +, +, −
4. −, +, +

Correct answer: −, +, −

Solution

The combustion of octane is an exothermic reaction, so ΔH is negative. The reaction produces more gaseous molecules (CO2 and H2O) than it consumes, increasing randomness, so ΔS is positive. Since the reaction is spontaneous, ΔG is negative.

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