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The combustion of ethanol is represented by the equation: C2H5OH(l) + 3O2(g) -> 2CO2(g) + 3H2O(l), with enthalpy change ΔH = -1366.5 kJ/mol. Calculate the change in internal energy (ΔU) for this reaction at 27°C.

1. -1371.5 kJ
2. -1369.0 kJ
3. -1364.0 kJ
4. -1361.5 kJ

Correct answer: -1364.0 kJ

Solution

With Δng = -1 and T = 300 K, ΔU = -1366.5 - (-1)(8.314 * 10⁻³)(300) = -1366.5 + 2.494 = -1364.0 kJ. The positive correction reduces the magnitude because fewer moles of gas are produced than consumed.

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