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Five moles of each of the following ideal gases are heated from 300 K to 301 K. In which case is the enthalpy change the greatest?

1. O2(g)
2. XeF4(g)
3. CO2(g)
4. Atomic chlorine

Correct answer: XeF4(g)

Solution

Enthalpy change = n*Cp*delta_T. Atomic Cl (monatomic): Cp = 5/2 R. O2 (diatomic linear): Cp = 7/2 R. CO2 (triatomic linear): Cp = 7/2 R. XeF4 (pentaatomic, square planar, non-linear): Cp = 4R (for non-linear polyatomic, Cp = 4R using classical equipartition). XeF4 has the highest Cp, so the enthalpy change is greatest for XeF4.

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