Chemistry

Chemistry

Problem

Naphthalene has a molar enthalpy of sublimation of 72  kJ mol−172\;\text{kJ mol}^{-1} at its sublimation temperature. What is the heat change, including sign, when 0.50  mol0.50\;\text{mol} of gaseous naphthalene deposits as a solid under these conditions? a) − ⁣72  kJ-\!72\;\text{kJ} b) + ⁣36  kJ+\!36\;\text{kJ} c) − ⁣36  kJ-\!36\;\text{kJ} d) − ⁣144  kJ-\!144\;\text{kJ}
Intuitive explanation

Deposition is the reverse of sublimation—a phase change from gas to solid that releases energy. Since sublimation is endothermic (+72  kJ mol−1+72\;\text{kJ mol}^{-1}), deposition must be exothermic with the opposite sign. You just need to scale that molar enthalpy by the number of moles and flip the sign to reflect the direction of the process.

Intuitive explanation

Deposition is the reverse of sublimation—a phase change from gas to solid that releases energy. Since sublimation is endothermic (+72  kJ mol−1+72\;\text{kJ mol}^{-1}), deposition must be exothermic with the opposite sign. You just need to scale that molar enthalpy by the number of moles and flip the sign to reflect the direction of the process.

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