Naphthalene, C₁₀H₈, is an aromatic organic compound that is a solid at room temperature and pressure. The enthalpy change of fusion, ∆Hfusion, of naphthalene is the enthalpy change that occurs when naphthalene melts. It cannot be directly measured. C₁₀H₈(s) ⇌ C₁₀H₈(l) The melting point of naphthalene depends on its purity. When other compounds dissolve in liquid naphthalene its melting point is lowered. Diphenylamine, (C₆H₅)₂NH, dissolves in liquid naphthalene. By varying the amount of diphenylamine and measuring the melting points of the different mixtures, the enthalpy change of fusion of naphthalene can be calculated. (a) The equation that links the melting point of a naphthalene-diphenylamine mixture with its enthalpy change of fusion, ∆Hfusion, is shown. log Y = A - ∆Hfusion / (2.30 x RTm) Y = mole fraction of naphthalene R = molar gas constant, 8.31 J K⁻¹ mol⁻¹ Tₘ = melting point of naphthalene in K A is a constant