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Enthalpies of melting and boiling for pure elements versus temperatures of transition, demonstrating Trouton's rule. In thermodynamics, the enthalpy of fusion of a substance, also known as (latent) heat of fusion, is the change in its enthalpy resulting from providing energy, typically heat, to a specific quantity of the substance to change its state from a solid to a liquid, at constant pressure.
Latent heat is energy released or absorbed by a body or a thermodynamic system during a constant-temperature process. Two common forms of latent heat are latent heat of fusion and latent heat of vaporization . These names describe the direction of energy flow when changing from one phase to the next: from solid to liquid, and liquid to gas.
J.A. Dean (ed), Lange's Handbook of Chemistry (15th Edition), McGraw-Hill, 1999; Section 6, Thermodynamic Properties; Table 6.4, Heats of Fusion, Vaporization, and Sublimation and Specific Heat at Various Temperatures of the Elements and Inorganic Compounds
L = latent heat of fusion (in [J·kg −1]), k = thermal conductivity of the mold (in [W·m −1 ·K −1)]), ρ = density of the mold (in [kg·m −3]), c = specific heat of the mold (in [J·kg −1 ·K −1]), ρ m = density of the metal (in [kg·m −3]), c m = specific heat of the metal (in [J·kg −1 ·K −1]).
In some texts, the heats of phase transitions are called latent heats (for example, latent heat of fusion). Molar enthalpy of zinc above 298.15 K and at 1 atm pressure, showing discontinuities at the melting and boiling points. The ΔH°m of zinc is 7323 J/mol, and the ΔH°v is 115 330 J/mol.
Here, if T = 400 K, the right hand side of the equation equals 10.5R, and we find the original formulation for Trouton's rule. ... "On Molecular Latent Heat".
where ΔH fus is the heat of fusion which is always positive, and ΔV fus is the volume change for fusion. For most substances Δ V fus is positive so that the slope is positive. However for water and other exceptions, Δ V fus is negative so that the slope is negative.
Substituting into the Clapeyron equation =, we can obtain the Clausius–Clapeyron equation [8]: 509 = for low temperatures and pressures, [8]: 509 where is the specific latent heat of the substance. Instead of the specific, corresponding molar values (i.e. L {\displaystyle L} in kJ/mol and R = 8.31 J/(mol⋅K)) may also be used.