Enthalpy of vaporization - wikipedia Enthalpy of vaporization Temperature - dependency of the heats of vaporization for water, methanol, benzene, and acetone. The enthalpy of vaporization, ( symbol ∆ H ) also known as the ( latent ) heat of vaporization or heat of evaporation, is the amount of energy ( enthalpy ) that must be added to a liquid substance, to transform a quantity of that substance into a gas. The enthalpy of vaporization is a function of the pressure at which that transformation takes place. The enthalpy of vaporization is often quoted for the normal boiling temperature of the substance ; although tabulated values are usually corrected to 298 K, that correction is often smaller than the uncertainty in the measured value. The heat of vaporization is temperature - dependent, though a constant heat of vaporization can be assumed for small temperature ranges and for reduced temperature T r ( displaystyle T_ ( r ) ) ≪ 1 ( displaystyle ll 1 ). The heat of vaporization diminishes with increasing temperature and it vanishes completely at a certain point called the critical temperature ( T r = 1 ( displaystyle T_ ( r ) = 1 ) ). Above the critical temperature, the liquid and vapor phases are indistinguishable, and the substance is called a supercritical fluid. Contents 1 Units 2 Enthalpy of condensation 3 Thermodynamic background 4 Vaporization enthalpy of electrolyte solutions 5 Selected values 5.1 Elements 5.2 Other common substances 6 See also 7 References Units Values are usually quoted in J / mol or kJ / mol ( molar enthalpy of vaporization ), although kJ / kg or J / g ( specific heat of vaporization ), and older units like kcal / mol, cal / g and Btu / lb are sometimes still used, among others. Enthalpy of condensation The enthalpy of condensation ( or heat of condensation ) is by definition equal to the enthalpy of vaporization with the opposite sign : enthalpy changes of vaporization are always positive ( heat is absorbed by the substance ), whereas enthalpy changes of condensation are always negative ( heat is released by the substance ). Thermodynamic background The enthalpy of vaporization can be written as Δ H v a p = Δ U v a p + p Δ V ( displaystyle Delta ( ) H_ ( mathrm ( vap ) ) = Delta ( ) U_ ( mathrm ( vap ) ) + p Delta , V ) It is equal to the increased internal energy of the vapor phase compared with the liquid phase, plus the work done against ambient pressure. The increase in the internal energy can be viewed as the energy required to overcome the intermolecular interactions in the liquid ( or solid, in the case of sublimation ). Hence helium has a particularly low enthalpy of vaporization, 0.0845 kJ / mol, as the van der Waals forces between helium atoms are particularly weak. On the other hand, the molecules in liquid water are held together by relatively strong hydrogen bonds, and its enthalpy of vaporization, 40.65 kJ / mol, is more than five times the energy required to heat the same quantity of water from 0 ° C to 100 ° C ( c = 75.3 J K mol ). Care must be taken, however, when using enthalpies of vaporization to measure the strength of intermolecular forces, as these forces may persist to an extent in the gas phase ( as is the case with hydrogen fluoride ), and so the calculated value of the bond strength will be too low. This is particularly true of metals, which often form covalently bonded molecules in the gas phase : in these cases, the enthalpy of atomization must be used to obtain a true value of the bond energy. An alternative description is to view the enthalpy of condensation as the heat which must be released to the surroundings to compensate for the drop in entropy when a gas condenses to a liquid. As the liquid and gas are in equilibrium at the boiling point ( T ), Δ G = 0, which leads to : Δ v S = S g a s − S l i q u i d = Δ v H / T b ( displaystyle Delta , _ ( v ) S = S_ ( gas ) - S_ ( liquid ) = Delta , _ ( v ) H / T_ ( b ) ) As neither entropy nor enthalpy vary greatly with temperature, it is normal to use the tabulated standard values without any correction for the difference in temperature from 298 K. A correction must be made if the pressure is different from 100 kPa, as the entropy of a gas is proportional to its pressure ( or, more precisely, to its fugacity ) : the entropies of liquids vary little with pressure, as the compressibility of a liquid is small. These two definitions are equivalent : the boiling point is the temperature at which the increased entropy of the gas phase overcomes the intermolecular forces. As a given quantity of matter always has a higher entropy in the gas phase than in a condensed phase ( Δ v S ( displaystyle Delta , _ ( v ) S ) is always positive ), and from Δ G = Δ H − T Δ S ( displaystyle Delta , G = Delta , H-T Delta , S ), the Gibbs free energy change falls with increasing temperature : gases are favored at higher temperatures, as is observed in practice. Vaporization enthalpy of Electrolyte Solutions Estimation of the enthalpy of vaporization of electrolyte solutions can be simply carried out using equations based on the chemical thermodynamic models, such as Pitzer model or TCPC model. Selected values Elements Enthalpies of vaporization of the elements 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Group → ↓ Period H0.44936 He 0.0845 Li 145.92 Be 292.40 B489.7 C355.8 N2.7928 O3.4099 F3.2698 Ne 1.7326 Na 96.96 Mg 127.4 Al 293.4 Si 300 P12.129 S1.7175 Cl 10.2 Ar 6.447 K79.87 Ca 153.6 Sc 314.2 Ti 421 V452 Cr 344.3 Mn 226 Fe 349.6 Co 376.5 Ni 370.4 Cu 300.3 Zn 115.3 Ga 258.7 Ge 330.9 As 34.76 Se 26.3 Br 15.438 Kr 9.029 5 Rb 72.216 Sr 144 Y363 Zr 581.6 Nb 696.6 Mo 598 Tc 660 Ru 595 Rh 493 Pd 357 Ag 250.58 Cd 100 In 231.5 Sn 295.8 Sb 77.14 Te 52.55 I20.752 Xe 12.636 6 Cs 67.74 Ba 142 Hf 575 Ta 743 W824 Re 715 Os 627.6 Ir 604 Pt 510 Au 334.4 Hg 59.229 Tl 164.1 Pb 177.7 Bi 104.8 Po 60.1 At 27.2 Rn 16.4 7 Fr n / a Ra 37 Rf n / a Db n / a Sg n / a Bh n / a Hs n / a Mt n / a Ds n / a Rg n / a Cn n / a Nh n / a Fl n / a Mc n / a Lv n / a Ts n / a Og n / a La 414 Ce 414 Pr n / a Nd n / a Pm n / a Sm n / a Eu n / a Gd n / a Tb n / a Dy n / a Ho n / a Er n / a Tm n / a Yb n / a Lu n / a Ac n / a Th 514.4 Pa n / a Un / a Np n / a Pu n / a Am n / a Cm n / a Bk n / a Cf n / a Es n / a Fm n / a Md n / a No n / a Lr n / a Enthalpy in kJ / mol, measured at their respective normal boiling points 0 -- 10 kJ / mol 10 -- 100 kJ / mol 100 -- 300 kJ / mol > 300 kJ / mol The vaporization of metals is a key step in metal vapor synthesis, which exploits the increased reactivity of metal atoms or small particles relative to the bulk elements. Other common substances Enthalpies of vaporization of common substances, measured at their respective standard boiling points : Compound Boiling Point at normal pressure Heat of vaporization ( J mol ) Heat of vaporization ( J g ) Acetone 329 K, 56 ° C, 133 ° F 31300 538.9 Aluminium 2792 K, 2519 ° C, 4566 ° F 294000 10500 Ammonia 240 K, − 33.34 ° C, − 28 ° F 23350 1371 Butane 272 -- 274 K, − 1 ° C, 30 -- 34 ° F 21000 320 Diethyl ether 307.8 K, 34.6 ° C, 94.3 ° F 26170 353.1 Ethanol 352 K, 78.37 ° C, 173 ° F 38600 841 Hydrogen 20.271 K, - 252.879 ° C, - 423.182 ° F 460 451.9 Iron 3134 K, 2862 ° C, 5182 ° F 340000 6090 Isopropyl alcohol 356 K, 82.6 ° C, 181 ° F 44000 732.2 Methane 112 K, − 161 ° C, − 259 ° F 8170 480.6 Methanol 338 K, 64.7 ° C, 148 ° F 35200 1104 Propane 231 K, - 42 ° C, − 44 ° F 15700 356 Phosphine 185 K, − 87.7 ° C, − 126 ° F 14600 429.4 Water 373.15 K, 100 ° C, 212 ° F 40660 2257 See also Enthalpy of fusion ( Specific heat of melting ) Enthalpy of sublimation Joback method ( Estimation of the heat of vaporization at the normal boiling point from molecular structures ) References This article has an unclear citation style. The references used may be made clearer with a different or consistent style of citation and footnoting. ( March 2016 ) Jump up ^ Ge, Xinlei ; Wang, Xidong ( 20 May 2009 ). `` Estimation of Freezing Point Depression, Boiling Point Elevation, and Vaporization Enthalpies of Electrolyte Solutions ''. Industrial & Engineering Chemistry Research. 48 ( 10 ) : 5123 -- 5123. doi : 10.1021 / ie900434h. Jump up ^ Ge, Xinlei ; Wang, Xidong ( 2009 ). `` Calculations of Freezing Point Depression, Boiling Point Elevation, Vapor Pressure and Enthalpies of Vaporization of Electrolyte Solutions by a Modified Three - Characteristic Parameter Correlation Model ''. Journal of Solution Chemistry. 38 ( 9 ) : 1097 -- 1117. doi : 10.1007 / s10953 - 009 - 9433 - 0. ISSN 0095 - 9782. Jump up ^ NIST CODATA Key Values for Thermodynamics Gmelin, Leopold ( 1985 ). Gmelin - Handbuch der anorganischen Chemie / 08 a ( 8., völlig neu bearb. Aufl. ed. ). Berlin ( u.a. ) : Springer. pp. 116 -- 117. ISBN 3 - 540 - 93516 - 9. NIST Chemistry WebBook Young, Francis W. Sears, Mark W. Zemansky, Hugh D. ( 1982 ). University physics ( 6th ed. ). Reading, Mass. : Addison - Wesley. ISBN 0 - 201 - 07199 - 1. 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Question: latent heat of vaporization for water in j/kg
Answer: 40.65 kJ/mol