热力学第一定律 热化学( ThermoChemistry) 1.吸热反应&放热反应 2标准态:指定温度(通常是29815K),一个标准大气压 标准焓变ΔH°变化前后物质都处于标准态 3.热化学方程式:注明状态 标准摩尔焓变AHn;标准反应焓变ΔrH° CH4g)+2o2(9) Co2 (g)+2H2 O() 4, Ho=-890 kJ mol △rH°=2△Hm°(H2O)+△Hn(CO2)-△Hn。(CH4)-2△Hn(O2)
热化学 (ThermoChemistry) 1. 吸热反应 & 放热反应 2. 标准态: 指定温度(通常是298.15K), 一个标准大气压. 标准焓变 H: 变化前后物质都处于标准态. 3. 热化学方程式: 注明状态 标准摩尔焓变 Hm ; 标准反应焓变rH rH = 2 Hm (H2O) + Hm (CO2 ) - Hm (CH4 ) - 2 Hm (O2 ) 热力学第一定律
热力学第一定律 盖斯定律( Hess law) The standard enthalpy of an overall reaction is the sum of the standard enthalpies of the individual reactions into which the overall reaction may be divided -Individual steps may not be"real" reactions, but must balance △/kJmo C3H6(g)+H2(g) C3Ha(g) 124 C3H8(g)+502(g)3CO2(g)+4H2O() 2220 H2O() H2(g)+O2(g) +286 C3H6(g)+(9/2)O2(g) 3cO2(g)+3H2O() 2058
盖斯定律 (Hess’ Law) 热力学第一定律
热力学第一定律 标准摩尔生成焓 standard enthalpy of formation,△HP° 反应焓:标准态的稳定单质→标准态的某物质 6C(s, graphite)+ 3H2 - ChOo elements △H=+49.0kJmo ∑w1H°-∑ reactants products reactants △H products
标准摩尔生成焓 反应焓: 标准态的稳定单质 →标准态的某物质 热力学第一定律
热力学第一定律 Group Contributions Sometimes it is difficult to exactly thermodynamically break enthalpies of formation down into contributions from individual atoms and bonds mean bond enthalpies,△H(A-B)平均键焓 enthalpy change associated with breaking of a specific bond, A-B A-B(g) A(g)+ b(g) △H(A-B) This is an unreliable method, since AH(A-B )are average values for series of unrelated compounds(different geometries, isomers, etc. thermochemical groups atom or physical group of atoms bound to at least two other atoms enthalpy of formation is associated with sum of contributions associated with all of the thermochemical groups into which the molecule can be divided-sometimes called Benson thermochemical groups
平均键焓 热力学第一定律
热力学第一定律 Benson Group Contributions Group △HP/kJmo1Cpm°JK1mo C(H)2C)-42.17 259 C(H)2C)2-20.7 22.8 C(H)C3 -6.19 18.7 C(C) +8.16 18.2 Example C(C)4 C(H)2C)2 Estimate standard enthalpy of formation for hexane(g& I)at 298 K Decomposition: two C(H)3(C) groups and four C(H)2(C)2 groups △H(C。H1429) 2(-42.17)+4(-20.7 =-167.1kJmo1 △a(CH14)=289kJmo1 △H(CAH 167.1-289 196.0 kJ mol- Experimental:-1987 kJ mol
热力学第一定律