(2)Quantityof heatat constantpressure,Qp:P△Vsurroundings = P, = P2, W=P,surroundings福△U=Q,-P·△VQp=△U + P·△V=U,-U, +P(V,-V)= (U, +PV,) -(U,+PV)Supposing H =U +PV, which is called enthalpy(烩). Thereare some explanations on enthalpyIt has no clear physical meaning,justfor convenience②Its absolute valuecan not be obtained.Foridealgas,itsenthalpyobeyH=f(T)16
16 （2） Quantity of heat at constant pressure, Qp： Psurroundings = P1 = P2，W = Psurroundings·△V △U = Qp－ P·△V Qp =△U + P·△V = U2－U1 + P (V2－V1 ) = （U2 + PV2）－(U1 + PV1 ) Supposing H ≡ U + PV，which is called enthalpy(焓). There are some explanations on enthalpy. ① It has no clear physical meaning, just for convenience ② Its absolute value can not be obtained. ③ For ideal gas,its enthalpy obey H = f (T)
H= U+PVT一定， △U=0PV= constant, △(PV) = 0: △H= △U + △(PV)= 0H=f(T)Q,=H, -H,=△HIt means that at constant pressure, the heat that thesystem absorbed is totally used to increaseitsenthalpy.17
17 H = U +PV T一定，△U = 0 PV = constant，△(PV) = 0 ∴ △H = △U + △(PV) = 0 H = f (T) Qp =H2 -H1 =△H It means that at constant pressure, the heat that the system absorbed is totally used to increase its enthalpy
(3)RelationbetweenQ,andQ,:constantpressure,△H,,△U,→productsIn,、P,、V,、Treactants n,PV,TIAH, AU②constantvolume,△H,△U,→productsIIn2、P2、ViT:AH, =AH+AH= △U, + △(PV), + △H= △U, + △(PV)2=△U,+P·△V+△P.V: △H, = △U, + △PVH, = U, + n(products-ractant) RT·RT:. Q,=Q, + n(products-reactants)18
18 （3）Relation between Qp and Qv : ①constant pressure，△H1，△U1 → productsⅠn2、P1、V2、T reactants n1、P1、V1、T ③ ↑ △H 3，△U3 ②constant volume，△H2，△U2 → productsⅡn2、P2、V1、T ∵△H1 = △H2 + △H3 = △U2 + △(PV)2 + △H3 = △U2 + △(PV)2 = △U2 + P·△V + △P·V ∴ △H1 = △U2 + △P·V △H1 = △U2 + △n(products－reactants)·RT ∴ Qp = Qv + △n(products－reactants)·RT
1-3-2 Reaction rate of progressthemolarandenthalpy change of reaction(1)Reactionrate of progress,YAA+B→GG+YEFABn1Define:OO.hnnnNGnh-nBBFA≥0YFYABGunit:mol19
19 1-3-2 Reaction rate of progress and the molar enthalpy change of reaction （1）Reaction rate of progress,ξ： γA A + γB B → γG G + γF F t = 0 nº A nº B nº G nº F t nA nB nG nF Define： unit：mol 0 − = − = − = − = F F F G G G B B B A nA nA n n n n n n
Example : Put 10mol N (g)and 20mol H2(g) together, when2mol NH(g) were produced, calculate the reaction rate ofprogress according to the following two equations(a)N2(g) + 3H(g) → 2NH3 (g)(b)1/2 N,(g) + 3/2 H(g) → NH(g)Solution:(a) N2(g) + 3H2(g) → 2NH, (g)(b) 1/2 N2(g) + 3/2 H2(g) → NHs(g)0010201020t=092917217tsa = (2一0) / 2 =1(mol)b= (2一0)/ 1= 2 (mol)20
20 Example ：Put 10mol N2 (g) and 20mol H2 (g) together，when 2mol NH3 (g) were produced, calculate the reaction rate of progress according to the following two equations. (a) N2 (g) + 3H2 (g) → 2NH3 (g) (b) 1/2 N2 (g) + 3/2 H2 (g) → NH3 (g) Solution： (a) N2 (g) + 3H2 (g) → 2NH3 (g) (b) 1/2 N2 (g) + 3/2 H2 (g) → NH3 (g) t = 0 10 20 0 10 20 0 t 9 17 2 9 17 2 ξa = (2－0)/ 2 = 1(mol) ξb = (2－0)/ 1 = 2 (mol)