2.2.4 Enthalpy change and Thermochemical reaction equationa.thermal effect during isochoric processes(Qv)a closed system without non volumetric workV=0,W=0,U=Qv The physical meaning is that the heat absorbed by a isochoric process is equal to the U value in a closed system without non volume work.1.enthalpy&enthalpy changes It is numerically equal to the heat(Qv)value of absorption or releasing in thermodynamics for a closed system without non volume work.A state function(U)of thermodynamics is only related to the initial and final states,in which,the constant volume heat(Qv)is only related to the process,but doesnt matter with the approaches.QV U b thermal effect during isobaric processes(QP)p2=p1=pexternal=pBased on the first law of thermodynamics:U=Q+WThe external work W of the system:W=-PV=-p（V2 V1）U=QP+W=QP p（V2 V1）U2 U1=QP p（V2 V1）QP=（U2+p2V2）-（U1+p1V1）a closed system without non volumetric workThe physical meaning is that the heat absorbed by a isobaric process is equal to the H value in a closed system without non volume work.In which,the U,P,and V belong to the state functions of the system,(U+PV)also belongs to a state function.(U+PV)is defined as an enthalpy in thermodynamics,using H to represent,H UpVSo,Qp H2H1 H(enthalpy change）Noted:H 0 for endothermic reaction,and vice versa,H 0 for endothermic reactionH 0 for exothermic reactionRelationship between Qp and QV(I)isobaric process H1Product I T1 p2V1 n2Product T1 p1V2 n2ReactantT1 p1V1 n1(II)Isochoric process H2 H3(III)H1 H2 H3 H1 U2 (p2V1p1V1)U3(p1V2p2V1)H1 U2 (p1V2p1V1)U3