厦门大学：《物理化学 Physical Chemistry》课程电子教案（PPT教学课件，英文版）chapter02 The First Law of Thermodynamics

Physical Chemistry Chapter 2 ≡: Enthalpy H≡U+Pp (245) For any change of state, the enthalpy change AH △H=H2-H1=(2+P2V2)-(1+P1)=△U+△(P) △H≡AU+P△ at constant p 248) Uand v are extensive, h is extensive The molar enthalpy of a pure substance 、HU+PV U+Py 17

17 For any change of state, the enthalpy change H m Um PVm n U PV n H H = + + = = U and V are extensive, H is extensive. H U + PV (2.45)* ( ) ( ) ( ) H = H2 − H1 = U2 + P2 V2 − U1 + P1 V1 = U + PV (2.47) H  U + PV at constant P (2.48) The molar enthalpy of a pure substance Enthalpy Physical Chemistry Chapter 2

Physical Chemistry Chapter 2 ≡: Enthalpy et qy be the heat adsorbed in a constant-volume process in a closed system, if it can do only p-V work, then dw=- pdV=0 Since dv=0 Then dw=0 SO w=0 From the first law AU=g+w AU=qr(closed syst, P-V work only, V const )(2.49 18

18 U = qV (closed syst., P-V work only, V const.) U = q + w Let qV be the heat adsorbed in a constant-volume process in a closed system, if it can do only P-V work, then dw = - PdV = 0 Since dV = 0 Then dw = 0 So w = 0 From the first law (2.49) Enthalpy Physical Chemistry Chapter 2

Physical Chemistry Chapter 2 ≡: Enthalpy △H≡△U+P△ at constant p(248) For a reaction involving a perfect gas △H≡△U+△RT Example Caco(s)>Cao(s)+CO,(8) An=+l(1 mole of gaseous CO, is created) △H≡△U+△mRT=△U+RT=△U+248k/morl at 298 K 19

19 H  U +nRT For a reaction involving a perfect gas ( ) ( ) ( ) CaCO3 s →CaO s +CO2 g n = +1 (1 mole of gaseous CO2 is created) 1 2.48 − H  U + nRT = U + RT = U + kJmol Example at 298 K H  U + PV at constant P (2.48) Enthalpy Physical Chemistry Chapter 2

Physical Chemistry Chapter 2 Exothermic and endothermic .Heat change in system Process Value of 4H Heat loss Exothermic Negative (heat lost to the (4H<0) surroundings) Heat gain Endothermic Positive (heat gained from the (41H>0 surroundings) The sign of enthalpy change indicates the direction of heat flow 20

20 Exothermic and Endothermic Heat change in system Process Value of H Heat loss (heat lost to the surroundings) Heat gain (heat gained from the surroundings) Exothermic Endothermic Negative (H < 0) Positive (H > 0) The sign of enthalpy change indicates the direction of heat flow Physical Chemistry Chapter 2

Physical Chemistry Chapter 2 s. Heat Capacities pr (250) heat capacity at constant pressure c (isobaric heat capacity) (21)* heat capacity at constant volume Cy (isochoric heat capacity) (252) 21

21 Heat Capacities heat capacity at constant volume CV (isochoric heat capacity) heat capacity at constant pressure CP (isobaric heat capacity)          dT dq C pr pr (2.50)*        dT dq C V V (2.52)*        dT dq C P P (2.51)* Physical Chemistry Chapter 2