School of Chemical & Biological Engineering, Konkuk University

Transcription

1 Schl f Cheical & Bilgical Engineeing, Knkuk Univesity

2 Lectue 7 Ch. 2 The Fist Law Thecheisty Pf. Y-Sep Min Physical Cheisty I, Sping 2008 Ch. 2-2

3 The study f the enegy tansfeed as heat duing the cuse f cheical eactins is called thecheisty. The syste in the thecheisty is a eactin vessel and its cntents. Caliety is used t easue the enegy supplied discaded as heat by a eactin. U q V q p at cnst V, n additinal wk at cnst p, n additinal wk Extheic pcess: the eleased enegy heats the suundings. < 0 at cnst P. Endtheic pcess: the absbed enegy cls the suundings. > 0 at cnst P. Pf. Y-Sep Min Physical Cheisty I, Sping 2008 Ch. 2-3

4 Standad enthalpy change ( ): the change in enthalpy f a pcess in which the initial and final substances ae in thei standad states. The standad state f a substance at a specified tepeatue is its pue f at 1 ba. Ex) the standad state f liquid ethanl at 298 K is pue liquid ethanl at 298 K and 1 ba. Standad enthalpy f vapizatin ( vap ): the enthalpy change pe le when a pue liquid at 1 ba vapizes t a gas at 1 ba. 1 2O (l) 2O (g) vap (373 K) kj l Standad enthalpies ay be epted f any tepeatue but cnventinally at K (25 C). Pf. Y-Sep Min Physical Cheisty I, Sping 2008 Ch. 2-4

5 Standad enthalpy f tansitin ( ts ): the standad enthalpy change accpanying a change f physical state. Standad enthalpy f vapizatin ( vap ): liquid t vap Standad enthalpy f fusin ( fus O (s) ): slid t liquid 1 2 2O (l) fus (273 K) 6.01kJ l F subliatin (slid t vap): Pf. Y-Sep Min Physical Cheisty I, Sping 2008 Ch O (s) O (g) sub? Because enthalpy is a state functin, a change in enthalpy is independent f the path between tw states. sub fus vap (f pcesses ccuing at the sae tepeatue)

6 Because enthalpy is a state functin, the standad enthalpy change f a fwad pcess and its evese diffe in sign. (A B) (B A) Pf. Y-Sep Min Physical Cheisty I, Sping 2008 Ch. 2-6

7 Thee ae tw ways t ept the change in enthalpy accpanying a cheical eactin. One is t wite the thecheical equatin which is a cbinatin f a cheical equatin and the cespnding change in standad enthalpy. C (g) 2 O2(g) CO2(g) 2 2O (l) kj is the change in enthalpy when eactants in thei standad states change t pducts in thei standad states. Except in the case f inic eactins in slutin, the enthalpy changes accpanying ixing and sepaatin ae insignificant in cpaisn with the cntibutin f the eactin itself. Pf. Y-Sep Min Physical Cheisty I, Sping 2008 Ch. 2-7

8 Altenatively, we wite the cheical eactin and then ept the standad eactin enthalpy ( ). C -1 4(g) 2 O2(g) CO2(g) 2 2O (l) 890 kj l F the eactin, 2 A B 3C D the standad eactin enthalpy is 3 (C) (D) 2 (A) (B) whee (J) is the standad la enthalpy f species J at the tepeatue f inteest. In geneal, Pducts Reactants whee is the stichietic cefficients. Pf. Y-Sep Min Physical Cheisty I, Sping 2008 Ch. 2-8

9 Se standad eactin enthalpies have special naes. Standad enthalpy f cbustin ( c ): standad eactin enthalpy f the cplete xidatin f an ganic cpund (C,, O) t CO 2 gas and liquid 2 O. F the ganic cpund cntaining C,, O and N, N 2 gas als included. Pf. Y-Sep Min Physical Cheisty I, Sping 2008 Ch. 2-9

10 The standad enthalpy f an veall eactin is the su f the standad enthalpies f the individual eactins int which a eactin ay be divided. The individual steps need nt be ealizable in pactice: they ay be hypthetical eactins. The thedynaic basis f the ess s law is the pathindependence f. (See Ex 2.5) Pf. Y-Sep Min Physical Cheisty I, Sping 2008 Ch. 2-10

11 Standad enthalpy f fatin ( f ): standad eactin enthalpy f the fatin f the cpund f its eleents in thei efeence states. The efeence state f an eleent is its st stable state at the specified tepeatue and 1 ba. Ex) Liquid benzene at 298 K 6 C (s, gaphite) 3-1 2(g) C66 (l) f 49.0 kj l The f f eleents in thei efeence state ae ze at all tepeatues, because they ae the enthalpies f such null eactins as N 2 (g) N 2 (g). Pf. Y-Sep Min Physical Cheisty I, Sping 2008 Ch. 2-11

12 The Standad enthalpy f fatin f ins in slutin pses a special pble because it is ipssible t pepae a slutin f catins alne f anins alne. The pble is slved by defining ne in, cnventinally +, t have ze standad enthalpy f fatin at all tepeatues: f (, aq) 0 Ex) (aq) B (aq) B (aq) if (B(aq)) 122 kj l -1 f (B, aq) f kj l Pf. Y-Sep Min Physical Cheisty I, Sping 2008 Ch. 2-12

13 In geneal, the standad eactin enthalpy is: Pducts Reactants Cnceptually, we can egad a eactin as pceeding by decpsing the eactants int thei eleents and then fing thse eleents int pduct. Theefe, standad eactin enthalpy is: Pducts f Reactants f (See Illustatin 2.7) Pf. Y-Sep Min Physical Cheisty I, Sping 2008 Ch. 2-13

14 Can we calculate standad enthalpies f fatin f a knwledge f the cheical cnstitutin? Thee is n thedynaically exact way t expess enthalpies f fatin in tes f cntibutins f individual ats and bnds. In the past, appxiate pcedues based n ean bnd enthalpies [ ( A B) ], which is the aveage enthalpy change assciated with the beaking f a specific A-B bnd, have been used. A B(g) A (g) B(g) (A B) weve, this pcedue is ntiusly uneliable, in pat because the ( A B) ae aveage value f a seies f elated cpunds. This appach des nt distinguish between geetical ises. Pf. Y-Sep Min Physical Cheisty I, Sping 2008 Ch. 2-14

15 Nwadays, cpute-aided lecula delling has lagely displaced this e piitive appach. Cecial sftwae packages use the pinciples develped in Chapt. 11. These technique can distinguish between the diffeent cnfatins f the sae lecules Ex) ethylcyclhexane (equatial vesus axial) Calculated cnfatinal enegy diffeence: 5.9 ~ 7.9 kj l -1 Expeiental value: ~7.5 kj l -1 (equatial e stable) weve, gd ageeent between calculated and expeiental values is elatively ae. Pf. Y-Sep Min Physical Cheisty I, Sping 2008 Ch. 2-15

16 If thee is n epted values f standad enthalpies easued at a desied tepeatue, (T) ay be calculated f heat capacities and the eactin enthalpy epted at a tepeatue (aybe 25 C) When a substance is heated f T 1 t T 2, d C p dt ( T2 ) ( T1 ) d CpdT T T 1 2 C p dt (Unde assuing n phase tansitin in the tepeatue ange f inteest) Pf. Y-Sep Min Physical Cheisty I, Sping 2008 Ch. 2-16

17 ( T2 ) ( T1 ) Because this equatin applies t each substance in the eactin, the standad eactin enthalpy changes f t: 2 T ) ( T ) C ( 2 1 T T 1 p dt Kichhff s law T T 1 2 C p dt ( T 1 ) whee C p is the diffeence f the la heat capacities f pducts and eactants as belw: C p Pduct C p, (See Ex 2.6) Reactant C If C p is independent f T, T ) ( T ) ( T T ) ( p, knwn unknwn Pf. Y-Sep Min Physical Cheisty I, Sping 2008 Ch C p

18 Reading: page 57 ~ 67 The 1 st Exa: Ap. 11 (Fi) 19:00 p R 별232 Chapt. 1 & 2 Pf. Y-Sep Min Physical Cheisty I, Sping 2008 Ch. 2-18