Contact, information, consultations

Transcription

1 ontact, nfomaton, consultatons hemsty A Bldg; oom 07 phone: cellula: E-mal: wojtek_c@pg.gda.pl Offce hous: Fday, 9-0 a.m. A quote of the week (o camel of the week): hee s no expedence to whch a man wll not go to avod the labo of thnkng. homas A. Edson Moe about enthalpy hange n enthalpy of a system, whee no chemcal eacton occus (only physcal changes, lke heatng o coolng, expanson o compesson) s gven by: Hence: dh n; dh dh dq Enthalpy can change n any type of pocess, whlst only n sobac pocesses t s equal to the heat exchanged. 3

2 I law of themodynamcs H. Helmholtz (847) If n an solated system cetan knd of enegy dsappeas, then at the same tme, some othe knd of enegy must appea n exactly equvalent amount. It s mpossble to buld the pepetuum moble of the fst type. otal enegy of an solated system s constant 4 Intenal enegy In closed systems a state functon exsts, whose ncement at passng fom the ntal to fnal state s equal to enegy exchanged wth the suoundngs dung ths passage. otal enegy of a system. he sum of all knds of enegy contbutng to ts oveall enegy. Symbol u o U du dq dw; u q w q and w ae NO state functons (no Δ sgn at the symbols) 5 Intenal enegy () When heat s exchanged at constant volume (n an sovolumc o sochoc pocess), ths heat s equal to the ntenal enegy of the system: Hence: du n ; du du dq Intenal enegy can change n any type of pocess, whlst only n sovolumc pocesses t s equal to the heat exchanged. 6

3 State functons () Mathematcal consequence of the fact that cetan quantty s a state quantty s that ths quantty must be a complete dffeental of the state paametes. u=f(,) u=f(,) u=f(,) du du du d d du du du d d du du du d d d d 7 Intenal enegy n sochoc pocesses Mola heat capacty at constant volume, a b c... (fo computatonal puposes at cuent level we wll assume ndependence of tempeatue, whch s smplfcaton) 3 8 Intenal enegy of sobac pocesses Izobac (=const.) dw=-d w=-( f - )=- then: u=q - u f -u =q -( f - ) q =(u f + f )-(u + ) h=u+ 9 3

4 Enthalpy once moe q p =h = h f -h lub Q p =H = H f -H dh du d d whee (du/d) s known as ntenal pessue (molecula nteactons ncease n U wth ) 0 Mola heat capactes Fo the pefect gas: p - =(d/) because =R/, then (d/) =R/ and p - =R heat was not exchanged Joule s expement tempeatue has not changed gas dd not pefom any wok (expanson to vacuum) ntenal enegy of a gas does not depend on ts volume at = const. Joule s expement - conclusons du 0 du du du du d d 0 d d theefoe: du d d du and: 0 d QED du d d u 4

5 q 0 At the same tme: R d fnally: Revesble adabatc pocess du dw d And because - =R du n nr nr then: n d afte ntegaton: f f f ln R ln f f ln ( )ln ln f f and: f f const 3 and We wll dscuss t late n detals, but fo the tme beng, let s assume that fo the pefect (deal) gas: gas molecule = +R = / monoatomc 3 / R 5 / R 5 / 3 =.67 datomc 5 / R 7 / R 7 / 5 =.40 4 olytopc pocesses Geneal fom of equaton: x const pocesses, dependng on the value of x. descbes dffeent value of x pocess descbed 0 sobac sothemal κ adabatc sochoc 5 5

6 olytopc pocesses on a - plane 6 Back to themochemsty. H and U of a eacton Fst pncple of themodynamcs says: Fo an sobac pocess: heefoe: q H U H U q w w he same must be tue, when a chemcal eacton occus n the system U H H ng R Because: ng R n g ng, p ng, e Subscpt g n n g and Δ n g means that what counts ae gas phase (gaseous) poducts and eactants. 7 Kchoff s law How to calculate heat of eacton at dffeent tempeatue? Reactants at H, oduct at H heat. eact. > H cool. pod. Reactants at H, oducts at 8 6

7 Kchoff s law () Enthalpy s a state functon, hence: H H H H H H heat. eact. cool. pod. H H H heat H. eact. cool. pod., e, p H, p, e H H ( ) 9 Kchoff s law (3) H H ( ) p = n - n,, p,, e Fo a eacton expessed as: aa + bb +... = ll + mm +... H, ll mm... aa bb... 0 Kchoff s law (4) When: the geneal fomula: may be smplfed to: and fnally to: p f ( ); H H ( ) H const H H H Kchoff s law may be appled to ntenal enegy of eactons, ΔU, (they ae state functons, too) usng nstead of. 7