CHNG VIIONG HOA HOC(3T)

7.1. i tng nghin cuong hoa hoc (HH): bo mon hoa hoc nghien cu c ch toc o cua phan ng va can bang hoa hoc. HH giup cho viec thiet ke cong nghe hp l hn, lam tang hieu qu cua san xuat ( iu khin cc qt ha hc xy ra vi vn tc mong mun v hn ch cc qt khng c li)

7.2. Mt s khi nim v phn ng ha hc:H s t lng NO, H2, N2O v H2O ln lt l 2, 1, 1, 1. H s t lng ca p: l ch s nguyn t, phn t v ion ca cc cht tham gia tng tc.2NO + H2 N2O + H2O7.2.1. Phn ng n gin v phn ng phc tp- P n gin l p xy ra mt giai on.- P phc tp l p xy ra qua nhiu giai on. - Mi giai on c gi l phn ng s cp. Tp hp cc p s cp xy ra trong p phc tp gi l c ch p.

Bac phan ng: so phan t cac chat phan ng ma nong o quyet nh toc o cua phan ng (TP). Bang tong so mu cua nong o cac chat phan ng trong bieu thc tnh TP viet theo nh luat tac dung khoi lng. 7.2.2. Phn t s v bc phn ngPhn t s ca phn ng: l s cc phn t tham gia vo mt giai on c bn, gia cc phn t ny c tng tc ln nhau to ra phn ng ha hc.

Phn loi ng hc phn ngDa vo phn t sDa vo bc phn ng n phn t Lng phn t Tam phn t Bc 1 Bc 2 Bc 3 Bc khngBc phn ng c th l nguyn dng, phn s hoc s m. P phc tp: bc p c quyt nh bi g chm nht

VD3: 2NO + H2 N2O + H2O co v = k [NO]2 [H2] BP bang 3. BP c xac nh t thc nghiem, ch bang tong he so cac chat phan ng trong phng trnh phan ng oi vi nhng p n gian.VD2: CH3COOH + C2H5OH CH3COOC2H5 + H2O v: v = k[CH3COOH].[C2H5OH] BP bang 2

Gom nhieu giai oan, chuyen qua mot so tieu phan trung gian, ke tiep nhau trc khi ti cac tieu phan san pham. VD5: 2N2O5 4NO2 + O2 gom 2 giai oan: N2O5 N2 O3 + O2 N2O3 + N2O5 4NO2 Nhng p xay ra nh the c goi la p phc tapVD4: H2 + I2 2HI C che phan ng: con ng theo o mot p hoa hoc xay ra. Mot p thng: Co the ch la mot giai oan, chuyen trc tiep t cac tieu phan (nguyen t, phan t, ion) chat phan ng ti cac phan t san pham. Nhng p xay ra nh the c goi la p n gian.

7.2.3. Phan ng ong the va phan ng d thePhan ng ong the la p trong o cac chat tham gia p va san pham p eu cung 1 pha.VD6: p tong hp kh NH3 t khi hydro va kh nit: N2 (k) + 3H2 (k) 2 NH3 (k)Phan ng d the la p trong o cac chat tham gia p va san pham p 1 vai pha khac nhau.VD7: p hoa tan kem trong dd axit HCl la p d the v no xay ra trong he gom 3 pha khac nhau la ran (Zn), dd (HCl, ZnCl2) va kh (H2).Zn (r) + 2HCl (l) ZnCl2 (l) + H2 (k)

Toc o phan ng hoa hoc la ai lng ac trng cho dien bien nhanh hay cham cua mot phan ng hoa hoc. TP thng c xac nh bang o thay oi nong o (mol/lt) cua chat tham gia p trong mot n v thi gian (phut, giay). /v p: aA + bB cC + dD7.2.4. Toc o phan ng hoa hoc TP trung bnh (tnh trong khong thi gian no ):

Tng qut: Tnh theo sn phm: nu nng cht C hay D thi im t1 l C1, thi im t2 l C2 th TP trung bnh tnh trong khong thi gian ny l: Tnh theo cht p: nu nng cht A hay B thi im t1 l C1, thi im t2 l C2 th TP trung bnh tnh trong khong thi gian ny l:(du cht p; du + sn phm p)

TP tc thiCac yeu to chnh anh hng quan trong /v TP la: nong o, nhiet o , xuc tac.T trung bnh:T tc thi: /v p TQ: aA + bB cC + dD

7.3.1. Anh hng cua nong o chat phan ng:Trong he ong the, tai nhiet o xac nh, TP moi thi iem t le thuan vi tch so nong o cac tac chat vi so mu bang he so hp thc cua cac chat phan ng Noi dung cua nh luat the hien trong bieu thc toc o p: aA + bB = cC + dD :(7.1) a, nh luat tac dung khoi lng (Guldberg & Waage)V = k. CAm.CBn /v p TQ:7.3. Mt s yu t nh hng n tc p

v: toc o tc thi thi iem nhat nh; C: nong o chat p thi iem o; k: he so ty le va c goi la hang so toc o p; m, n: bac ring chat p; m+n: bac p cua p Nong o chat p tang lam tang TP Can lu y rang nh luat tac dung khoi lng ch ap dung chnh xac cho p ong the n gin - la nhng p ch dien ra qua 1 giai oan m = a, n = b

Nhng /v cac p phc tap la nhng p dien ra qua nhieu giai oan, nh luat tac dung khoi lng khong ap dung c cach nghiem nhat. Trong nhieu p loai nay so mu nong o cac chat p trong bieu thc toc o p, nghia la bac p, theo cac chat p co gia tr khac vi he so ty lng cua chung.

S d nh vay la v toc o cua p phc tap c xac nh bi giai oan cham nhat, ma giai oan nay co the co he so ty lng cua cac chat p khac vi he so ty lng cua p tong cong.

Bc p v biu thc TP trong thc t c xac nh bang thc nghiem.

VD8: 2 N2O5 = 4 NO2 + O2 (1) 2 NO + O2 = 2NO2 (2) p (1) gom 2 giai oan: N2O5 N2 O3 + O2 (cham) (I) N2O3 + N2O5 4 NO2 (nhanh) (II)P (1) v (2) eu la p phc tap v chung eu dien ra qua 2 giai oan, nhng da tren thc nghiem ngi ta xac nh c:va do o co BP la 1 (khong trung vi he so ty lng cua N2O5)

= = K1 [N2O2] = K1[NO]2 v= k2K1[NO]2[O2], biu thc TP thu c ph hp vi thc nghimDo [N2O2]tg kh xc nh, nn c th da vo CB nhanh ca giai on (I) tnh:

V giai on (II) xy ra chm, nn TP c xc nh da trn giai on ny, V = k2[N2O2][O2] NO + NO N2O2 (CB nhanh) (I)N2O2 + O2 = 2NO2 (chm) (II)Trng hp ny do c ch 2 giai on ca p nh sau:v= kC2 C BP la 3 (trung vi he so ty lng cua NO va O2).NOO2 p (2): 2NO + O2 = 2NO2 tuy cng l p phc tp, nhng c : K

b. Hang so toc o phan ng:Hang so t le k c goi la hang so toc o, gia tr cua no ch phu thuoc ban chat cac chat p va nhiet o p. k la ai lng ac trng cho TP k cang ln th TP cang nhanh. Theo tnh ton l thuyt: k = z . e - E*/ RT. eS*/R = . e - E*/ RT Z- h s t l va chm gia cc cht p; E*, S*: nng lng hot ha, entropi hot ha ca p R- hang so kh ; T- nhiet o tuyet oi

/v p nht nh = const nn k ch ph thuc vo nhit k l hng s ti nhit nht nh.Y ngha vat ly cua k: khi CA = CB = 1 mol/lt k = v k la toc o rieng cua phan ng. Cch xc nh k:K c xac nh bang cach ly tch phn cc phng trnh ong hoc tng ng vi cac p co bac khac nhau.

VD /v p bac 1: v= kC= - = - kdt lnC = - kt + lnC0 K = ln [sec -1 ]C0: nong o (mol/lt) cua chat p thi iem t = 0.C: nong o cua chat p sau khoang thi gian t. HST K khong phu thuoc vao n v bieu th nong o, no co th nguyen nghch ao thi giandCdtdCCLy tch phn vi gii hn t t =0 (C0) n t (C)

c. Ly thuyet hoat hoa va nang lng hoat hoa, entropi hoat hoa: Nang lng hoat hoa (E*): Theo ly thuyet hoat hoa muon va cham co hieu qua (xay ra p) th cac tieu phan va cham phai co nang lng d toi thieu nao ay so vi nang lng trung bnh cua cac tieu phan trong he e thang c lc ay gia cac lp vo electron cua cac tieu phan, e co the va cham nhau, pha huy lien ket cu, tao lien ket mi. Nang lng d toi thieu o c goi la nang lng hoat hoa cua tieu phan, con tieu phan co nang lng d toi thieu o c goi la tieu phan hoat ong.

Bang tong NLHH cua cac chat p: E* = aEA* + bEB* = Ett - Eb Bang hieu so gia nang lng toi thieu ( Ett) can thiet cua he p e tng tac xay ra va nang lng ban au (Eb) cua he p.Tom lai:NLHH cua chat la nang lng toi thieu can cung cap cho cac tieu phan e chung tr thanh hoat ong,NLHH cang nho, p cang de xay ra, TP cang ln.VD: /v p : aA + bB = cC + dDNang lng hoat hoa (NLHH) cua p:

C che p: Theo quan niem cu: p dien ra theo con ng tat ca lien ket cu phai b pha v het roi lien ket mi mi c tao thanh. Con ng nay oi hoi NLHH ln.VD: Theo c che nay p tong hp HI t H2 va I2 oi hoi NLHH ln: E* = E + E = 104 + 36 = 140kcal/molplH2plI2nen p kho xay ra, nhng thc te p nay tng oi de xy ra.

Theo quan niem mi: a so p dien ra qua giai oan tao thanh hp chat trung gian hoat ong (phc chat hoat ong ); hp chat nay khong ben se phan huy tao thanh san pham. S tao thanh phc chat hoat ong oi hoi NLHH nho nen p de xay ra. Nh vay theo c che nay: NLHH la nang lng can thiet e chuyen cac chat p sang trang thai phc chat hoat ong. Trong khi cac lk cu cha b pha v het th lk mi a c tao thanh.

Xet gian o nang lng cua p tong hp HI . Theo c che mi: Etn* = Et* - En* =(Htg - H) ( Htg - Hc)= Hc- H Etn*= Hc- H = H0 =40 - 44= 4 kcal/molVD: p tong hp HI theo c che nay co E* = 4 kcal/molS o bieu dien s hnh thanh phc chat hoat ong H2.I2

ng phan ng NLHH cua p thuan nghch co tr so bang hieu ng nhiet cua p thuan nghchTheo c che cu: Etn*= Et* - En*=(Htg - H) ( Htg - Hc)= Hc- H Etn*= Hc- H = H0 =140 - 144= 4 kcal/molGin nng lng p tng hp HI

NLHH cua p co the tnh c khi biet 2 gia tr cua hang so TP k (k1, k2) 2 nhiet o khac nhau (T1, T2):VD: Hng s tc ca p phn hy N2O5 thnh NO2 v O2 250C l 3,7.10-5 s-1 v 650C l 5,2.10-3 s-1. Hy tnh nng lng hot ha ca p.Gii: p dng cng thc trn, ta c:

Entropi hoat hoa (S*): e xay ra p, cac tieu phan phai co nang lng can thiet va phai co c s nh hng thuan li nhat nh khi va cham. Khong ch NLHH ma con mot so yeu to khac na, nh s nh hng trong khong gian khi va cham cua cac tieu phan, cung co anh hng en hieu qua cua va cham, nghia la co anh hng en toc o p.VD: /v p tong hp HI noi tren e tao thanh phc chat hoat ong H2---I2 th s nh hng cua cac phan t H2 va I2 khi va cham theo trng hp (a) se thuan li hn trng hp (b). Entropi hoat hoa actrng cho xac suat nh hng co hieu qua khi va cham:

Xet gian o entropi cua p tong hp HI theo c che mi:Stn* = St* - Sn* = (Stg - S) ( Stg -Sc) = Sc - S = S entropi hoat hoa cua p thuan nghch bang o bien oi entropi cua p thuan nghch.

NLHH E*nho, nhng S*ln th k van rat be va p xay ra cham;ngc lai E* ln, nhng S* be th p van xay ra nhanh.Tom lai theo thuyet hoat hoa: khi tang nong o chat p lam tang so tieu phan hoat ong, lam tang so va cham co hieu qua, dan en tang TP.hang so toc o k, nghia la TP, c quyet nh bi 2 yeu to: nang lng hot ha E*(yu t nng lng entanpi H) s nh hng trong KG thun li (yu t hnh hoc entropi S)A l tha s trc ly tha, c trng cho s nh hng trong KG thun li khi va chm gia cc tiu phn p.

7.3.2. Anh hng cua nhiet o:a) Quy tac (kinh nghiem) Van't Hoff: Khi tang nhiet o len 100C, TP trung bnh tang t 2 4 lan". Gia s t1 0C, mot p co toc o v1, t2 0C, p co toc o v2 Theo Van't Hoff, lien he gia v1 va v2 nh sau:So lan tang nay c goi la he so nhiet o cuaTP, c ky hieu: ay la quy tac kinh nghiem, nen co sai so ang ke (khoang 20%), tuy nhien co the dung e tnh gn ng hang so tc k, toc o v tng bao nhiu ln.

VD: Xac nh hang so toc o k 300C va tnh toc o p tang len bao nhieu lan khi nhiet o tang len 1000 /v p phan huy N2O5 thanh NO2 va O2, biet rang so k cua p 00 va 600 C tng ng bang 7,9.10-7 va 2,57.10-3Giai:Tnh hang so k 300C: = (3,86)3 k30 = k0 x (3,86)3 = 4,54.10-5 Tnh o tang toc o p khi tang nhiet o len 1000:

Gia HST k vi nhiet o T tai o p xay ra co lien he:Ly logarit t nhin PT trn v chuyn thnh logarit thp phn:

b)Phng trnh (thc nghim) Arrhenius B hng s tch phn

a. Khai niem ve xuc tac:7.3.3. Anh hng cua chat xuc tac: Chat xuc tac la chat lm tng TP, hoc gy nn p nu v mt nguyn tc p c th xy ra. Cht c ch l cht lm cho TP chm li.VD: glyxerin l cht c ch ca p oxyha Na2SO3 Na2SO4 Cc c im ca cht xc tc:Lng cht xc tc dng t hn cht p rt nhiu ln.Cht xc tc khng thay i v lng cng nh v thnh phn v tnh cht ha hc sau p (v l thuyt).Mi cht xc tc thng ch c tc dng /v mt p nht nh.

P xuc tac ong the l p c cht xc tc cng pha vi hn hp p, trai qua nhieu giai oan trong o NLHH cua giai oan nao cung tng oi nho. co the trai qua hai giai oan vi chat trung gian la NO2:P tuy phai trai qua nhieu bc, song chat xuc tac a lam giam NLHH cua he khien cho TP tang nhanh. nh ngha: Chat xuc tac la nhng chat lam tng TP do tham gia vo tng tc ha hc vi cc cht p giai on trung gian, lam giam nang lng hoat hoa, nhng sau p, chat xuc tac khong b bien oi ve so lng cung nh ban chat.VD:

Chat xuc tac d the hap phu t nhat mot trong nhng chat p. P hoa hoc dien ra tren be mat xuc tac tai nhng v tr xac nh (goi la tam hoat ong) thng la nhng nguyen t co hoa tr cha bao hoa.VD: p hidro hoa hp chat hu c (nh anken) co xuc tac kim loai niken, platin...(k hieu chung la M). Khi cho hon hp tac chat qua bot kim loai tren, kim loai hap phu mot lng ln kh hidro, ket hp vao mang li tinh the kim loai va chuyen thanh cac nguyen t hidro.Nguyen t hidro co hoat tnh manh hn phan t hidro, do o p hidro hoa xay ra:

P xc tc ng th l p c cht xc tc cng pha vi hn hp p, cn khi cht xc tc c pha khc vi hn hp p th l p xc tc d thP xc tc enzim mt loi anbumin c bit), l p ha hc xy ra di tc dng xc tc ca cc cht albumin c bit c gi l cc enzim (men). Nhng p ny l nhng p ha hc xy ra trong c th sinh vt.u im ca xc tc enzim l c tnh chn lc cao v cc p xc tc enzim xy ra nhit thng, c hiu sut lnVD: p thy phn ure vi xc tc enzim ureaza:(NH2)2CO + H2O = CO2 + 2 NH3

P dy chuyn: l p gm nhiu tng tc ha hc (tc dng n gin) xy ra lin tip nhau, lp i lp li (cc mc xch) vi s tham gia ca nhng tiu phn c bit c gi l cc gc t do.Gc t do l nhng tiu phn khng bo ha ha tr (H , O, OH, Cl,) do cc phn t b ph v di tc dng chiu sng, t nng, phng in, to thnh, c kh nng p rt ln v khi mt gc t do tham gia p mt i th s xut hin nhiu gc t do mi tip tc p to ra dy chuyn. VD: cc qu trnh ha hc nh s oxi ha (chy, n) cracking, trng hp,P dy chuyn c nng lng hot ha rt nh, c khi bng khng.P dy chuyn bao gm 3 giai on c bn: giai on khi mo to gc t do. giai on pht trin mch. giai on ngt mch.

P quang ha: l p ha hc xy ra di tc dng ca nh sng. nh sng khng phi l cht xc tc cho p m l tc nhn cung cp NL cho p to ra gc t do. P quang ha m tng tc ha hc xy ra t l thun vi lng nh sng hp th c, ngha l khi cht p hp th 1 photon nh sng ch sinh ra ti a 1 gc t do, s xy ra cch bnh thng.

VD: p phn hy AgBr di tc dng ca nh sng thnh Ag v Br2 trong k thut nh; p quang hp ca cy xanh,.P quang ha, trong s hp th nh sng ch ng vai tr kch thch p xy ra, cn sau p t xy ra khng ph thuc vo lng nh sng hp th, ngha l khi cht p hp th 1 photon li sinh ra c nhiu gc t do, s xy ra theo kiu dy chuyn.VD: p tng hp HCl t H2 v Cl2 khi c chiu sng s xy ra theo kiu dy chuynCl:Cl 2 Cl Cl + H:H HCl + H

hhH + Cl:Cl HCl + Cl.h

(a): Khong xuc tac: Chat xuc tac lam giam NLHH p bng cch thay i c ch png phan ngH(b): Co xuc tac: A + BAB+ KA + B A-- -B AB , E1* A + K A---K AK, E2* AK + B AK---B AB + K, E3*AK + BNanglngA---BAK--BE*3E*1A+B+KA--KABE*xtE*2b. Tc dng xc tc: /v qu trnh xc tc ng th: cht xc tc tham gia to phc cht hot ng mi lm gim NLHH p. C E2* v E3* u nh hn E1* lm cho cc p khi c xc tc u xy ra nhanh hn khi khng c xc tc.Cht xc tc lm gim nng lng hot ha pXt p: A + B = ABH

VD: p CH3CHO(k) CH4(k) + CO (k) khi khng dng xc tc c E* = 45,5 kcal/mol, cn khi dng hi iot lm xc tc c E*= 32,5 kcal/mol. Nh s gim NLHH ny m TP tng ln 500000 ln 5000K (nhng tiu phn khng bo ha ha tr) to ra p dy chuyn, l p xy ra rt mnh lit (n) v c NLHH rt nh hoc bng khng.VD: TP 2CO + O2 = 2CO2 tng ln mnh khi dng hi nc lm xc tc l do hi nc cung cp cc gc t do H , O, OH to ra p dy chuyn :OH + CO = CO2 + H H + O2 = OH + OO + CO = CO2nhng nguyn nhn dn n s gim NLHH p xc tc cha c gii thch tng tn cho tt c cc trng hp, c th l do c s chuyn dch electron v phn b li NL, lm cho cc lin kt b yu i hoc b phn cc hn nn p k tip ch cn 1 NLHH thp. Cung cp gc t do