Repulsive Force

Transcription

1 ctivation nrgis (back to collision thory...) Potntial and Kintic nrgy during a collision + + ngativly chargd lctron cloud Rpulsiv Forc ngativly chargd lctron cloud - as colliding molculs approach th rpulsion slows thm down so kintic nrgy dcrass. - as thy push against th rpulsiv forc potntial nrgy incrass (lik comprssing a spring) is convrtd to - so: Kintic Enrgy Potntial Enrgy KE + PE = Total E (stays constant) if on gos down, th othr gos up. Potntial nrgy diagrams s molculs approach ach othr, KE is convrtd to PE Molculs form a tmporary, unstabl spcis calld th ctivatd Complx rarrangs to form th PRODUCT molculs Potntial Enrgy (kj) (High KE, Low PE) Product Molculs mov apart and spd up. PE is convrtd to KE. (High KE, Low PE) Progrss of Raction

2 Think of this as a barrir which must b ovrcom bfor th raction can tak plac Potntial Enrgy (kj) THE CTIVTION ENERGY BRRIER Raction Procds If colliding molculs don t hav nough KE to convrt to PE to mak it ovr th ctivation Enrgy Barrir, it is an UNSUCCESSFUL collision and thr is NO raction. Th molculs will just bounc off of ach othr unchangd. Potntial Enrgy (kj) THE CTIVTION ENERGY BRRIER Raction Procds CTIVTION ENERGY (Ea) - Th minimum nrgy rquird for a succssfull collision. (or) Th minimum nrgy racting molculs must hav in ordr to form th ctivatd Complx. Th ctivatd Complx can b dfind as a vry short-livd, unstabl combination of ractant atoms that xists bfor products ar formd.

3 NOTE: Th ctivation Enrgy (Ea) is fixd by th natur of th ractants (# s and strngths of bonds in ractants.) Ea is NOT affctd by Δtmpratur or Δ concntration.! Ea = 85 - = 35 kj Progrss of Raction Tmpratur s rol - th tmpratur dtrmins how many (or what fraction of th) molculs will hav nrgy > Ea (to mak it ovr th barrir & hav a succssful collision) Rcall KE distributions: g.) t a LOW tmpratur: Numbr of Molculs Curv at Lowr Tmp. (T 1 ) ctivation Enrgy (E a ) Kintic Enrgy t Tmpratur T 1 (lowr tmp), th molculs rprsntd by this ara do NOT hav sufficint KE for a succssful collision t Tmpratur T 1 (lowr tmp.), only ths molculs hav sufficint nrgy for a succssful collision

4 Most of us don t hav th nrgy to mak it ovr th barrir! Yaaay! fw of us mad it!!! Potntial Enrgy Progrss of Raction Notic in th diagrams on th prvious pag and abov, that only a small fraction of th molculs had nough nrgy to ovrcom th ctivation Enrgy barrir. Now, at a Highr Tmpratur: ctivation Enrgy (E a ) Numbr of Molculs Curv at Highr Tmp. (T 2 ) t a highr Tmpratur (T 2 ), thr ar lss molculs which don t hav nough KE for a succssful collision. Kintic Enrgy t Tmpratur T 2 (highr tmp.), thr ar mor molculs which hav sufficint nrgy for a succssful collision t th highr tmpratur, a gratr fraction of th molculs hav sufficint nrgy to mak it ovr th ctivation Enrgy barrir. (i. a gratr fraction of th molculs posss nough nrgy to form th ctivatd Complx):

5 fw of us don t hav th nrgy to mak it ovr th barrir! Yaaay! gratr fraction of us mad it this tim! Potntial Enrgy Progrss of Raction Looking at th diagram abov, you can s that at a highr tmpratur, a gratr fraction of th molculs hav sufficint nrgy to mak it ovr th barrir. Thrfor th raction is fastr. So if you study th graphs on th prvious pags, you will s that: Incrasing th tmpratur incrass th fraction of molculs which hav sufficint nrgy to form th ctivatd Complx (i. sufficint nrgy to mak it ovr th activation nrgy barrir.) This is on rason that incrasing th tmpratur will INCRESE th rat of raction. lso, NOTICE that a chang in tmpratur dos NOT chang th Potntial Enrgy diagram at all. Tmpratur dos NOT affct th ctivation nrgy or th ΔΗ!! Considr two ractions T THE SME TEMPERTURE: Ea E a Progrss of Raction Raction Progrss of Raction Raction B Which raction is fastr? Explain why.

6 Collision Gomtry (corrct alignmnt) g. for th rx. 2 + B 2 2B: + B B th abov collision has unfavourabl alignmnt (nd highr nrgy for collision to b ffctiv) B B B + + B B B Ractant Molculs ( 2 & B 2 ) PPROCH ECH OTHER Thy collid and form an ( 2 B 2 ) Th C braks apart to form th (2B) In th abov collision, th ractants hav favourabl alignmnt (lss nrgy ndd for an ffctiv collision) Potntial nrgy diagram Rout with UNFVOURBLE Collision Gomtry (lignmnt) POTENTIL ENERGY Ractants Rout with FVOURBLE Collision Gomtry (lignmnt) Products RECTION PROCEEDS

7 To Summariz Collision Thory so far: For any succssful collision (on rsulting in a raction): 3 Rquirmnts: 1.) - particls must collid 2.) - thy must collid with sufficint nrgy > Ea 3.) - thy nd to hav corrct alignmnt (collision gomtry) (to kp Ea as low as possibl) Ea, ΔΗ and bond strngths for forward and rvrs ractions POTENTIL ENERGY Ea Enrgy ndd for Ractants to form th ctivatd Complx ΔΗ- Enrgy Diffrnc btwn Ractants and Products. (In this cas ΔΗ is ngativ so Rx. is Exothrmic Progrss of Raction Try this qustion: Forward Raction Progrss of Raction

8 Using th graph abov, find: Ea (forward rx.) = kj ΔΗ (forward rx. ) = kj -Considring rvrs rx. This forward raction is thrmic 2 00 POTENTIL ENERGY Ea (rvrs rx.) ΔΗ (rvrs rx.) Rvrs Raction Ea (rvrs rx.) = kj Progrss of Raction ΔΗ (rvrs rx. ) = kj This rvrs raction is thrmic Givn th following Potntial Enrgy Diagram for th Raction: 2 + B2 2B POTENTIL ENERGY B B B Progrss of Raction

9 a) Ea (forward) = KJ b) Enrgy ndd to brak bonds in 2 & B 2 - B-B KJ c) Ea (rvrs) = KJ d) Enrgy ndd to brak bonds in B (-B) KJ ) Which has th strongr bonds 2 & B 2 or 2B? f) On a PE diagram, spcis with strongr bonds (mor stabl) ar (low/high) r on th graph g) Which st of spcis ( 2 & B 2, 2 B 2, or 2B) hav th wakst bonds?. This spcis is th most stabl. It is calld th h) Which st of spcis has th highst PE? i) Which st of spcis has th highst KE? j) Draw a graph of KE vs. raction procds for th sam forward rx. KINETIC E N E R G Y Progrss of Raction