AP Chemistry--Chapter 12: Chemical Kinetics

Transcription

1 AP Chemisry--Chaper 12: Chemical Kineics I. Reacion Raes A. The area of chemisry ha deals wih reacion raes, or how fas a reacion occurs, is called chemical kineics. B. The rae of reacion depends on he seps aken by reacans o become producs. This series of seps is called he reacion mechanism. * Oher more obvious facors ha affec he rae of a reacion are concenraion of reacans ([A] increases wih increased concenraion) emperaure (k increases wih increased emperaure, Arrhenius eq) surface area caalys (lowers E a ) C. Reacion rae is defined as he change in a paricular reacan or produc per uni ime 1. Reacion raes are always posiive, so in he case of reacans, he rae is he rae of disappearance (negaive added o equaion o make resul posiive). 2. For producs, rae is considered he rae of appearance (should urn ou posiive, so no negaive included) 3. Also, raes of 1:1 (coefficiens) subsances in a reacion are equal, if 2:1, he 2 subsance s rae is double he 1 subsance s rae 4. Reacion raes decrease over ime 5. Equaion for rae: rae = concenraion of A a ime 2 concenraion of A a ime or rae = Δ[A] Δ where A is he reacan or produc being considered 6. For insananeous rae, rae a a paricular ime, he slope of he line angen o he curve a ha poin is used II. Rae Laws A. (Differenial) Rae Law 1. The differenial rae law (called jus he rae law) shows how he rae of a reacion depends on concenraions 2. I has he general forma rae = k[a] n [B] m a. [A] and [B] are he concenraions of he reacans (maybe have only one reacan in he rae law, or hree, or whaever) b. k is he rae consan (i is deermined experimenally and is used o correc labels on he wo sides of he equaion)--k depends on emperaure, no on he concenraion of reacans, so k changes when emperaure of he reacion changes (laer in noes) c. n and m are he order of he reacion wih respec o he corresponding reacan (hey are deermined experimenally and involve he various seps aken by reacans o become producs)

2 B. Inegraed Rae Law shows how he concenraion of species in he reacion depends on ime (various equaions will be presened laer) C. Knowing he rae law for a reacion is imporan mainly because we can usually infer he individual seps involved in he reacion from he specific form of he rae law. III. Deermining he Form of he Rae Law A. Deermining he form of he rae law means o basically deermine he order of he reacion (recall rae = k[a] n [B] m ) B. The overall order of a reacion is he sum of n + m + (if any more) 1. n and m are NOT COEFFICIENTS, hey are experimenally deermined from daa and in he rae law become exponens 2. n and m are referred o as he order wih respec o he paricular corresponding reacan 3. n and m can be 1, 2, ec, even 0, ½, ½ a. rae = k[x][y], 2 nd order overall, 1 s order WRT X and Y b. rae = k[x] 2, 2 nd order overall, 2 nd order WRT X C. k can also be deermined from experimenal daa 1. once he rae law is deermined, plug in concenraions from any experimen o solve for k 2. he unis of k for differen reacions vary since he overall order of a reacion and he rae law of a reacion also vary; he unis of he rae consan need o allow he rae law expression o = M/s (usually shown as mol L 1 s 1 or mol L 1 s 1 ) 3. rae order does no change wih increasing emperaure, k changes IV. The Inegraed Rae Law A. Firs Order Rae Laws (sum of n + m + ec. = 1) 1. Recall inegraed rae laws express concenraion as a funcion of ime 2. The equaion below is for firs order reacions (you may also see differen forms of i) *Noe [ ] = molariy, usually shown as mol L 1 a. [A] is he concenraion of a paricular reacan afer ime b. [A] 0 is he iniial concenraion of he reacan c. k is he rae consan d. is ime ( afer ime ) y = mx + b form: ln[ A] k ln[ A] 0 Given on your green shee as: ln[ A] ln[ A] 0 k Skech of graph of 1 s order reacion: Wha would you need plo o ge a sraigh line? Wha would his graph look like? Wha would he slope of his line equal?

3 B. Second Order Rae Laws (sum of n + m + ec. = 2) 1 1 y = mx + b form: k [ A] [ A] on your green shees: k [ A] [ A] 0 Skech of graph of 2 nd order reacion: Wha would you need plo o ge a sraigh line? Wha would his graph look like? Wha would he slope of his line equal? C. Zero Order Rae Laws (sum of n + m + ec. = 0) and half-life 1. zero order reacions occur when he rae of he reacion is consan, no maer if iniial concenraions are changed or no 2. an example of a zero order reacion would be he decomposiion of a subsance when i comes in conac wih a surface; only hose a he surface can reac so more wouldn maer 3. zero-order rae law: rae = k Skech of graph of zero order reacion: D. The inegraed rae laws for reacions beyond second order are found by seing cerain condiions in he reacion 1. for example, one reacan has a large concenraion compared o he oher in he reacion and informaion concerning order wih respec o ha reacan are found 2. he reacion, when simplified by his crieria may reduce o firs order, for example V. Reacion Mechanisms A. The process by which a reacion occurs is called he reacion mechanism. A reacion mechanism can describe how bonds are broken and formed and he way in which he aoms move during a reacion.

4 B. If a reacion occurs in one single even or sep, i is called an elemenary sep. For insance, when NO combines wih O 3 o form NO 2 and O 2 (one oxygen moves from O 3 o NO). C. The number of molecules ha paricipae in a reacion is referred o as he moleculariy of he sep. 1. If a single molecule is involved, he reacion is said o be unimolecular. 2. If wo reacan molecules collide during he reacion, he reacion is bimolecular. 3. Three molecules is called ermolecular, hese are rare (why?) D. A mulisep mechanism involves a molecule ha is neiher a reacan nor a produc in he overall reacion, bu is produced and consumed during he reacion. Example: NO 2 + NO 2 NO 3 + NO + NO 3 + CO NO 2 + CO 2 NO 2 + CO NO + CO 2 During his reacion, NO 3 is produced and consumed before he final producs are formed. The NO 3, herefore, is called an inermediae. E. Caalyss can also be idenified in a reacion mechanism. They are a reacan in he beginning, a produc a he end, and do no appear in he final reacion. Example: Sep 1: Ce 4+ + Mn 2+ Ce 3+ + Mn 3+ Sep 2: Ce 4+ + Mn 3+ Ce 3+ + Mn 4+ Sep 3: Mn 4+ + Tl + Tl 3+ + Mn 2+ Wha is he caalys above? Is here an inermediae? Wha is he overall reacion? F. Furher definiion of reacion mechanism--i is a series of elemenary seps ha mus saisfy wo requiremens: 1. The Soichiomery: he sum of he elemenary seps mus give he overall balanced equaion for he reacion 2. The Rae Law: he mechanism mus agree wih he experimenally deermined rae law (his where he rae-deermining sep can be deermined) VI. A Model for Chemical Kineics A. The collision model 1. molecules mus collide o reac 2. a higher emperaures, molecules are colliding more ofen and a higher speeds, and i is shown ha nearly all reacions occur faser a higher emperaures a. fas reacions have low acivaion energy and weaker reacan bonds b. slow reacions have high acivaion energy and sronger reacan bonds

5 B. Since emperaure does no affec concenraion very much, he rae consan in a rae law mus change wih increasing emperaure. C. The Arrhenius equaion allows you o calculae he rae consan, k, a some differen emperaure when he acivaion energy and rae consan are known a anoher emperaure. 1. he acivaion energy (E a ) is he minimum energy a reacion mus overcome o proceed; i is he energy required o form he ransiion sae in a chemical reacion 2. i is lowered when a caalys is used a. spark from a spark plug provides energy for combusion reacion b. enzyme caalase o quickly break down H 2 O 2 3. i arises from he energy needed o break bonds, rearrange molecules, ec. Arrhenius Equaion (as given on your green shee) Ea 1 ln k ln A where R = 8.31 J mol 1 K 1, T is emp in K R T Noe here ha he above is already in he y = mx+b form; a graph of 1 ln k vs gives a sraigh line wih slope = T R E a Sample graph o find E a Wha would his graph look like? 4. So, you can also use he Arrhenius Equaion o find acivaion energy by measuring he rae of a reacion a various emperaures