QKinetics Prem Sattsangi Copyright 2009 http jchemed chem

Q-Kinetics Prem Sattsangi Copyright 2009 http: //jchemed. chem. wisc. edu/JCEsoft/cca/c ca 3/main/clockrx/page 1. htm

#2 -Chemical Kinetics: (p. 574) Study of the Rate of a Chemical Process • Reactions happen because, molecules collide with each other. • Reactions proceed faster, if the following are increased: 1. Reactant concentration. [Increased concentration, more collisions per second. ] 2. Temperature. [Faster moving molecule, more collisions per sec. ]

#3 -Average rate (M/s) (p. 576) • Time (s) • 0. 0 • 100 • 200 [C 4 H 9 Cl] (M) 0. 1000 0. 0820 0. 0671 Calculate average rate between 100 sec -200 sec Av. Rate =- {[0. 0671]final – [0. 0820 M]init. } 200 secfinal – 100 secinitial = 1. 49 x 10 -4 M/s

#4 -Rate and Stoichiometry (p. 579) • Reactants Products • N 2(g) + 3 H 2(g) 2 NH 3(g) • • As reaction proceeds, concentration of: Reactants decrease (minus sign), -D[R]/Dt Products increase, D[P]/Dt To equate the rates, divide by the coefficient, in the balanced equation.

#5 -Study of Reaction Rate as a function of Concentration at 25 o. C (p. 580) [one reagent a a time] • Reactants Products • NH 4+(aq) + NO 2 -(aq) N 2(g) + 2 H 2 O(l) • No. [NH 4+] [NO 2 -] Rate • Constant • 1. • 2. 0. 0100 0. 0200 • Constant 0. 200 5. 4 x 10 -7 10. 8 x 10 -7 • Doubling the [NH 4+] doubles the rate. • 3. • 4. 0. 200 0. 0202 0. 0404 10. 8 x 10 -7 21. 6 x 10 -7 • Doubling the [NO 2 -] doubles the rate.

#6 -Rate Law Equation (p. 581) • Rate = k [NH 4+]1 x [NO 2 -]1 • “k” = Rate constant, • (Value increases at higher temperature. ) • Exponents “ 1” are the order of reaction. • This reaction is First order in NH 4+ and • First order in NO 2 • Overall reaction order = 1 + 1 = 2

#7 -Figuring out Order of a reaction (p. 581) [Doubling Reactant Concentration] [R]r (order “r”) is related to Rate Doubling the concentration, “[2 R]r” and its effect on Rate: Rate Equation “r” Order UNCHANGED [2 R]0 = 1 x Rate “ 0” 2 x [2 R]1 = 2 x Rate “ 1” 4 x [2 R]2 = 4 x Rate “ 2” 8 x [2 R]3 = 8 x Rate “ 3”

#8 -Figuring out Order of a reaction [Tripling Reactant Concentration] [R]r (order “r”) is related to Rate Tripling the concentration, “[3 R]r” and its effect on Rate: Rate Equation “r” Order UNCHANGED [3 R]0 = 1 x Rate “ 0” 3 x [3 R]1 = 3 x Rate “ 1” 9 x [3 R]2 = 9 x Rate “ 2” 27 x [3 R]3 = 27 x Rate “ 3”

#9 -Orders (0, 1, 2, or 3) Trial • 1 • 2 [A](M) 0. 273 [B](M) 0. 763 1. 526 Rate (M/s) 2. 83 x 10 -6 • 3 0. 819 0. 763 2. 547 x 10 -5 • In 1 and 2 [A] is constant, [B] is doubled • Rate remained same, Order in B = 0 • In 1 and 3 [B] is constant, [A] is tripled • Rate changed to 9 x or 32. • Order in A = 2

#10 -Calculating the value of “k” (p. 581) at 25 o. C • NH 4+(aq) + NO 2 -(aq) N 2(g) + 2 H 2 O(l) • No. [NH 4+] • 1 0. 0100 [NO 2 -] 0. 200 Rate 5. 4 x 10 -7 • Rate = k [NH 4+]1 x [NO 2 -]1 k = __Rate Ms-1___ [NH 4+]1 x [NO 2 -]1 = 2. 7 x 10 -4 M-1 s-1 = __ 5. 4 x 10 -7 Ms-1___ [0. 0100 M ]1 x [0. 200 M]1

#11 Units of “k” [M] in Rate (p. 581) cancels M in the denominator. • Rate law k= Unit • Rate = k[A]1 x [B]1 __Rate Ms-1___ [A M]1 x [B M]1 M-1 s-1 • Rate = k[A]1 x [B]2 __Rate Ms-1___ [A M]1 x [B M]2 M-2 s-1 • Rate = k[A]2 x [B]1 __Rate Ms-1___ [A M]2 x [B M]1 M-2 s-1 • Rate = k[A]2 x [B]2 __Rate Ms-1___ [A M]2 x [B M]2 M-3 s-1

#12 -Energy of activation • Data: Set Temp(o. C) time(s) • D (T’) 23 (296 K) (Dt’) 350 • G (T) 13 (286 K) (Dt) 650 (Average) • Temperatures must be in K R= 8. 31 J/K. mol Ea = _________ = 43. 6 k. J/mol