Unformatted text preview: needed for the concentration of a reactant to decrease by half for a second order reaction: [A]
o [A] = [A]0 at t = 0 1 1 = + kt [A]t [A]o at t 1 [A]t = 1 [A]o 2 t1 =
2 [A] [A] = [A]t at t = t 2 therefore
Temps Time 1 [A]ok for second order reactions, the half-life is dependent on the concentration CHM1311 Chemical Kinetics 43 Summary Table Zero Order Rate expression Rate Law Integrated Rate Law Units of k Plot to get k rate = k[A]0 1st Order k[A] 2nd Order k[A]^2 rate = ! d[A] =k dt [A]t = [A]0 - kt mol L1s1 [A] vs. t 1/s 1/(M x s) 1/[A] vs t ln [A] vs t CHM1311 Chemical Kinetics 44 Your Turn... Reaction A has a rate constant which is equal to 16.2 L mol-1 s-1. A plot relating the concentrations of the reactant with time is plotted for Reaction B to the right. reaction order A: 2 order Enter the numerical order for Reactions A and B, respectively. For example, if you wish to answer "Reaction A is zero-order and Reaction B is second-order, please enter "02" Con. vs time vs B reaction B: 0 order [X] t order: 2 0 CHM1311 Chemical Kinetics 45 Theoretical Models for Chemical Kinetics How are reactants converted to products at the molecular level? We want to connect the RATE LAW MECHANISM CHM1311 Chemical Kinetics 46 Kinetic Molecular Theory...again KMT can be used to calculate collision density For example: gases undergo 1032 collisions/Ls IF every collision yielded product, then theoretical rate = 106 M/s ACTUAL rates are several orders of magnitude less typical actual rate = 104 M/s This tells us that they are colliding but only a small amount of collisions yield products CONCLUSION: Only a fraction of collisions yield a reaction! CHM1311 Chemical Kinetics 47 Collision Theory Reactions require activation energy correct geometry factors that affect rate of reaction that is why rate of reactions are much smaller O3(g) + NO(g) O2(g) + NO2(g) NOT ENOUGH ENERGY WRONG ORIENTATION CORRECT ENERGY& ORIENTATION CHM1311 Chemical Kinetics 48 Transition State Theory N2O + NO --> NO2 + N2 The activated complex is a hypothetical species lying between reactants and products at a point on the reaction profile called the transition state. (half bonds, one bond is being broken
while the other is being formed) Reaction profile = diagram o f t h e e n e r g y v e r s u s reaction progress Some exothermic reactions go very slow
--> transition state theory exothermic reaction activation energy for endothermic barrier is the ACTIVATION ENERGY, which is independent of the enthalpy of the reaction. CHM1311 Chemical Kinetics 49 Your Turn... Energy A particular reaction was found to have forward and reverse activation energies of 60 and 140 kJ mol-1, respectively. The enthalpy change for the reaction is, (do not use a calculator) A. H = 60 kJ mol-1 B. H = 60 kJ mol-1 C. H = 80 kJ mol-1 D. H = 80 kJ mol-1 E. H = 140 kJ mol-1 R P
reaction coordinate dH= Ea (forward) - Ea (reaction) CHM1311 Chemical Kinetics 50 Your Turn... The reaction between A and B is determined to be a fairly fast reaction and slightly exothermic. Which of the following potential energy surfaces fit this description? A.
Energy A+B B.
Energy X+Y X+Y A+B reaction coordinate reaction coordinate C.
Energy A+B X+Y reaction coordinate D.
Energy A+B X+Y reaction coordinate CHM1311 Chemical Kinetics 51 Average Kinetic Energy and T Recall from kinetic molecular theory: If activation barrier is high, only a few molecules have sufficient kinetic energy and the reaction is slower. As...
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