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Unformatted text preview: Ch 14 – Recap Kinetics The reaction rate rate = − Δ [A] Δ t = Δ [B] Δ t A → B Δ [x] Δ t dx dt lim Δ t → The instantaneous rate time rate here, lower slope, slower rate rate here, higher slope, faster rate [ ] But rate will change as the concentrations of reactants and products change! Ch 14 – Recap Kinetics For a general reaction, a normalized rate can be obtained: aA + bB → cC + dD rate = − Δ [A] Δ t 1 a Δ [B] Δ t 1 b = − = Δ [C] Δ t 1 c = Δ [D] Δ t 1 d The rate law is the expression which ties all of the rate dependence together. rate = k [A] x [B] y Practice problem #41 For the reaction F 2 (g) + 2 ClO 2 (g) → 2 FClO 2 (g), the rate law is experimentally determined to be: rate = k [F 2 ] [ClO 2 ]. From the exponents in the rate law: 1 st order in F 2 (exponent is 1) 1 st order in ClO 2 (exponent is 1) 2 nd order overall (sum of exponents is 2) What is the order with respect to F 2 , ClO 2 , and overall? Ch 14 – Recap Kinetics Experimentally determining rate laws – method of initial rates • Make initial conditions isolate the dependence of one reactants • Use the ratio between two of the trials can be used to determine the order of the reaction with respect to the isolated reactant! Practice problem #42 Using the general reaction A + B → C and the data below, determine the order of the reaction with respect to A and B, and the value of the rate constant, k, for this reaction. Trial [A] [B] initial rate, M s − 1 1 2 3 0.01 0.01 0.02 0.01 0.02 0.02 4 × 10 − 4 4 × 10 − 4 11.3 × 10 − 4 Idea: solve for dependences in changes of concentration between runs compare trials 1 and 2 for dependence of B compare trials 2 and 3 for dependence of A Ch 14 – Recap Kinetics Practice problem #42 Using the general reaction A + B → C and the data below, determine the order of the reaction with respect to A and B, and the value of the rate constant, k, for this reaction. Looking at the reaction, the rate law takes the general form: rate = k [A] x [B] y Trial [A] [B] initial rate, M s − 1 1 2 3 0.01 0.01 0.02 0.01 0.02 0.02 4 × 10 − 4 4 × 10 − 4 11.3 × 10 − 4 Let’s make a ratio of the first two rate expressions: rate 1 rate 2 = k [A] x [B] y k [A] x [B] y = k (0.01) x (0.01) y k (0.01) x (0.02) y = (0.01) y (0.02) y = (0.5) y From this: rate 1 rate 2 = (0.5) y 4 × 10 − 4 4 × 10 − 4 = (0.5) y 1 = (0.5) y y = 0 “zero order” in B Practice problem #42 Using the general reaction A + B → C and the data below, determine the order of the reaction with respect to A and B, and the value of the rate constant, k, for this reaction. Trial [A] [B] initial rate, M s − 1 1 2 3 0.01 0.01 0.02 0.01 0.02 0.02 4 × 10 − 4 4 × 10 − 4 11.3 × 10 − 4 Ok, now let’s make a ratio of the last two rate expressions: rate 2 rate 3 = k [A] x [B] y k [A] x [B] y = k (0.01) x (0.02) y k (0.02) x (0.02) y = (0.01) x (0.02) x = (0.5) x From this: rate 2 rate 3 = (0.5) x 4 × 10 − 4 11.3 × 10 − 4 = (0.5) x...
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 Spring '09
 Farmer
 Kinetics, Rate equation

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