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Unformatted text preview: rate = ! rate = ! d[H2O2 ] dt = ! slope of tangent =! =! [H2O2 ]2 ![H2O2 ]1 t 2 ! t1 initial reaction rate Time (s) 0 M ! 0.630 M 570 s ! 0 s = 0.00111 M/s CHM1311 Chemical Kinetics 13 Reaction Rates initial reaction rate: instantaneous reaction rate at t = 300 s average reaction rate from t = 0 to t = 300 s initial reaction rate "[H2O2 ] "t where t # 0 d[H2O2 ] rate = ! dt = ! slope of tangent [H O ] ![H2O2 ]1 =! 2 2 2 t 2 ! t1 rate = ! 0 M ! 0.882 M 275 s ! 0 s = 0.00321 M/s =! Time (s) CHM1311 Chemical Kinetics 14 Factors Affecting Rates 1. 2. 3. 4. Concentrations of reactants Contact surface between reactants Addition of Catalyst Temperature CHM1311 Chemical Kinetics 15 Factors Affecting Rates 1. Concentrations of reactants the more concentration available for the reactants, the probability for a reaction is increased, increasing reaction rate. Rate with 0.3 M HCl Rate with 6.0 M HCl CHM1311 Chemical Kinetics 16 Factors Affecting Rates 2. Contact surface between reactants similar to likely hood of collision (when reactant is in a powder, you increase S.A , increasing the reaction rate) CHM1311 Chemical Kinetics 17 Factors Affecting Rates while not being consumed in the reaction, can accelerate the reaction by providing a faster way of getting reactants to products 3. Addition of a catalyst CHM1311 Chemical Kinetics 18 Factors Affecting Rates 4. Temperature proportional: the higher the temp, the faster the rate CHM1311 Chemical Kinetics 19 Rate Laws for the general reaction: aA + bB ! cC + dD rate = k[A]x[B]y N.B. usually: x ! a, y ! b x and y must be determined experimentally! x and y can be positive, negative, or fractional values! CHM1311 Chemical Kinetics 20 Rate Laws aA + bB ! cC + dD rate = k[A] [B] k is the rate constant (1) the reaction (2) the temperature k is proportional to temperature (3) the presence of a catalyst. the UNIT of k depends on the overall order of the reaction: the sum of all exponents in the rate law (in this example, x + y) in this example, the reaction is said to be x order in A and y order in B LOWER CASE k!!!!!!!!!!!!!!!! x y This is the rate law, not expression!!! A & B are the ones stoichiometry dependant the VALUE of k depends on: in genral: BIG k means fast small k --> SLOW CHM1311 Chemical Kinetics 21 The Method of Initial Rates this is an experimental method to determine rate laws by examining initial rates of reaction [F2] initial (M) 0.10 0.10 0.20 [ClO2] initial (M) 0.010 0.040 0.010 initial rate (M/s) 0.0012 0.0048 0.0024
<--- initial rate comes from graph for the reaction: F2(g) + 2 ClO2(g) 2 FClO2(g) CHM1311 Chemical Kinetics 22 The method of initial rates [F2] initial (M) 0.10 0.10 0.20 [ClO2] initial (M) 0.010 0.040 0.010 initial rate (M/s) 0.0012 0.0048 0.0024 when [F2] is fixed, we observe that the initial rate increases by a factor of 4 if [ClO2] is increased by Rate is directly proportional to concentration of [ClO2] a factor of 4 when [ClO2] is fixed, we observe that the initial rate increases by a factor of 2 if [F2] is increased Rate is directly proportional to concentration of [F2] by a factor of 2 CHM1311 Chemical Kinetics 23 The method of initial rates rate ![F2 ][ClO2 ] r...
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- Fall '08