Chapter 14 - Chemical Kinetics Reaction Rates Rate Laws Rate Constants Effect of Concentration on Reaction Rate
Kinetics The study of the factors that affect the speed of a reaction and the mechanism by which a reaction proceeds
Why Study Kinetics?
Rate How much a quantity changes in a given period of time Rate of a Chemical Reaction How much the concentration of a reactant decreases in a given period of time How much the concentration of a product increases in a given period of time
Reaction Rate Over Time As time passes, the reaction rate generally slows down because the concentration of the reactants decreases. At some time the reaction stops either because the reactants run out or the reaction reaches equilibrium.
Reaction Rates and Stoichiometry In most reactions, the coefficients in the balanced equation are not the same: H 2 (g) + I 2 (g) 2 HI (g) ons, the change in the nu The change in concentration of one substance is a multiple of the concentration of another. Rate of a reaction, therefore, can be described in terms of reactants or products:
Average Rate The average rate is the change in measured concentration in a particular time period. Instantaneous Rate The instantaneous rate is the change in concentration at any one particular time.
Average Rate H 2 (g) + I 2 (g) 2 HI (g) ons, the change in the nu Avg. Rate, M/s Avg. Rate, M/s Time (s) [H 2 ], M [HI], M -Δ[H 2 ]/Δt 1/2 Δ[HI]/Δt 0.000 1.000 0.000 10.000 0.819 0.362 0.0181 0.0181 20.000 0.670 0.660 0.0149 0.0149 30.000 0.549 0.902 0.0121 0.0121 40.000 0.449 1.102 0.0100 0.0100 50.000 0.368 1.264 0.0081 0.0081 60.000 0.301 1.398 0.0067 0.0067 70.000 0.247 1.506 0.0054 0.0054 80.000 0.202 1.596 0.0045 0.0045 90.000 0.165 1.670 0.0037 0.0037 100.000 0.135 1.730 0.0030 0.0030
Instantaneous Rate H2 (g) + I2 (g) 2 HI (g) Using [H2], the (g)2 (g) 2 (g) (g) Using [H ], the 2 2 (g) (g) instantaneous Using [H2], rate the at instantaneous rateatat 50 s is: instantaneous rate 50 50ss is: is: H2 H + I+ I 2 2HI HI Using [HI], the instantaneous rate at Using [HI], the 50 s is: Using [HI], the instantaneous rate at instantaneous rate at 50 s is: 50 s is: H2 (g) + I2 (g) 2 HI (g) Using [H2], the instantaneous rate at
Measuring Reaction Rates Continuous Monitoring (for short periods of time) Periodic Sampling
Factors Affecting Reaction Rates Nature of Reactants Small vs large molecules Gases vs liquids vs solids Powders vs solid blocks of substance Reduction/Oxidation potentials Ions vs molecules Temperature of Reaction Catalysts Reactant Concentration
Rate Law The mathematical relationship between the rate of the reaction and the concentration of reactants The rate of a reaction is directly proportional to concentrations of the reactant raised to a power. For the reaction aa + bb products Rate = k[a] n [B] m Rate constant Reaction order for each reactant
Reaction Order The sum of the exponents on the reactants For the reaction 2 NO (g) + O2 (g) 2NO2 (g) Rate = k[no] 2 [O2] The reaction is second order with respect to [NO], first order with respect to [O 2 ] and third order overall. The rate law is determined experimentally and is not related to the coefficients in the balanced equation!!!
Sample Rate Laws
Graphical Representation of Kinetic Data Zero Order Reaction First Order Reaction Second Order Reaction
Deriving a Rate Law from Rate Data The method involves observing the effect on the initial rate of a reaction when the initial concentration of only one reactant is changed. For zero order reactions, changing the concentration has no effect on the rate. For 1st order reactions, the rate changes by the same factor as the concentration (i.e., Doubling the initial concentration doubles the rate.). For 2nd order reactions, the rate changes by the square of the factor the concentration changes (i.e., Doubling the initial concentration quadruples the rate.).
1) Determine the rate law and rate constant for the reaction: NH4 + + NO2-1 N2 + 2 H2O given the data below. Expt. Number [NH4 + ], M [NO2 - ], M Rate, (x 10-7 ), M/s Reaction is 1st order with respect to NH4 + 1. 2. 0.0200 0.0600 10.8 32.3 Reaction is 1st order with respect to NO2 3. 0.0202 10.8 4. 0.0404 21.6 Rate = k [NH4 + ][NO2 - ]
1) Determine the rate law and rate constant for the reaction: NH4 + + NO2-1 N2 + 2 H2O given the data below. Expt. Number [NH4 + ], M [NO2 - ], M Rate, (x 10-7 ), M/s Rate = k[nh4+][no2-] 1. 0.0200 10.8 2. 0.0600 32.3 3. 0.0202 10.8 4. 0.0404 21.6 10.8 x 10-7 M/s = k(0.0200 M)( M) k = 10.8 x 10-7 M/s (0.0200 M)( M) = 2.70 x 10-4 M-1 s-1 21.6 x 10-7 M/s = k( M)(0.0404 M) k = 21.6 x 10-7 M/s ( M)(0.0404 M) = 2.70 x 10-4 M-1 s-1
2) Determine the rate law and rate constant for the reaction: NO2 + CO NO + CO2 given the data below. Expt. Number [NO2], M [CO], M Rate, M/s Reaction is 2nd order with respect to NO2 1. 2. 0.100 0.100 0.100 0.0021 0.0082 Reaction is 0 order with respect to CO 3. 0.0083 4. 0.400 0.100 0.033 Rate = k [NO2] 2 [CO] 0
2) Determine the rate law and rate constant for the reaction: NO2 + CO NO + CO2 given the data below. Expt. Number [NO2], M [CO], M Rate, M/s 1. 0.100 0.100 0.0021 Rate = k [NO2] 2 [CO] 0 2. 0.100 0.0082 Rate = k [NO2] 2 3. 0.0083 4. 0.400 0.100 0.033 0.0021 M/s = k[no2] 2 0.0021 M/s = k(0.10 M) 2 k = 0.0021 M/s (0.10 M) 2 = 0.21 M-1 s-1 0.033 M/s = k[no2] 2 0.033 M/s = k(0.400 M) 2 k = 0.033 M/s (0.40 M) 2 = 0.21 M-1 s-1
3) Determine the rate law and rate constant for the reaction: O2 + 2 NO 2 NO2 given the data below. Expt. Number [O2], M [NO], M Rate, M/s Reaction is 1st order with respect to O2 1. 2. 1.10 x 10-2 2.20 x 10-2 1.30 x 10-2 1.30 x 10-2 3.21 x 10-3 6.40 x 10-3 Reaction is 2nd order with respect to NO 3. 1.10 x 10-2 2.60 x 10-2 12.8 x 10-3 Rate = k [O2][NO] 2 4. 3.30 x 10-2 1.30 x 10-2 9.60x 10-3 5. 1.10 x 10-2 3.90 x 10-2 28.8 x 10-3 9.6 x10-3 M/s = k[o2][no] 2 9.6 x10-3 M/s = k(3.30 x 10-2 )(1.30 x 10-2 ) 2 k = 9.60 x10-3 M/s (3.30 x 10-2 M)(1.30 x 10-2 M) 2 = 1.72 x 10 3 M-2 s-1
Deriving a Rate Law Alternative Method for Partial Reaction Order Rate = k[a] n [B] m Rate1 ( ) = [B]1 Rate2 [B]2 n log ( Rate1 ) = n Rate2 log ( ) [B]1 [B]2 log ( Rate1 ) Rate2 n = log ( ) [B]1 [B]2
Deriving a Rate Law Alternative Method for Partial Reaction Order 4) Determine the rate law and rate constant for the reaction: NO + NO3 2 NO2 given the data below. Expt. Number [NO], M [NO3], M Rate, M/s 1. 1.25 x 10-3 1.25 x 10-3 2.45 x 10 4 2. 1.50 x 10-3 1.25 x 10-3 2.94 x 10 4 3. 1.50 x 10-3 2.00 x 10-3 4.71 x 10 4 Reaction is 1st order with respect to NO Reaction is 1st order with respect to NO3 n=1