Kinetics
Introduction Rate of Reaction – the rate at which reactants are consumed and products are formed. Chemical Kinetics – the study of the rates of reactions.
Factors that affect Reaction Rates Nature of the Reactants – surface area and color Concentration Temperature Presence of a Catalyst
Rate = k[A]x[B]y Zero order – the concentration of the reactant does not affect the rate of production of product. The concentration of reactant is doubled, the rate of production of product not affected. First order – the concentration of the reactant does affect the rate of production of product. The concentration of reactant is doubled, the rate of production of product double.
Second order – the concentration of the reactant does affect the rate of production of product. The concentration of reactant is doubled, the rate of production of product quadruple. Overall order = sum of all orders
Example 1 Expt # [CH3CHO] Rate (M/sec) 1 0.10 0.020 2 0.20 0.080 3 0.30 0.180 4 0.40 0.320
Example 2 Expt # [CO] [NO2] Rate (M/hr) 1 5.1 x 10-4 0.35 x 10-4 3.4 x 10-8 2 5.1 x 10-4 0.70 x 10-4 6.8 x 10-8 3 5.1 x 10-4 0.18 x 10-4 1.7 x 10-8 4 1.0 x 10-3 0.35 x 10-4 6.8 x 10-8 5 1.5 x 10-3 0.35 x 10-4 1.02x 10-7
2A + B C + D Expt [A] [B] Rate (M/sec) 1 0.10 0.05 6.0 x 10-3
2A + B + C D + E Expt [A] [B] [C] Rate (M/min)
C2H4 + O3 2CH2O + ½ O2 Expt [O3] [C2H4] Rate (M/sec) 1 5.0 x 10-8 1.0 x 10-8 1.0 x 10-12 2 1.5 x 10-7 1.0 x 10-8 3.0 x 10-12 3 1.0 x 10-7 2.0 x 10-8 4.0 x 10-12
First Order Reactions ln([A]0/[A]) = akt A0 = Initial Concentration A = Concentration at some point a = Coefficient of A k = Rate Constant t = Time
Half Life of a First Order Rxn t1/2 = ln2/ak
Compound A decomposes to form B and C in a reaction that is first order with respect to A and first order overall. At 25C, the specific rate constant for the reaction is 4.50 sec-1. What is the half life of A? A B + C
The reaction 2N2O5 2N2O4 + O2 obeys the rate law: rate = k[N2O5], in which the specific rate constant is 0.00840 sec-1 at a certain temperature. If an initial molarity is 4.2M, what would the molarity of it be after 1 minute?
How long would it take for it to decrease to 0.5M?
Second Order Rxn 1/[A] - 1/[A]0 = akt t1/2 = 1/(ak[A]0)
Compound A reacts to form C and D in a reaction that was found to be second order overall. The rate constant is 0.622 M-1 min-1. What is the half life of A when 4.10 x 10-2 M A is reacted.
The gas phase decomposition of NOBr is second order in [NOBr] with k = 0.810 M-1sec-1 at 10C. We start with 4.00 x 10-3M NOBr in a flask at 10C. How many seconds does it take to reduce the concentration of NOBr to 2.5 x 10-3 M? 2NOBr 2 NO + Br2
Consider the reaction in the previous problem. If we start with 6 Consider the reaction in the previous problem. If we start with 6.2 M NOBr, what concentration of NOBr will remain after 5.00 minutes of reaction?
Rate Determining Step A reaction can never proceed faster than its slowest step. Intermediates – Substances that are produced by one reaction and later consumed by another reaction. Catalyst – Substances that start a reaction and later produced by another reaction
Example 1 NO2 + NO2 N2O4 (fast) N2O4 + CO NO + CO2 + NO2 (slow) What are the intermediates? What is the rate law for this reaction?
Example 2 NO + Br2 NOBr2 (slow) NOBr2 + NO 2NOBr (fast) What are the intermediates? What is the rate law for this reaction?
2B C + D Expt [B] Rate (M/sec) 1 0.40 6.0 x 10-3 2 0.80 1.2 x 10-2 3 1.20 1.8 x 10-2 What is the order with respect to B? What is the rate law? What is k? If the initial concentration of B is 4.2, how long would it take for it to reach 0.5M? Which of the following mechanisms correctly justifies your rate law? Justiify B C + E (Slow) B + E D (Fast) 2. A + B C + E (Fast) B + E A + D (Slow)
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