Unit 6 Chemical Kinetics

The Rate of a Reaction The reaction rate is the increase in concentration of a product per unit time or decrease in concentration of a reactant per unit time. Kinetics is the study of rates of chemical reactions, the factors that affect rates and the mechanisms by which they occur. A reaction mechanism is the series of molecular steps by which a reaction occurs.

The Rate of a Reaction Thermodynamics (Chapter 15) determines if a reaction can occur. Kinetics (Chapter 16) determines how quickly a reaction occurs. Some reactions that are thermodynamically feasible are kinetically so slow as to be imperceptible.

The Rate of a Reaction Understanding & controlling the rates of rxns is critically important in many areas Sustaining living systems Chemical engineers use the chemical kinetics concepts to: develop conditions to produce the desired product at an economical rate slow unwanted rxns avoid dangerously high rates that may lead to explosions

aA(g) + bB(g)  cC(g) + dD(g) The Rate of Reaction Consider the hypothetical reaction, aA(g) + bB(g)  cC(g) + dD(g) equimolar amounts of reactants, A and B, will be consumed while products, C and D, will be formed as indicated in this graph:

[A] is the symbol for the concentration of A in M ( mol/L). Note that the reaction does not go entirely to completion. The [A] and [B] > 0 plus the [C] and [D] < 1.

The Rate of Reaction Mathematically, the rate of a reaction can be written as: aA(g) + bB(g)  cC(g) + dD(g) Refer to “Problem Solving Tip” on pg 610 for a useful analogy

Factors that affect Rxn Rates Nature of reactants See Section 16-2 Identity of elements or compounds Extent of subdivision (surface area)e.g. solid form vs powder State of matter e.g. liquid gasoline burns smoothly burn gasoline vapours can burn explosively. Concentration of the reactants Temperature The presence of a catalyst

Conc. of Reactants: Rate Law-Expression The rate-law expression or rate law for a reaction describes how its rate depends on the concentration of reactants at constant temperature. NOTE: The rate-law is experimentally determined The rate-law expression for a reaction in which A, B…..are reactants has the general form: Rate = k[A]x[B]y [A] and [B] are the concentrations of A and B in moles/L (M) respectively. k is the specific rate constant for the rxn at a particular temperature The values of the exponents (x and y) bear no necessary relationship to the coefficients in the balanced chemical equation. These must be EXPERIMENTALLY determined

Important things to note about k… Experimental data must be used to determine rxn orders and then the value of k for the rxn. k is for a specific rxn, represented by a balanced equation. The units of k depend on the overall order of the rxn. The value of k does not change with changing conc. of either reactants or products. If temp. changes, k changes. The value of k, changes if there is a catalyst and the nature of that catalyst.

Conc. of Reactants: Rate Law-Expression Important terminology for kinetics. The order of a reaction can be expressed in terms of either: each reactant in the reaction or the overall reaction. Order for the overall reaction is the sum of the orders for each reactant in the reaction. For example:

Conc. of Reactants: Rate Law-Expression A second example is:

Conc. of Reactants: Rate Law-Expression A final example of the order of a reaction is:

Conc. of Reactants: Rate Law-Expression Given the following one step reaction and its rate-law expression. Remember, the rate expression would have to be experimentally determined. Because it is a second order rate-law expression: If the [A] is doubled the rate of the reaction will increase by a factor of 4. 22 = 4 If the [A] is halved the rate of the reaction will decrease by a factor of 4. (1/2)2 = 1/4

Conc. of Reactants: Rate Law-Expression Example 16-1: The following rate data were obtained at 25oC for the following reaction. What are the rate-law expression and the specific rate-constant for this reaction? 2 A(g) + B(g) ® 3 C(g) Experiment Number Initial [A] (M) Initial [B] Initial rate of formation of C (M/s) 1 0.10 2.0 x 10-4 2 0.20 0.30 4.0 x 10-4 3

Conc. of Reactants: Rate Law-Expression

Conc. of Reactants: Rate Law-Expression

Conc. of Reactants: Rate Law-Expression

Conc. of Reactants: Rate Law-Expression Example 16-2: The following data were obtained for the following reaction at 25oC. What are the rate-law expression and the specific rate constant for the reaction? 2 A(g) + B(g) + 2 C(g) ® 3 D(g) + 2 E(g) Experiment Initial [A] (M) Initial [B] Initial [C] Initial rate of formation of D (M/s) 1 0.20 0.10 2.0 x 10-4 2 0.30 6.0 x 10-4 3 4 0.60 0.40 1.8 x 10-3

Conc. of Reactants: Rate Law-Expression

Conc. of Reactants: Rate Law-Expression Experiment Initial [A] (M) Initial [B] Initial [C] Initial rate of formation of D (M/s) 1 0.20 0.10 2.0 x 10-4 2 0.30 6.0 x 10-4 3 4 0.60 0.40 1.8 x 10-3

Conc. of Reactants: Rate Law-Expression Experiment Initial [A] (M) Initial [B] Initial [C] Initial rate of formation of D (M/s) 1 0.20 0.10 2.0 x 10-4 2 0.30 6.0 x 10-4 3 4 0.60 0.40 1.8 x 10-3

Conc. of Reactants: Rate Law-Expression

Conc. of Reactants: Rate Law-Expression Example 16-3: consider a chemical reaction between compounds A and B that is first order with respect to A, first order with respect to B, and second order overall. From the information given below, fill in the blanks. You do it! Experiment Initial Rate (M/s) Initial [A] (M) Initial [B] 1 4.0 x 10-3 0.20 0.050 2 1.6 x 10-2 ? 3 3.2 x 10-2 0.40

Conc. of Reactants: Rate Law-Expression

Conc. of Reactants: Rate Law-Expression

Conc. of Reactants: Rate Law-Expression

See Pages 618 and 621 for important Problem Solving Tips