Kinetics Part II: Rate Laws & Order of Reaction

1. Kinetics Part II:Rate Laws & Order of Reaction Jespersen Chap. 14 Sec 3 Dr. C. Yau Fall 2014 1

2. Rate of Rxn vs. Rate Law Rate of reaction is based on one component (reactant or product) of the reaction: disappearance of a reactant or formation of a product. Rate law is a rate expression that includes all reactants. LEARN THESE TERMS SO YOU KNOW WHAT IS BEING ASKED FOR!! 2

3. The Rate Law Depends On The Concentrations Used Rate= k [reactant]order • k is a reaction rate constant, a measure of time efficiency(not to be confused with “Rate”). • High values of k mean high efficiency.(Reaction goes fast.) • k must be determined experimentally. • Each experiment has its own rate law. • Rate law must be determined experimentally.

4. A + B products Rate = k [A]m[B]n where m and n are the "orders of reaction" and are found by experiment, NOT based on the coefficients of the chemical equation, and k is the "rate constant." This expression is called the "rate law."

5. H2SeO3 + 6I- +4H+ Se + 2I3- + 3H2O Rate = 5.0x105 L5mol-5 s-1 [H2SeO3][I-]3[H+]2 5.0x105 mol-5 s-1 is the rate constant (k). We speak of the reaction as being… first orderwith respect to H2SeO3, third orderwith respect to I-(Nothing to do with 6 in eqn) second orderwith respect to H+, and the overall order of reaction is 6 (sum of all the orders). Learn this terminology! What is the unit of Rate in the equation shown above? Do Practice Exercises 7, 8 & 9 on p.648.

6. What is a rate law used for? Rate changes with concentrations. The rate law allows us to determine the rate for various concentrations of the reactants. Example: The rate law for the reaction 2A +B→3C is Rate = 0.045M-1s-1 [A][B] If the concentration of A is 0.2M and that of B is 0.3M, what will be the reaction rate? rate = 0.045 M-1 s-1 [0.2M][0.3M] rate = 0.0027 M/s Do Practice Exercises 5 & 6 p.646

7. Chlorine dioxide, ClO2, is a reddish-yellow gas that is soluble in water. In basic solution it gives ClO3- and ClO2- ions. ClO2(aq) + OH(aq) ClO3(aq) + ClO2(aq) + H2O (l) The rate law is Rate=k[ClO2]2[OH-]. What is the value of the rate constant given that when [ClO2]=0.060M, [OH-] = 0.030M, the reaction rate is 0.0248 M/s • 0.02 M-1 /s • 0.02 M/s • 0.02 s- • None of these 2.3x102 M-2 s-1

8. Orders… • are indicated for each reactant, • the overallreaction order is the sum of individual reactant orders, • may be zero, negative, fractional or integers, but in this course we will usually encounter positive integers, and • must be determined from experimental data.

9. Determining The Rate Law: • Run reaction under the same conditions, varying only the concentrations of reactants (not the temperature). • A ratio of rate laws for each experiment allows us to determine the order of each reactant. • The rate law is unique to temperature and concentration conditions. Therefore, when a rate law is stated, it must include the temperature at which it is determined.

10. Use Rate Laws To Determine Orders : 2NO(g) + O2(g)→ 2NO2(g) Select 2 rate laws that vary in concentration for only one of the substances (NO). Hint: Write the fractions with the larger R on top.

11. Use Rate Laws To Determine Orders : 2NO(g) + O2(g)→ 2NO2(g) Next choose 2 rate laws where the concentration for the other component (O2) changes. x=2, y = 1 so….. Rate = k [NO]2[O2]

12. Determining The Value Of k Finally we can solve for k. Use any rate law and the orders that we have determined. rate = k[NO]2[O2] 0.048M/s =k [0.015M]2[0.015M] 1.4×104 M-2s-1 =k Do Example 14.5, 14.6 p.651, Exercises 10 thru 14p.651+.

13. Determine The Rate Law From Given Data x=1 y=1 z=0 rate=k[A][B] Note that changing the concentration of C had no effect on the rate. We say it is “zero order with respect to C.”

14. Effect of Order of Rxn on Rate Consider Rate = k[A]n If n = 0, change in conc has no effect on rate. If n = 1, Rate = k[A]1 and when conc is 2x, rate is 2x. If n = 2, Rate = k[A]2 and when conc is 2x, rate is 4x If n = 2, when conc is tripled, rate is …? rate is 9x If n = 3, and conc is doubled, rate is…? rate is 8x

15. Visual Determination of Reaction Order • Once you understand how you can predict effect of a change in concentration on rates (as in the previous slide), you can often determine the rxn order visually without doing complicated calculations. • HOWEVER, that is only if the conc were neatly doubled or tripled, etc. (See next 2 slides.)

16. p. 648 What is the rate law? Rate = k[A]?[B]? Rate = k[A]1[B]2

17. For the following data, determine the order of NO2 in the reaction at 25°C 2 NO2(g) + F2(g)→ 2 NO2F(g): • 0 • 1 • 2 • 3 • not enough information given

18. When Visual Determination Fails... We cannot always determine the rxn order visually. For example, if we ended with 32.1=3.18x How do we determine what x is? In high-level chemistry courses, x might even be a fraction!