This document discusses the effects of temperature on reaction rates and provides an explanation using collision theory and activation energy. It introduces the Arrhenius equation and shows how to use it to determine activation energy from rate constants measured at different temperatures. Catalysts are discussed as lowering the activation energy of reactions without being consumed. Enzymes are described as biological catalysts that regulate metabolic reaction speeds. An example problem determines activation energy for a temperature-dependent firefly flashing process using rate data.

1. General Chemistry II CHEM 152 Week 3

2. Week 3 Reading Assignment Chapter 13 – Sections 13.5 (temperature), 13.7 (catalysts)

3. Temperature and Rate We have seen that, generally, as temperature increases so does the reaction rate. But the powers in the rate law do NOT change This means that k is temperature dependent . k Temperature We need a microscopic model to explain this

4. Three conditions must be met at the molecular level if a reaction is to occur: The molecules must collide ; They must be positioned so that the reacting groups are together in a transition state between reactants and products; The collision must have enough energy to form the transition state and convert it into products. Collision Rate Model

5. Molecules must collide with the correct orientation and with enough energy to cause bond breakage and formation. Right Orientation

6. Activation Energy There is a minimum amount of energy required for reaction: the activation energy , E a . Just as a ball cannot get over a hill if it does not have enough energy, a reaction cannot occur unless the molecules possess sufficient energy to get over the activation energy barrier.

7. The higher the temperature, the more molecules have energy to overcome the activation energy barrier. Low T High T Extra molecules at high T that exceed the E a Enough Energy

8. Transition State

9. Reaction Coordinate Diagrams Reaction coordinate diagrams help visualize energy changes throughout a process. Reaction Coordinate CH 3 NC CH 3 CN rearrangement of methyl isonitrile.

10. Reaction Coordinate Diagrams E Reactants E Products ∆ E reaction E activated complex E a Notice E a is not related to ∆E

11. Find E a and ∆E in each case. Identify endothermic and exothermic processes. Your Turn

12. SOLUTION A key reaction in the upper atmosphere is The E a(fwd) is 19 kJ, and the  H rxn ( ∆E) for the reaction is -392 kJ. Draw a reaction energy diagram for this reaction, postulate a transition state, and calculate E a (rev) . Your Turn O 3 ( g ) + O( g ) 2O 2 ( g ) transition state E a = 19kJ  H rxn = -392kJ E a (rev) = (392 + 19)kJ = 411kJ

13. The Arrhenius Equation ln k = ln A - E a /RT where k is the rate constant at T E a is the activation energy R is the energy gas constant = 8.3145 J/(mol K) T is the Kelvin temperature A is the collision frequency factor Temperature Effects ln k 2 k 1 = E a R - 1 T 2 1 T 1 -

14. ln k = -E a /R (1/T) + ln A Graphical Analysis . Y = m X + b

15. Typical Problems Suppose a chemical reaction has an activation energy of 76 kJ/mol. By what factor is the rate of reaction at 50 o C increased over its rate at 25 o C? k 2 =rate constant @ 50ºC k 1 =rate const @ 25ºC k 2 /k 1 = 10.7 over 10 times faster! = -76000 J 8.3145 J/mol K ln k 2 k 1 = 2.37 ln k 2 k 1 1 323.15K 1 298.15K -

16. The decomposition of hydrogen iodide, E a = ___________ kJ/mol 2HI( g )  H 2 ( g ) + I 2 ( g ) has rate constants of 9.51x10 -9 L/mol*s at 500. K and 1.10x10 -5 L/mol*s at 600. K. Find E a .

17. SOLUTION The decomposition of hydrogen iodide, has rate constants of 9.51x10 -9 L/mol*s at 500. K and 1.10x10 -5 L/mol*s at 600. K. Find E a . E a = 1.76 x 10 5 J/mol = 176 kJ/mol Answer / 2HI( g ) H 2 ( g ) + I 2 ( g ) ln k 2 k 1 = E a - R 1 T 2 1 T 1 - ln 1.10x10 -5 L/mol*s 9.51x10 -9 L/mol*s 1 600K 1 500K - E a = - (8.314J/mol*K)

18. Series of plots of concentra-tion vs. time Initial rates Reaction orders Rate constant ( k ) and actual rate law Integrated rate law (half-life, t 1/2 ) Rate constant and reaction order Activation energy, E a Plots of concentration vs. time Overview Find k at varied T Determine slope of tangent at t 0 for each plot Compare initial rates when [A] changes and [B] is held constant and vice versa Substitute initial rates, orders, and concentrations into general rate law: rate = k [A] m [B] n Use direct, ln or inverse plot to find order Rearrange to linear form and graph Find k at varied T

19. Catalysts speed up reactions by altering the mechanism to lower the activation energy barrier, but they are not consumed. MnO 2 catalyzes decomposition of H 2 O 2 2 H 2 O 2  2 H 2 O + O 2 Catalysis Uncatalyzed reaction Catalyzed reaction

20. Imagine trying to get a bunch of cattle to where you want them to go without any help… “ Cattle-ists” Will they get there very quickly on their own? Will they take the shortest path do get there? Is there a way to help?

21. What if there was someone there to herd the cattle and guide them along a more efficient path? “ Cattle-ists” The end result is the same – but the reaction takes a more efficient path.

22. The metal-catalyzed hydrogenation of ethylene

23. Catalytic Converters 13.6 CO + Unburned Hydrocarbons + O 2 CO 2 + H 2 O catalytic converter 2NO + 2NO 2 2N 2 + 3O 2 catalytic converter

24. Catalysis 3) Enzymes — biological catalysts Enzymes are specialized organic substances, composed of polymers of amino acids ( proteins ), that act as catalysts to regulate the speed of the many chemical reactions involved in the metabolism of living organisms.

25. Enzyme Catalysis

26. Enzymes uncatalyzed enzyme catalyzed

27. Enzymes Saturation Effects Denaturization

28. Summary Activity: Fireflies flash at a rate that is temperature dependent. At 29 ˚C the average firefly flashes at a rate of 3.3 flashes every 10. seconds. At 23 ˚C the average rate is 2.7 flashes every 10. seconds. Use the Arrhenius equation to determine the activation energy (kJ/mol) for the flashing process.

29. The activation energy of the firefly flashing process is: E a = ___________ kJ/mol

30. Do you know the equation for the force of gravity? Yes No Rotate your group spokesperson for today. Spokesperson: go to www.rwpoll.com – LOG IN and enter the session ID. Then – Answer this question: