1. CHE 311 Organic Chemistry I Dr. Jerome K. Williams, Ph.D. Saint Leo University

2. Chapter 4. The Study of Chemical Reactions Chemical Reactions Equilibrium Constants Thermodynamics –Free Energy –Enthalpy –Entropy –Bond Dissociation Energies Kinetics –Rate Laws –Reaction Profile Diagrams –Mechanisms

3. Chemical Reactions: A Review Overall reaction: reactants  products To learn more about a reaction: –Thermodynamics is the study of the energy changes that accompany chemical and physical transformations. – Kinetics is the study of reaction rates. Mechanism: Step-by-step description of how the reaction happens. Chapter 43

4. Equilibrium Constant K eq = [products] [reactants] For CH 4 + Cl 2  CH 3 Cl + HCl K eq = [CH 3 Cl][HCl] = 1.1 x 10 19 [CH 4 ][Cl 2 ] Large value indicates reaction “goes to completion.” Chapter 44

5. Free Energy Change   G = (energy of products) - (energy of reactants)   G is the amount of energy available to do work. A reaction with a negative  G is favorable and spontaneous.  G o = -RT(lnK eq ) = -2.303RT(log 10 K eq ) where R = 8.314 J/K-mol and T = temperature in kelvins. Chapter 45

6. Factors Determining  G  Free energy change depends on: –Enthalpy  H  = (enthalpy of products) - (enthalpy of reactants) –Entropy  S  = (entropy of products) - (entropy of reactants)  G  =  H  - T  S  Chapter 46

7. Enthalpy  H o = heat released or absorbed during a chemical reaction at standard conditions. Exothermic (-  H): Heat is released. Endothermic (+  H): Heat is absorbed. Reactions favor products with the lowest enthalpy (strongest bonds). Chapter 47

8. Entropy  S o = change in randomness, disorder, or freedom of movement. Increasing heat, volume, or number of particles increases entropy. Spontaneous reactions maximize disorder and minimize enthalpy. In the equation  G o =  H o - T  S o, the entropy value is often small. Chapter 48

9. Calculate the value of  G° for the chlorination of methane.  G° = –2.303RT(log K eq ) K eq for the chlorination is 1.1 x 10 19, and log K eq = 19.04 At 25 °C (about 298 K), the value of RT is RT = (8.314 J/kelvin-mol)(298 kelvins) = 2478 J/mol, or 2.48 kJ/mol Substituting, we have  G° = (–2.303)(2.478 kJ/mol)(19.04) = –108.7 kJ/mol (–25.9 kcal/mol) This is a large negative value for  G°, showing that this chlorination has a large driving force that pushes it toward completion. Solved Problem 1 Solution Chapter 49

10. Bond-Dissociation Enthalpies (BDEs) Bond dissociation requires energy (+BDE). Bond formation releases energy (-BDE). BDE can be used to estimate  H for a reaction. BDE for homolytic cleavage of bonds in a gaseous molecule. Homolytic cleavage: When the bond breaks, each atom gets one electron. Heterolytic cleavage: When the bond breaks, the most electronegative atom gets both electrons. Chapter 410

11. Homolytic and Heterolytic Cleavages Chapter 411

12. Enthalpy Changes in Chlorination CH 3 -H + Cl-Cl  CH 3 -Cl + H-Cl Bonds Broken  H° (per Mole) Bonds Formed  H° (per Mole) Cl-Cl+242 kJH-Cl-431 kJ CH 3 -H+435 kJCH 3 -Cl-351 kJ TOTAL+677 kJTOTAL-782 kJ  H° = +677 kJ + (-782 kJ) = -105 kJ/mol Chapter 412

13. Kinetics Kinetics is the study of reaction rates. Rate of the reaction is a measure of how the concentration of the products increases while the concentration of the starting materials decreases. A rate equation (also called the rate law) is the relationship between the concentrations of the reactants and the observed reaction rate. Rate law is determined experimentally. Chapter 413

14. Rate Law For the reaction A + B  C + D, rate = k r [A] a [B] b –Where k r is the rate constant –a is the order with respect to A –b is the order with respect to B –a + b is the overall order Order is the number of molecules of that reactant which is present in the rate- determining step of the mechanism. Chapter 414

15. Activation Energy The rate constant, k r, depends on the conditions of the reaction, especially the temperature: where A = constant (frequency factor) E a = activation energy R = gas constant, 8.314 J/kelvin-mole T = absolute temperature E a is the minimum kinetic energy needed to react. Chapter 415

16. Temperature Dependence of E a At higher temperatures, more molecules have the required energy to react. Chapter 416

17. Energy Diagram of an Exothermic Reaction The vertical axis in this graph represents the potential energy. The transition state (‡) is the highest point on the graph, and the activation energy (E a ) is the energy difference between the reactants and the transition state. Chapter 417

18. Rates of Multistep Reactions The highest points in an energy diagram are transition states. The lowest points in an energy diagram are intermediates. The reaction step with the highest E a will be the slowest step and will determine the rate at which the reaction proceeds (rate-limiting step). Chapter 418

19. Energy Diagram for the Chlorination of Methane Chapter 419

20. Rate, E a, and Temperature XE a (per Mole)Rate at 27 °CRate at 227 °C F5140,000300,000 Cl17130018,000 Br759 x 10 -8 0.015 I1402 x 10 -19 2 x 10 -9 Chapter 420

21. Conclusions With increasing E a, rate decreases. With increasing temperature, rate increases. Fluorine reacts explosively. Chlorine reacts at a moderate rate. Bromine must be heated to react. Iodine does not react (detectably). Chapter 421

22. Consider the following reaction: This reaction has an activation energy (E a ) of +17 kJ/mol (+4 kcal/mol) and a  H° of +4 kJ/mol (+1 kcal/mol). Draw a reaction-energy diagram for this reaction. We draw a diagram that shows the products to be 4 kJ higher in energy than the reactants. The barrier is made to be 17 kJ higher in energy than the reactants. Solved Problem 2 Solution Chapter 422

23. Skill Building: Practice Problems Problem 4-5 thru 4-17