Chapter 17 Reaction Rates and Equilibrium

1. Chapter 17 Reaction Rates and Equilibrium

2. Collision Theory When one substance is mixed with another, the two substances do not react on a macroscopic basis, but react as their individual particles (atoms, molecules, or ions) come together. Collision theory video

3. The factors that affect how these particles collide are the factors that influence the rate of a reaction.

4. Factors influencing the likelyhood of a reaction occurring: 1) Force of collision is sufficient

5. 2) Orientation of colliding particles during collision is exact

6. 3) Must meet the minimum activation energy requirement for the specific reaction

10. activated complex In order for the reaction to occur, the particles involved must collide with each other, the more often the particles collide, the faster the reaction occurs. Not every collision results in a reaction. The activated complex is the temporary arrangement of atoms as they change from reactants into products.(transition state)

11. Orientation Successful collision Unsuccessful collision

12. activation energy The colliding molecules must have enough energy to react and form an activated complex. This minimum amount of energy is called the activation energy.

13. Rate of a Reaction = speed of reactants turning into products Can be measured in : grams of reactant consumed/second or grams of product produced/second

15. Effect of the Nature of the Reactant The nature of the reactants involved will determine the kind of reaction that occurs. Reactions with bond rearrangements or electron transfer take longer than reactions without these changes. Ionic reactions (such as double displacement or neutralization) occur almost instantly. Simulation

16. Effect of Temperature on Reaction Rate • Increasing the temperature increases • the rate of a reaction. At higher temperatures, the particles have more energy, move faster and collide more frequently. • Increasing the temperature 10 degrees Celsius generally doubles the reaction rate. Simulation

17. Effect of Concentration on Reaction Rate Increasing the concentration of the reactants increases the rate of a reaction. When there are more particles per unit volume, they will collide more frequently, causing the rate to increase. Simulation

18. Effect of Particle Size/Surface Area Decreasing particle size/increasing surface area increases the rate of a reaction. When the surface area is increased, there is more contact between the reactants, the number of collisions increase, and therefore the rate of reaction also increases. Simulation

19. Simulation

20. Not Cattle List!!

21. Catalyst - any substance that is added to the reaction to INCREASE the rate. The catalyst DOES NOT react with any substance to produce product. Inhibitor - Any substance added to slow down the rate of reaction. Works against the catalyst.

22. C2H4 C2H2 H2 catalyst

23. A catalyst increases the rate of a reaction by lowering the activation energy.

25. Rate Law Expression Equation relating the rate of a reaction to the concentrations of the reactants and the specific rate constant. 2NO + 2H2 N2 + 2H20 Rate = k[NO]2[H2]2

26. Rate = k[NO]2[H2]2 Rate = speed of reactants turning into products [ ] = the concentration of k = specific rate constant for a reaction - if the rate is fast k will be high - if the rate is slow k will be low 2 (exponent)is used to predict the order of the reaction. The order reflects the affect of doubling the concentration of a reactant on the overall speed or rate of reactants becoming products.

27. Order of reaction affect on the rate of rxn when [ ] of reactant is “doubled” No affect 0 1st 2nd 3rd 4th Rate doubles (2)1 = 2x faster Rate quadruples (2)2 = 4x faster Rate octuples (2)3 = 8x faster Rate goes crazy (2)4 = 16x faster

28. Rate = k[NO]2[H2]2 What is the affect of doubling the concentration Of each of the reactants in the above expression? N2 + 3H2 2NH3 Write the rate law expression for the above reaction and predict the affect of doubling the concentration of each of the reactants. Also predict the overall reaction rate

29. In-Depth Rate law • Rate Laws

30. Reaction Mechanism Most reactions occur in a series of steps. Each step normally involves the collision of only two particles. There is little chance of three or more particles colliding with the proper position and sufficient energy to cause a reaction.

31. Reaction Mechanism If a reaction consists of several steps such as the following: A B B C C final product One of the steps will be slower than all the others. This step is called the rate determining step. The other faster steps will not affect the rate of the reaction. The series of steps that must occur for a reaction to go to completion is called the reaction mechanism.

32. Old Lady Driver or The rate of a reaction is determined by the slowest step in the reaction mechanism

33. Elementary steps -1st step is the rate determining step intermediate

34. Assignment 1 • Calculate tangents (rates of reaction) on Part C graphs of Lab 1 • Collision Theory Worksheet • Rate Expressions Worksheet • (see figure 17.8 as an example) • Do Q 4,6,21-28 Chp 17 • Do 17A • OBWS 1-4

35. Entropy A measure of the disorder of system Law of disorder Systems tend to go from a state of order(low entropy)To a state of maximum disorder (high entropy) spontaneously.

36. Entropy changes can be predicted I2 (s) I2 (g) Solids – low entropy Liquids –avg entropy Gases – high entropy Or You can predict the entropy by comparing The # of moles in the equation. 2H20(g) + 2Cl2(g) 4HCl(g) + O2(g) Fewer moles means lower entropy

37. Entropy( S) Calculations 2H2S + 3O2 2H2O + 2SO2 2mol (205.6J/Kmol) + 3 (205.O) = 2 (188.7) 2 (248.5) + 1026.2 J/K = 874.4J/K S = Products - Reactants S =874.4J/K - 1026.2J/K S = -151.8J/K A (-) entropy indicates a decrease in entropy

38. Spontaneous Reactions are reactions that are known to produce the written products. Copy table 17.2 page 408

39. A reaction tends to be spontaneous if : -the reaction is exothermic heat is released (- Enthalpy) -the entropy of the products is greater than the entropy of the reactants (+ Entropy)

40. Free Energy When a reaction occurs some energy known as free energy of the system becomes available to do work. Free energy may be available but is not always used efficiently. - Auto engine uses only 30% of free energy of burning gasoline. 70% is lost as friction and heat.

41. Spontaneous reactions release free energy and are said to be exergonic. (- Gibbs free energy)

42. (+ Gibbs free energy) Nonspontaneous reactions absorb free energy and are said to be endergonic.

43. Nonspontaneous reactions can be made spontaneous by: -increasing the temperature -by linking it to a spontaneous reaction that produces a large amount of free energy Which makes the nonspontaneous reaction go!!

44. C + O2 CO2 (0.0kj/mol) + (0.0kj/mol) = (-394.4kj/mol) 1mol (0.0kj/mol) + 1 (0.0kj/mol) = (-394.4kj/mol) 1 G = Products - Reactants G = -394.4kj - 0.0kj G = -394.4kj Gibb’s Free Energy Calculation A (-) free energy means the reaction is spontaneous

45. G = Gibb’s Free energy (kJ/mol) Table 17.4 Pg 414 H = Enthalpy (heat energy) (kJ/mol) Table 8.1 Pg 190 Free Energy Calculations G = H - T S

46. S = entropy (disorder) (j/K-mol) Table 17.1 pg 407 T = temperature (Kelvin) C + 273= K G = H - T S *All are calculated using Products – Reactants Method

47. G = H - T S CaCO3 CaO + CO2 at 25 C G = -178.5kJ/mol – [(298K)x(164.7J/Kmol)] G = -227.6 kJ/mol Is this reaction spontaneous? Try getting the Gibb’s Free energy. = -178.5 kJ/mol – 49080.6 J/mol = -178.5 kJ/mol – 49.1 kJ/mol Negative G means the reaction is spontaneous

48. Assignment 2 • 17B Entropy Problems • 17C Gibb’s Free Energy Problems

49. Reversible Reactions The conversion of reactants into products and the conversion of products into reactants occuring at the same time. A + B C Note double arrows

50. When the forward and reverse reactions are occurring at the same rate(speed) the reaction is said to be in a state of chemical equilibrium The equilibrium position can be shown by the double arrows and a 3rd arrow A B 1% 99% A B 99% 1%