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 Lab 1 Factors Affecting Rates of Reaction  Create Data Tables  Do Part C,B,D,A  Graph Parts C, D

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

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

6 2) Orientation of colliding particles during collision is exact

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

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11 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. activated complex The activated complex is the temporary arrangement of atoms as they change from reactants into products.(transition state)

12 Successful collision Unsuccessful collision Orientation

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

14 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

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16 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.

17 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 celcius generally doubles the reaction rate.

18 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.

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

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21 Not Cattle List!!

22 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.

23 C2H2C2H2 catalyst H2H2 C2H4C2H4

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

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26 Equation relating the rate of a reaction to the concentrations of the reactants and the specific rate constant. Rate Law Expression 2NO + 2H 2 N 2 + 2H 2 0 Rate = k[NO] 2 [H 2 ] 2

27 Rate = k[NO] 2 [H 2 ] 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.

28 Order of reaction affect on the rate of rxn when [ ] of reactant is “doubled” 0 1 st 2 nd 3 rd 4 th No affect 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

29 Rate = k[NO] 2 [H 2 ] 2 What is the affect of doubling the concentration Of each of the reactants in the above expression? N 2 + 3H 2 2NH 3 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

30 In-Depth Rate law  Rate Laws Rate Laws Rate Laws

31 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.

32 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. Reaction Mechanism

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

34 Elementary steps -1 st step is the rate determining step intermediate

35 Assignment 1  Calculate tangents (rates of reaction) on Part C graphs of Lab 1  (see figure 17.8 as an example)  Do Q 4,6,21-28 Chp 17  Do 17A  OBWS 1-4

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

37 Entropy changes can be predicted I 2 (s) I 2 (g) Solids – low entropy Liquids –avg entropy Gases – high entropy Or You can predict the entropy by comparing The # of moles in the equation. 2H 2 0 (g) + 2Cl 2(g) 4HCl (g) + O 2(g) Fewer moles means lower entropy

38 2H 2 S + 3O 2 2H 2 O + 2SO 2 (188.7)(205.O)(205.6 J/Kmol )(248.5)=+ + 2mol J/K=874.4J/K A (-) entropy indicates a decrease in entropy S = Products - Reactants S =874.4J/K J/KS = J/K Entropy( S) Calculations

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

40 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)

41 When a reaction occurs some energy known as free energy of the system becomes available to do work. Free Energy 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.

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

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

44 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!!

45 C + O 2 CO 2 (0.0kj/mol) +(0.0kj/mol) =(-394.4kj/mol) (0.0kj/mol) +(0.0kj/mol) =(-394.4kj/mol)1mol1 1 A (-) free energy means the reaction is spontaneous G = Products - Reactants G = kj - 0.0kj G = kj Gibb’s Free Energy Calculation

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

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

48 CaCO 3 CaO + CO 2 at 25 C G = kJ/mol – [(298K)x(164.7J/Kmol)] G = kJ/mol G = H - T S Negative G means the reaction is spontaneous = kJ/mol – J/mol = kJ/mol – 49.1 kJ/mol Is this reaction spontaneous? Try getting the Gibb’s Free energy.

49 Assignment 2  17B Entropy Problems  17C Gibb’s Free Energy Problems

50 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

51 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 3 rd arrow A B 1%99% A B 99%1%

52 Catalysts will have no effect on the equilibrium because they speed up both the forward and reverse reactions equally. A B catalyst

53 Assignment 3  Lab E1 : Analogy for an Equilibrium Reaction

54 Equilibrium constants (Keq) -Indicate if the reverse reaction will favor reactants or products at equilibrium aA + bB cC + dD Keq = [C] c [D] d [A] a [B] b Keq > 1 more products than reactants at eq Keq < 1 more reactants than products at eq Keq expression products reactants

55 Assignment 4  I initial C hange E quilibrium Section 16.3 Equilibrium Calculations for Homogeneous Systems pg ,665 (see Section 16.3 Summary pg 665) (see Section 16.3 Summary pg 665)  17D Equilibrium Calculations

56 Henry Louis Le Chatelier If a chemical system at equilibrium experiences a change in concentration, temperature, or total pressure; the equilibrium will shift in order to minimize that change.concentrationtemperature pressure

57 Le Châtelier’s Principle If a stress is applied to a system at equilibrium, the equilibrium will shift to relieve the stress.

58 Equilibrium = A state in which no net change takes place in a system

59 In a reaction equilibrium, both the forward and reverse reactions are taking place simultaneously, at the same rate, so that no net change occurs. reaction equilibriumreaction equilibrium N 2 (g) + 3H 2 (g) 2NH 3 (g)

60 Dynamic Equilibrium = An equilibrium in which two or more changes are taking place simultaneously, but at the same rate. Also called…

61 How can you shift a system that is already in equilibrium? By adding a stress on the system!! Why would anyone want to shift a system in equilibrium? To make more product!! LIKE…

62 Concentration Change Reactants Products Increase [reactant] = shifts to use up the added reactants and produce more products Shifts right Increase [product] = shifts to use up added product and produce more reactants Shifts left Reactants Products

63 Decrease [reactant] = shifts to produce more reactantsShifts left Decrease [product] = shifts to produce more product Shifts right Reactants Products

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65 Adding heat to increase stress

66 Effect of Temperature on a System In Equilibrium  Video Video

67 A + B C + heat Increase = favors the endothermic direction. The reaction shifts in the direction that uses up the added energy. Temperature Change Decreasing = favours the exothermic direction. The reaction shifts in the direction that produces energy.

68 NO (g) NO 2 (g) colorless brown

69 Pressure/Volume Change

70 Increasing Pressure or Decreasing Volume favors fewer gas molecules. The reaction shifts to relieve the pressure. 2A + B C

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72 Decreasing Pressure or Increasing Volume favors more gas molecules. The reaction shifts to restore the pressure. 2A + B C

73 Assignment 5  Online Lab : Le Chatelier’s Principle  Complete OBWS  Review Sheet 12  Test