AP Chemistry 12 Reaction Kinetics III Name: Date: Block: 1. Catalysts 2. Mechanisms Catalysts Catalysts provide an alternate reaction pathway in which a different, activated complex can form. Catalysts must be consumed in one step of a reaction mechanism and regenerated in a later step. There are two types of catalysts: Heterogeneous The catalyst is in a phase from the reactants. Typical example: solid catalyst with reactants as either liquids or gases Homogeneous The catalyst is in the phase as the reactants.
Mechanisms Elephant s Toothpaste Overall Reaction: Catalyst: Mechanism: Reaction pathways are called. A reaction mechanism is a series of steps that may be added together to give an overall chemical reaction. Cannot be determined by just looking at overall reaction. Deduced through much study and research (up to years) You will not be asked to come up with mechanism from scratch. Some mechanisms are known, however, many are yet to be discovered. Example: (known mechanism) 4 HBr + O 2 2 H 2O + 2 Br 2 Mechanism (determined from lots of research) step 1: HBr + O 2 HOOBr (found to be slow) step 2: HBr + HOOBr 2 HOBr (fast) step 3: 2 HOBr + 2 HBr 2 H 2O + 2 Br 2 (very fast) Each step is called an. ü An is a species that is formed in one step and consumed in a subsequent step and does not appear in the overall reaction. ü Catalysts must be in one step of a reaction mechanism and in a later step. Rate determining step (RDS) - the step in the mechanism. The only way to speed up an overall reaction is to speed up the RDS (eg. by increasing. the concentration of a reactant in the RDS) Speeding up a fast step (not RDS) will have no effect on the overall rate. (eg. adding HOOBr or HOBr has no effect) ü The reactants in the RDS are in the rate law for the reaction.
To determining overall reaction, cancel stuff, which is identical on both sides - add up what s left. 1.) A + 2X AX 2 2.) AX 2 + X AX + X 2 3.) AX + A A 2 + X Overall reaction: Intermediates: Catalysts: The following reaction occurs in a 3- step mechanism. Determine the 3 rd step of the mechanism: 2A 4+ + B + 2A 3+ + B 3+ step 1: A 4+ + C 2+ C 3+ + A 3+ step 2: A 4+ + C 3+ C 4+ + A 3+ step 3: Consider the following reaction for the formation of HCl in the presence of light. Determine the 2 nd step of the mechanism. Cl 2 + CHCl 3 HCl + CCl 4 The following is the proposed reaction mechanism: Step 1: Cl 2 Cl + Cl Step 2: Step 3: Cl + CCl 3 CCl 4
Returning back to 4 HBr + O 2 2 H 2O + 2 Br 2 step 1: HBr + O 2 HOOBr (found to be slow) step 2: HBr + HOOBr 2 HOBr (fast) step 3: 2 HOBr + 2 HBr 2 H 2O + 2 Br 2 (very fast) PE diagram for the reaction mechanism AC (step 1) AC (step 2) AC (step 3) PE HOOBr HOBr HBr + O2 STEP 1 STEP 2 STEP 3 H2O + Br2 REACTION PROCEEDS Notes: Ø Each bump is a step Ø The higher the bump, (greater Ea) the the step Ø The highest bump (from the reactants level) is for the. Ø AC s at of bumps, intermediates in valleys, products in the valley Ø The Ea for the forward overall reaction is vertical distance from to top of highest bump. Given the following Potential Energy Diagram for a reaction mechanism: 1. This mechanism has steps. PE (KJ) Number each step. 80 70 60 50 2. Ea for overall reaction = kj 3. Step is the RDS 4. Step is the fastest step. 5. The overall reaction is thermic. 6. ΔΗ = kj 7. ΔΗ for reverse rx. = kj Reaction Proceeds 8. Ea (reverse rx.) = kj 9. RDS for reverse reaction is step
Draw a Potential Energy Diagram for a reaction mechanism with 2 steps. The first step is fast and the second step is slow. The overall reaction is exothermic. With labeled arrows show the overall Activation Energy (E a ) and the ΔΗ for the forward reaction. PE Reaction Proceeds 1. In the following reaction mechanisms, identify: i. The catalyst ii. The reaction intermediates iii. The overall reaction a) 2 NO à N 2O 2 N 2O 2 + H 2 à N 2O + H 2O N 2O + H 2 à N 2 + H 2O i. ii. iii. b) A + B à C C + D à E + F E + B à D + F i. ii. iii. 2. All catalyzed reaction mechanisms have more than one step. Why must this statement be true? 3. Supposed a catalyzed reaction is occurring in a reaction container. If the catalyst were removed would the reaction stop completely? Explain your answer.
4. Consider the following reaction: 2 NO (g) + O 2 (g) 2 NO 2 (g) The rate law is found through experiments to be rate = k[no] 2. A valid mechanism is consistent with the overall equation and the experimentally- determined rate law. Which of the following proposed mechanisms is consistent with the reaction data? Explain your answer. Mechanism 1: (1) NO + O 2 NO 2 + O slow (2) NO + O NO 2 fast Mechanism 2: (1) NO + O 2 D NO 3 fast equilibrium (2) NO 3 + NO 2 NO 2 slow Mechanism 3: (1) 2 NO N 2O 2 slow (2) N 2O 2 + O 2 2 NO 2 fast Complete PE Diagram Worksheet Hebden Workbook Pg. 36 #62, 63