Collision Geometry (comparing alignment)

Transcription

1 Collision Geometry (comparing alignment) Hebden # consider the rxn: 2 + B 2 2B: E.g. 1) + B B NO RXN E.g. 1collision has alignment (need E for collision to be effective) E.g. 2) B B B + + B B B Reactant Molecules ( 2 & B 2 ) PPROCH ECH OTHER They collide and form an COMPLEX ( B ) The C breaks apart to form the (2B) In the collision of e.g. 2 above, the reactants have alignment (less energy needed for an collision) E needed for e.g. 2 < E needed for e.g. 1 Summary of Collision Theory so far: For any successful collision there are 3 Requirements: Hebden # ) - particles must 2.) - they must collide with energy > Ea 3.) - they need to have correct ) (to keep Ea as low as possible) Ea and for forward and reverse reactions Reactants Products If products can attain E then the reaction is possible. 17

2 Eg..) Potential Energy RECTNTS Forward Reaction CTIVTED COMPLEX PRODUCTS Progress of Reaction 1) From the graph above, find: E a (fwd rx.) = kj (fwd rxn. ) = kj This fwd rxn is thermic 2. Considering reverse rxn: Use a different color pen and draw arrows for and E a for the reverse reaction. E a (rvs rxn.) = kj (rvs rxn ) = kj This rvs rxn is thermic Hebden # Reaction Mechanisms every long journey begins with a Most reactions (other than simple 2 particle collisions eg. g + + Cl - gcl(s) ) take place in a series of simple steps Reaction # of Reacting Particles Probability of Reaction Occuring in a Single Step 5C 2 O MnO H % 18

3 Reaction Mechanism: the (sequence) of steps by which a rxn takes place. cannot be determined by just at overall rxn. deduced through much study and (up to years) you will not be asked to come up with mechanism from scratch. some mechanisms are known, many are yet to be discovered. eg) overall rxn: 4HBr + O 2 2H 2 O + 2Br 2 5 reactant particles..does NOT take place in a single step The Mechanism (determined from lots of research): step 1: HBr + O 2 HOOBr (found to be slow) step 2: HBr + HOOBr 2HOBr (fast) step 3: HOBr + HBr H 2 O + Br 2 (very fast) Each step is called an Rate Determining Step (RDS): the step in the mechanism. Notes: the overall rxn can be faster than the RDS the only way to speed up an overall rxn is to speed up the RDS eg.) in the rxn above, increasing [HBr] or [O 2 ] would: - up Step 1 (the RDS) - and the overall. BUT speeding up a fast step (not the RDS) will have on the overall rate. (ie. [HOOBr] or [HOBr] has no effect on rate) Determining overall rxn given the steps (mechanism) Simply cancel things which appear on both sides and add up what s left. eg.) 1.) HBr + O 2 HOOBr 2.) HBr + HOOBr 2HOBr 3.) 2HBr + 2HOBr 2H 2 O + 2Br 2 overall rxn: 4HBr + O 2 2H 2 O + 2Br 2 19

4 eg.) 1.) + 2X X 2 2.) X 2 + X X + X 2 3.) X X overall rxn: Question: the following rxn occurs in a 3 step mechanism: B B 3+ this is the overall rxn step 1: 4+ + C 2+ C step 2: 4+ + C 3+ C step 3:? find step 3. nother Example: Consider the following rxn for the formation of HCl in the presence of light. Cl 2 + CHCl 3 HCl + CCl 4 The following is the proposed rxn mechanism: Step 1: Cl 2 Cl + Cl Step 2: Step 3: Cl + CCl 3 CCl 4 Determine Step 2 of the rxn mechanism. Step 2: Reaction intermediate -a species (atom, molecule or ion) which is in one step and up in a later step. (appears on right & also lower on left) 20

5 eg.) For the mechanism: 1) HBr + O 2 HOOBr 2) HBr + HOOBr 2HOBr 3) 2HBr + 2HOBr 2H 2 O + 2Br 2 intermediates are & Notes: an intermediate does not (like a product) because as soon as it is formed, it gets used up again intermediates are not necessarily (may last long) an activated complex(c) is very and short-lived. (doesn t usually have real bonds) has very high.only a temporary arrangement diagram for a Reaction Mechanism Hebden # C (step 1) see p. 26 SW C (step 2) see p.27 C (step 3) see p. 27 HOOBr HOBr HBr + O 2 STEP 1 STEP 2 STEP 3 H 2 O + Br 2 RECTION PROCEEDS Notes: each bump is a the higher the bump the Ea the slower the step the highest bump (measured from the reactants level) is the Cs are at of bumps, in middle valleys, at the end the Ea for the fwd overall rxn. is the vertical distance from reactants level to top of highest bump. Label the RDS on the last diagram. Draw an arrow label the following to show: 1) Ea (overall rxn) 2) Ea for Step 1. 3) for the overall rxn 21

6 NOTICE: E a is the difference in energy between the and the top of the peak. E a (Overall Rx.) RECTION PROCEEDS Question: Given this Diagram: Hebden # (KJ) Reaction Proceeds 1. This mechanism has steps 2. Ea for overall rx = kj 3. Step is the RDS 4. Step is the fastest step. 5. The overall rx. is thermic 6. = kj 7. for reverse rx. = kj 8. Ea (reverse rx.) = kj 9. RDS for reverse rx. is step 22

7 Catalysts: how they work - to avoid a hill, build a catalyst- an introduced substance which produce an mechanism with a Ea DIGRM SHOWING UNCTLYZED ND THE CTLYZED RXN 200 (kj) E a (uncatalyzed rx.) Uncatalyzed route 12 0 Catalyzed route RECTNTS E a (catalyzed rx.) PRODUCTS Progress of Reaction Notes the E required (Ea) is with the catalyst at same T, more molecs can make it over the barrier so rxn rate catalyzed rxns usually involve more steps but the highest Ea (highest bump) in catalyzed rxn is as high as in the uncatalyzed reaction a catalyst NEVER changes the of reactants of products - only the between them. (no change in ) an uncatalyzed rxn still at its own slow rate when a catalyst is added. reactions aer occurring. (the uncatalyzed rxn is usually insignificant compared to catalyzed rate) if catalyst speeds up rxn, it also speeds-up (reduces Ea for) the reverse rxn. 23

8 Hebden # Compare the ctivation Energies for the fwd and rvs uncatalyzed and catalyzed rxns: DIGRM SHOWING UNCTLYZED ND CTLYZED RXNS 2 00 (kj) Ea(f) (uncatalyzed ) Uncatalyzed route 12 0 Catalyzed route RECTNTS Ea(f) (catalyzed) PRODUCTS Ea (r) (Uncatalyzed ) Progress of Reaction Ea (r) (Catalyzed ) How Catalysts Work 1. May provide a whose spacing of atoms is just right to a reactant molecule and hold it for an attack from another reactant. Hebden # Read p Hebden # Helping to form an which can react more to form products. eg.) 2H 2 O 2(l) 2H 2 O (l) + O 2(g) (very slow uncatalyzed) Catalyzed Mechanism: step 1) H 2 O 2 + I - ---> H 2 O + OI - (The catalyst I- is put in.) step 2) H 2 O 2 + OI - ---> H 2 O + O 2 + I - (The catalyst I- is regenerated.) overall rx. 2H 2 O 2 ---> 2H 2 O + O 2 - Unit Test Coming Soon - 24