UNIT I Reaction Mechanisms

R EACTION M ECHANISMS Definition: the series (sequence) of steps by which a reaction takes place Ex.) 5C 2 O MnO H +  involves 23 reacting particles chance of this taking place in one step is almost “0”

R EACTION M ECHANISMS Even a 3 particle collision… 2H 2 (g) + O 2 (g)  probably doesn’t take place in a single step 1,000 times less probable than a 2 particle collision Most reactions (other than simple 2 particle collisions eg. Ag + + Cl -  AgCl(s) ) take place in a series of simple steps

P ROVINCIAL E XAM Q UESTIONS

R EACTION M ECHANISMS the series (sequence) of steps by which a reaction takes place 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 a mechanism from scratch some mechanisms are known, many are yet to be discovered

R EACTION M ECHANISMS Example For the overall reaction: 4HBr + O 2  2H 2 O + 2Br 2 5 reactant particles Doesn’t take place in a single step!

R EACTION M ECHANISMS 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)

R EACTION M ECHANISMS Each step is called an Elementary Process. The Rate Determining Step is the slowest step in the mechanism. The overall reaction can never be faster than the RDS. 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).

R EACTION M ECHANISMS In this case, increasing [HBr] or [O 2 ] would speed up Step 1 (the RDS) and hence the overall rate. Speeding up a fast step (not RDS) will have no effect on the overall rate. For example, adding HOOBr or HOBr has no effect.

D ETERMINING O VERALL R EACTION Cancel stuff which is identical on both sides and add up what’s left. Example 1 1. HBr + O 2  HOOBr 2. HBr + HOOBr  2HOBr 3. HOBr + HBr  H 2 O + Br 2 _______________________________

D ETERMINING O VERALL R EACTION Example 2 1.)A + 2X  AX 2 2.)AX 2 + X  AX + X 2 3.)AX + A  A 2 + X _______________________

D ETERMINING O VERALL R EACTION Example 3 The following reaction occurs in a 3 step mechanism. This is the overall reaction: 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:? Find step 3.

D ETERMINING O VERALL R EACTION Example 4 Consider the following reaction for the formation of HCl in the presence of light. 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 Determine Step 2 of the reaction mechanism.

P ROVINCIAL E XAM Q UESTIONS

R EACTION I NTERMEDIATE A species (atom, molecule or ion) which is produced in one step and used up in a later step. The reaction intermediate appears on the right first and then on the lower left. For the mechanism: 1. HBr + O 2  HOOBr 2. HBr + HOOBr  2HOBr 3. HOBr + HBr  H 2 O + Br 2 intermediates are ___________ & ___________

P ROVINCIAL E XAM Q UESTIONS

R EACTION I NTERMEDIATE Notes: An intermediate doesn’t accumulate (like a product) because as soon as it is formed, it gets used up again. Intermediates are not necessarily unstable (in other circumstances, they may last a while). An activated complex is very unstable and short-lived. It doesn’t usually obey bonding “rules.” Hebden Textbook Page 28 Questions #46-53

PE D IAGRAM FOR A R EACTION M ECHANISM AC (Step 1) AC (Step 2) AC (Step 3) HOOBr PE HBr + O 2 HOBr H 2 O + Br 2 Reaction Proceeds Label this diagram: RDS, Ea (Overall Rxn), Ea (Step 1), ∆H.

PE D IAGRAM FOR A R EACTION M ECHANISM Notes: each “bump” is a step the higher the bump (greater Ea), the slower the step the highest bump (from the reactants level) is for the RDS AC’s are found at top of bumps, intermediates in middle “valleys”, products in the final “valley” the Ea for the forward overall rxn is vertical distance from reactants to top of highest bump

PE D IAGRAM FOR A R EACTION M ECHANISM The Ea for the overall forward reaction is the difference in energy between the reactants and the top of the highest peak.

PE D IAGRAM FOR A R EACTION M ECHANISM Example: Given the following Potential Energy Diagram for a reaction mechanism: PE Reaction Proceeds

PE D IAGRAM FOR A R EACTION M ECHANISM 1. This mechanism has steps 2. Ea for overall rxn = ______kJ 3. Step is the RDS 4. Step is the fastest step. 5. The overall rxn is thermic 6.  H = kJ 7.  H for reverse rxn = kJ 8. Ea (reverse rxn) = kJ 9. RDS for reverse rxn is step ________

PE D IAGRAM FOR A R EACTION M ECHANISM Example: 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 (Ea) and the  H for the forward reaction. Hebden Textbook Page 30 Questions #54-55

C ATALYSTS Catalyst: an introduced substance which produces an alternate mechanism with a lower activation energy.

P ROVINCIAL E XAM Q UESTIONS

C ATALYSTS Notes:  energy required (Ea) is less with the catalyst, so at the same temperature, more molecules can make it over the “barrier” and reaction rate speeds up  catalyzed reactions usually involve more steps but it’s highest Ea (highest bump) is never as high as the uncatalyzed reaction  a catalyst NEVER changes the PE of reactants of products - only the route between them ( no change in  H! )  uncatalyzed reaction still continues at its own slow rate when a catalyst is added (usually insignificant compared to catalyzed rate)  if catalyst speeds up forward reaction, it also speeds up (reduces Ea for) the reverse reaction

C ATALYSTS

Catalysts sometimes work by:  providing a surface whose spacing of atoms is just right to break a reactant molecule and hold it for an attack from another reactant  helping to form an intermediate which can react more easily to form products enzymeAnimation ctions/projectfolder/animations/Catalyst2NOO2N 28.html ctions/projectfolder/animations/Catalyst2NOO2N 28.html

C ATALYSTS Example 2H 2 O 2 (l)  2H 2 O(l) + O 2 (g) (very slow uncatalyzed)  add some KI (I - ) Demonstration Catalyzed Mechanism : step 1) H 2 O 2 + I - ---> H 2 O + OI - step 2)H 2 O 2 + OI - ---> H 2 O + O 2 + I - overall 2H 2 O 2 ---> 2H 2 O + O 2

P ROVINCIAL E XAM Q UESTIONS

C ATALYSTS Read pages in Hebden Textbook Complete Questions #56-61 on page 34 Review examples of real catalysts on pages Complete Questions on page 36 Do Provincial Exam Questions on Unit 1 THIS IS THE END OF UNIT 1