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Topic: Reaction Mechanisms(Confused)  (Read 9002 times)

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777888

  • Guest
Reaction Mechanisms(Confused)
« on: January 18, 2005, 04:58:31 PM »
1.Consider the overall rxn: A2+ 2B-> 2AB  where 2 elements combine to form compound. State a possible mechanism, indicating the slowest(rete-determining)step.
I suggest this mechanism, is this correct?
A2 -> 2A (slow)
2A+2B->2AB (fast)
Can I make another one that has 3 or more steps?

2.Consider the following mechanism where A,B,E may be elements or compounds,and C,D,F are ompounds:
(1) 2A+B->C
(2) C->D
(3) D+E->F
Which step is most likely to be the rate-determining step?Explain.

I really have no idea how to answer this question...there is no rete information...

3.For a multistep rxn,a potential energy diagram is drawn. Determine the rate-determining step.
E(reactants)=0kJ
E(activated complex 1)=50kJ, this "peak" is labelled V
E(rxn intermediate 1)=30kJ, this "trough" is labelled W
E(activated complex 2)=60kJ, this peak is labelled X
E(products)=-35kJ

I am guessing, would the answer be W->X, or just  X, or X->reactants? Which part would be the rate-determining step?

The questions are quite short, but I don't get them. Please help me! Thank you so much!
« Last Edit: January 18, 2005, 11:18:53 PM by 777888 »

777888

  • Guest
Re:Reaction Mechanisms(Confused)
« Reply #1 on: January 18, 2005, 11:38:31 PM »
Is anyone here? :-\

Demotivator

  • Guest
Re:Reaction Mechanisms(Confused)
« Reply #2 on: January 18, 2005, 11:57:46 PM »
1) OK, but instead of
  2A+2B->2AB (fast)
  just
 A+B->AB (fast)
because this is a mechanism. Just two molecules are involved or collide, not four.

2) I guess step 1 because the probability of three molecules colliding successfully is smallest.( No guarantees.)

3) I would say the first step, reactants to V. It has the largest energy difference (50kj) to the activated complex. . Between W and X is just 30kj.

777888

  • Guest
Re:Reaction Mechanisms(Confused)
« Reply #3 on: January 19, 2005, 12:32:55 AM »
1) OK, but instead of
  2A+2B->2AB (fast)
  just
 A+B->AB (fast)
because this is a mechanism. Just two molecules are involved or collide, not four.

2) I guess step 1 because the probability of three molecules colliding successfully is smallest.( No guarantees.)

3) I would say the first step, reactants to V. It has the largest energy difference (50kj) to the activated complex. . Between W and X is just 30kj.
Thank you!

2.Do you look at the coefficients in the balanced chem. equations?

3.But "E(activated complex 2)=60kJ, this peak is labelled X" has the highest potential energy in the wholte graph...(the overall activation energy is then 60kJ)...
« Last Edit: January 19, 2005, 12:37:30 AM by 777888 »

Demotivator

  • Guest
Re:Reaction Mechanisms(Confused)
« Reply #4 on: January 19, 2005, 09:28:28 AM »
2. I look at the coefficients of the balanced mechanism equations.

3. This is a stepwise mechanism. In going to X, the intermediate W already has a potential energy of 30 and needs only 30 more (not 60) to get to X for that step.

777888

  • Guest
Re:Reaction Mechanisms(Confused)
« Reply #5 on: January 19, 2005, 03:25:12 PM »
2. I look at the coefficients of the balanced mechanism equations.

3. This is a stepwise mechanism. In going to X, the intermediate W already has a potential energy of 30 and needs only 30 more (not 60) to get to X for that step.
Oh i see!How about Q1, is there a restriction on the number of elementary steps? Or can you make as many elementary steps as you like?

Demotivator

  • Guest
Re:Reaction Mechanisms(Confused)
« Reply #6 on: January 19, 2005, 04:17:16 PM »
There is no restriction on the number of steps.
All mechanistic schemes are just hypothetical until experimental evidence decides which ones are plausible and which are not.

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