question about kinetics and effective collisions

[Bis-GMA111]

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it was my understanding that the more reactants you had, the more effective collisions there would be, leading to a higher reaction rate...

however, a destroyer question implies the opposite when it says that the more reactants you have, the less chances there are of having a "fruitful" collision. which is correct and why?

thanks.

 

[needzmoar]

it was my understanding that the more reactants you had, the more effective collisions there would be, leading to a higher reaction rate...

however, a destroyer question implies the opposite when it says that the more reactants you have, the less chances there are of having a "fruitful" collision. which is correct and why?

thanks.

does the question mention anything about the ratio of reactants?

 

[Bis-GMA111]

the question is...

"Consider the following elementary steps; which would be the least likely to occur?"

a. X+Y---->Z
b. 2X+Y--->Z
c. 2X+2Y--->W+Z
d. 2X-->X+Y
e. 2X+Y--->2W+Z

 

[needzmoar]

the question is...

"Consider the following elementary steps; which would be the least likely to occur?"

a. X+Y---->Z
b. 2X+Y--->Z
c. 2X+2Y--->W+Z
d. 2X-->X+Y
e. 2X+Y--->2W+Z

You're just comparing the molecularity of the elementary step. In order for a fruitful collision to occur, the molecules must collide with enough energy and at the right orientation. This is why trimolecular and tertmolecular collisions aren't as likely to occur. Choice A and D are both bimolecular but look at the species involved. Choice A is less likely than D because with A, you have molecules of X colliding with each other (Z is not produced) and molecules of Y colliding with each other (Z is not produced).

 

[jwnichols21]

What is the answer to this Q? OP implies something different than poster above me.

 

[mh0000]

As needz said, you need to consider what is happening at the molecular level. A trimolecular reaction requires three separate molecules/atoms to collide at the same time, with an appropriate amount of energy to overcome the activation energy barrier, and they need to collide with the correct orientation. The vast majority of collisions of even simple reactions (bimolecular) don't result in a reaction. So the more reactants involved, the less likely all of the stated conditions will occur. The probability of correct collision orientation and the fraction of collisions with sufficient thermal energy are incorporated into the rate-law constant. Now, increasing the concentration of the reactants doesn't change these requirements for a "fruitful" reaction, but rather increaes the number of collisions to can happen, so this will increase the rate. THis is reflected in the concentration terms of the rate-law.