Brandon L.

asked • 01/28/20

Which corresponds to a faster rate: a mechanism with a small activation energy or a mechanism with a large activation energy? Explain your answer

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John M. answered • 01/28/20

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In order to sufficiently answer this question, there are a few things that must be appropriately defined, particularly in context with each other.


Activation Energy - the amount of energy required (think heat, light, etc.) to initiate a chemical reaction

The important takeaway here is that in order for a chemical reaction to occur, there must be some energy that starts the reaction.


Rate of Reaction - the amount of time that it takes for a chemical reaction to occur


If a mechanism has a large activation energy, we can deduce that more energy must be put into the system to react when compared to a mechanism with a small activation energy. In other words, to make the reaction with a small activation energy occur, we could use less energy to make it occur, thus making it faster.


Hope this helps!

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Dr. Douglass E.

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The reaction rate is NOT just dependent on Activation energy, it is also related to concentration of each reactant, and order of the reaction. Your answer assumes that reaction rate is ONLY dependent on activation energy which is not accurate. This is what I posted for another student: This subject is covered usually in General Chemistry 2, in the Equilibrium and Kinetics chapters. The reaction rate is due to many factors, including concentrations of each reactant, reaction conditions (like temperature), order of the reaction (including whether each concentration is 1, 0 or squared or higher order). In your main question, if reaction rate was ONLY dependent on Activation energy, even that would depend. It is like pushing a bowling ball over a small or big hill. A big guy could do it easily, as in his normal push, either one would be easy and so under those conditions the reaction rate would perhaps be fast. A child might have more difficulty pushing the bowling ball over a small hill, because he can provide less energy to move the ball. (The analogy is like a reaction that easily occurs at room temperature, like burning paper if you have a match to start the fire, or the same fire started when temperature is 0 degrees. There is plenty of energy for the match to start it, irrespective as to whether the paper is soaked with lighter fluid or not). The catalyst is like adding lighter fluid, because in that case a fire is started in the fluid first (with less energy required) then the paper starts burning. But, with low activation energy, the reaction can go BOTH ways, like pushing a ball back and forth over a small hill, versus a large hill which is much less likely if the larger hill end point is much lower than the starting point before the ball was moved. The hill aspect is essentially more closer related to the concept of activation energy.
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01/28/20

J.R. S.

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Whereas you are absolutely correct in pointing out the other variables upon which reaction rate may depend, I believe the question is meant to see if the student understands the relationship between Ea and reaction rate, assuming all other variables are constant. Otherwise, I believe the question would have phrased "what variables affect reaction rate and explain how"? Just my opinion.
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01/29/20

Dr. Douglass E. answered • 01/28/20

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It depends, as a mechanism with a small activation energy will often have equilibrium issues, that will drive the reaction in both directions, essentially making the rate appear slower to completion. Whereas an activation with a large activation energy might have kinetic features that force it completion under certain reaction conditions like the temperature that the reaction takes place may provide enough energy for the reaction to occur fast with or without a catalyst, where catalysts often provide a pathway with a lower activation energy.

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Brandon L.

Can you possibly dumb this for me a little?
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01/28/20

Dr. Douglass E.

tutor
This subject is covered usually in General Chemistry 2, in the Equilibrium and Kinetics chapters. The reaction rate is due to many factors, including concentrations of each reactant, reaction conditions (like temperature), order of the reaction (including whether each concentration is 1, 0 or squared or higher order). In your main question, if reaction rate was ONLY dependent on Activation energy, even that would depend. It is like pushing a bowling ball over a small or big hill. A big guy could do it easily, as in his normal push, either one would be easy and so under those conditions the reaction rate would perhaps be fast. A child might have more difficulty pushing the bowling ball over a small hill, because he can provide less energy to move the ball. (The analogy is like a reaction that easily occurs at room temperature, like burning paper if you have a match to start the fire, or the same fire started when temperature is 0 degrees. There is plenty of energy for the match to start it, irrespective as to whether the paper is soaked with lighter fluid or not). The catalyst is like adding lighter fluid, because in that case a fire is started in the fluid first (with less energy required) then the paper starts burning. But, with low activation energy, the reaction can go BOTH ways, like pushing a ball back and forth over a small hill, versus a large hill which is much less likely if the larger hill end point is much lower than the starting point before the ball was moved. The hill aspect is essentially more closer related to the concept of activation energy.
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01/28/20

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