Can someone explain the Transition State Theory? I have looked through the wikipedia page but I am still confused. I understand what transition states are how the reaction progresses through the the reaction energy diagrams.
Transition State Theory
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I am having trouble with understanding the equation (I don't want to just randomly memorize it). I came across in the content review (not any specific practice question) and just trying to understand it.
Well, the Arrhenius equation gives you the reaction rate based on the energy of activation and temperature. So the best way to understand it is to dissect it. First, collision theory predicts that reactions occur when molecules collide. When reactants A and B collide, they form an activated complex that has enough energy to go over the energy barrier. But these activated complexes may not be in the right geometry to do so. Imagine an SN2 reaction where the nucleophile is on the wrong side of the sigma anti-bonding orbital that is attacked. So the colliding species must also be in the correct orientation. Finally, if collisions occur a lot, there is a greater chance for reaction as opposed to if collision frequency is low.
So the Arrhenius equation is k = A*e^-E/RT. The pre-exponential term A combines the frequency and orientation factors. So if frequency is high, the reaction is more likely to occur and occurs faster. The E takes into account the energy barrier. If the energy barrier is high, the rate approaches zero - what you would expect if molecules were in an infinite potential well. Finally, T takes into account temperature. If temperature is high, there is a greater chance that the colliding molecules will have enough energy to go over the energy barrier. Thus, as T increases, k also increases.
You can derive the Arrhenius equation but I won't do it here - just Google that.
The Eyring equation is : k = (kb*T/h)*e^(-delta H/RT)*e^(delta S/R), where the thermodynamic parameters refer to the transition state. So you can see that the Eyring equation has basically set the pre-exponential factor. In addition, the "transition state" referred to by these two equations are not identical but I believe that is beyond the scope of the MCAT. You can also derive the Eyring equation but it is not intuitive at all and definitely beyond the scope of the MCAT.
You can also parse through the terms of the Eyring equation as well to rationalize the roles enthalpy and entropy play in determining rate constant.
So the Arrhenius equation is k = A*e^-E/RT. The pre-exponential term A combines the frequency and orientation factors. So if frequency is high, the reaction is more likely to occur and occurs faster. The E takes into account the energy barrier. If the energy barrier is high, the rate approaches zero - what you would expect if molecules were in an infinite potential well. Finally, T takes into account temperature. If temperature is high, there is a greater chance that the colliding molecules will have enough energy to go over the energy barrier. Thus, as T increases, k also increases.
You can derive the Arrhenius equation but I won't do it here - just Google that.
The Eyring equation is : k = (kb*T/h)*e^(-delta H/RT)*e^(delta S/R), where the thermodynamic parameters refer to the transition state. So you can see that the Eyring equation has basically set the pre-exponential factor. In addition, the "transition state" referred to by these two equations are not identical but I believe that is beyond the scope of the MCAT. You can also derive the Eyring equation but it is not intuitive at all and definitely beyond the scope of the MCAT.
You can also parse through the terms of the Eyring equation as well to rationalize the roles enthalpy and entropy play in determining rate constant.
I thought the Eyring equation was k = (kb*T/h)*e^(-Ea/RT). How did you tie in the thermodynamic quantities into the equation?
Does the A from the arrhenius equation the same as (kb*T/h) from Eyring equation?
I am still confused about what the difference between the two is?
Does the A from the arrhenius equation the same as (kb*T/h) from Eyring equation?
I am still confused about what the difference between the two is?
Eyring equation has -delta G where you have "-Ea" above. So the thermodynamic quantities come from the definition of Gibbs energy. delta G and Ea are not the same in this case - that's the difference between transition state theory and the classical Arrhenius equation. That is likely beyond the scope of the MCAT.
The A in the Arrhenius equation is related to the pre-exponential in the Eyring equation but is not the same because, as noted above, the delta G and Ea are actually referring to two slightly different states and thus are not the same.
The difference between the Arrhenius equation and the Eyring equation is that A must be experimentally determined in the Arrhenius equation whereas the pre-exponential in the Eyring equation does not. It basically allows you to be able to predict the rate constant with less experimental information.
The distinction is likely not one you learned in gen chem and thus is not very relevant for the MCAT - you just need to understand what the equations are and what they mean.
The A in the Arrhenius equation is related to the pre-exponential in the Eyring equation but is not the same because, as noted above, the delta G and Ea are actually referring to two slightly different states and thus are not the same.
The difference between the Arrhenius equation and the Eyring equation is that A must be experimentally determined in the Arrhenius equation whereas the pre-exponential in the Eyring equation does not. It basically allows you to be able to predict the rate constant with less experimental information.
The distinction is likely not one you learned in gen chem and thus is not very relevant for the MCAT - you just need to understand what the equations are and what they mean.
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The Transition state theory is used to explain the reaction rates of the chemical reactions. The theory assumes a special type of chemical equilibrium (quasi-equilibrium) between reactants and activated transition state complexes.
I think the above explanation should help you.
I think the above explanation should help you.
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