Rate Laws

Introduction:

Origin of Rate Laws:

The Basics:

Thermodynamics versus Kinetics

Thermodynamics is all about the “if” of a reaction, such as whether the process can occur.  Kinetics is all about “how”, such as whether the reaction occurs fast or slow.  The kinetics of the process is how to overcome the energy barrier from the reactants to the products, while the thermodynamics ensures that the reaction is a favorable transformation.
For a process to occur, it must overcome the energy maximum, usually denoted E (activation energy).  The larger the barrier (Ea), which is the difference between the energy maximum and initial minimum, the more difficult the process to occur, which thus results in a slower rate.  For a reaction to occur, not only must the thermodynamically favored with a -ΔG (which could be referred to as the driving force), it must also be fast enough, which results from a small Ea.  
If a reaction is thermodynamically favored, that means that the products are in the lowest possible energy state.  If a reaction is kinetically favored, that means that the reaction went through the easiest possible direction, that is, it went in the direction with the lowest activation energy.  Given inifite amount of time, all reactions would proceed to the thermodynamically favored products.  However, if the reaction was only given a short time to proceed, some kinetically favored products would be in solution, however it is unlikely any thermodyamically favored products would, assuming the activation energy of the thermodynamically favored products is higher than that of the kinetically favored products.  The diagram below describes a possible reaction that is helpful in the understanding of thermodynamics versus kinetics. 

If you started with B and were asked what would be created, C or A, if the reaction was thermodynamically favored, the answer would be C.  Why?  C is at a lower, more stable energy than A, therefore, given infinite amount of time, all of B would eventually form C.  If you started with C and went in the reverse reaction, what would the rate determining step be? As stated before, the rate determining step is the one with the highest activation energy.  Therefore, the rate determining step would be from B to A, because that is the biggest energy “hump” to get over.

Below is another energy diagram that can be studied to help better understand the relationship, and difference, between kinetics and thermodynamics.   

Given the reactant D, what is the kinetically favored product, E or F?  The answer is E, but why?  Does that mean F is the thermodynamically favored product?  To answer the latter, E is the kinetically favored product because it has a lower activation energy i.e. the “hump” is smaller and easier to get over.  To answer the last question, F is not then, by process of elimination, the thermodynamically favored product.  Both products E and F have the same energy, therefore one product is not energetically favored versus the other.  However, if F was lower energy than E, even though the activation energy is higher than that of D to E, then F would be the thermodynamically favored product.  

Questions:

1. Kinetically favored reactions:

     a. have a comparitively high activiation energy

     b. have a comparitvely low activation energy

     c. are the lowest possible energy product

     d. a and c

     e. b and c

Answers: 1. b