For a long time, I am confused with the phrase thermodynamics and kinetics. In my opinion, thermodynamics reflect the possibility of interactions, and it is represented by parameters like enthalpy, entropy and free energy. Based on that, interaction forces and interaction sites can be deduced. Kinetics demonstrate the interaction rate and affinity. But I am confused when people say the reaction system is under thermodynamics or kinetics control. Can anyone help me?
From the greek origin of these words:
* dynamics deals with forces
* kinetics deals with velocities
Often however, dynamics is used to refer to any kind of time evolution in physics, such that kinetics and dynamics are often used meaning the same thing.
Thermodynamics is a macroscopic description of the corresponding potentials, as the ones you mentioned.
Chemical kinetics deals with the rates of chemical reactions.
Not to be confused with kinetic theory of gases, which gives a microscopic foundation of thermodynamics.
Thermodynamic control refers to the free-energy change between the reactants and the products. The lower the free energy of the products with respect to the reactants, the more thermodynamically favored a reaction is. Kinetic control refers to the peak free energy of the intermediate state between reactants and products. If this is far above the free energy of either reactants or products, few if any atoms or molecules can surmount it, and hence the reaction will be very slow even if the free energy of the products is much lower than that of the reactants and the reaction is therefore strongly thermodynamically favored.
Thermodynamic or kinetic control are terms used to describe the physical parameters governing a particular reaction that can afford multiple products. For example, you can either form a thermodynamic enolate or the kinetic enolate from the same molecule if it has multiple alpha protons.
The thermodynamic enolate is more stable (higher substituted olefin...lower ΔG) but the alpha proton to be deprotonated to form the enolate is more sterically encumbered and therefore the rate of the reaction will be slower to form this enolate (higher ΔG‡). The kinetic enolate is less stable (less substituted olefin...higher ΔG) but the alpha proton is more accessible and therefore the rate of reaction will be faster to form this enolate (lower ΔG‡).
If you conduct your reaction under kinetic control then you will predominately form the kinetic enolate because it has the faster rate of formation than the thermodynamic enolate. How do you conduct reaction under kinetic control? One example is cold temperatures. At cold temperatures (cold is relative, it depends on the activation energy), there is not enough energy in the system for reaction to be reversible. Therefore, at cold temperatures you will predominately get your kinetic enolate because it has faster rate of formation and once the enolate is formed it will not go in reverse direction. So when you think kinetic control, think irreversible. Another way to form kinetic enolate is to use bulky base. The bulky base will deprotonate the least sterically encumbered proton the fastest.
Under thermodynamic control you will predominately form thermodynamic enolate because it is the more stable enolate. To conduct under thermodynamic control, high temperatures are used (high is relative, it depends on activation energy). At high temperatures, there is enough energy in system for reaction to be reversible. So although the kinetic enolate still has faster rate of formation, because the reaction can now go in reverse, over time, you will start to form the more stable enolate. So when you think kinetic control, think reversible. Another way to form thermodynamic enolate is to use small base. The small base can access both alpha protons but since it is reversible there will be equilibrium between deprotonation reactions and under thermodynamic control the most stable species will dominate.
This example of kinetic and thermodynamic enolates is just one example. There are many reactions which can produce multiple products and the proportion of products can be altered by conducting reaction under thermodynamic or kinetic control.
Sometimes, the thermodynamic product (most stable) and kinetic product (forms the fastest) are the same and hence that product will always dominate even if there is another possibility.
Lastly, you can look up tunneling control if you are interested.
Li Zeng,
Suppose for argument's sake that the reaction is exoergic overall (free energy of products less than free energy of reactants). Imagine the free-energy landscape as topography, the higher the elevation, the greater the free energy. The reactants are in a metastable state, trapped in a high valley. To get to products --- the absolutely stable state, the low valley --- the system must cross a ridge. The peak of the ridge is the intermediate state between reactants and products.
All answers stated above give good explanations. However, there is a real difficulty to give a perfectly satisfactory answer to your question as the scientific concepts which explain what happens may diversify at the atomic level. I'll explain the difficulties rather than answering your question.
Think of a first order irreversible chemical reaction such that A decays to B. If the initial number of A is Ao, then rate equation gives A=Aoexp(-kt) where k is the rate constant. This relation says, A is never PERFECTLY converted to B; it needs infinite time for all A to go to B. Here is one of the logical difficulties; things are never perfectly converted into another thing in the universe. The rate constant k can be expressed in terms the activation energy, or chemical potentials of A and B, say mA and mB, where m stands for Greek letter mu. In thermodynamics the reaction stops when the Gibbs free energy difference (deltaG) is equal to zero;
deltaG=nA mA - nB mB=0 where n stands for number of molecules, A and B stand for A and B.
Here is another logical difficulty; the Gibbs free energy difference cannot be equal to zero if mB is zero. However, if mB=0 then from kinetic point of view all A can be converted to B even if it takes too long time. If mB is infinitesmall but not exactly zero then the Gibbs free energy difference can be equal to zero, but not all A is converted to B as the kinetic theory says so. Chemical reactions are reversible or irreversible at the macro state, i.e. when there is say an Avagadro number of molecules. When we have a single molecule it becomes difficult to interpret the changes in terms of kinetic and thermodynamic formalism.
Now let us go classical physics. Consider two trains running on the same railroad approaching each other at different velocities. We can calculate at what kilometer they hit each other. Now substitute the trains with the molecules, or much better with two electrons. They never hit each other because the Coulombic repulsion goes to infinity as the distance between them becomes zero, i.e. if they hit each other. An infinite amount of energy is needed for them to hit each other. This is somehow true also for two atoms; only the outmost-shell electrons rearrange each other on collision to give a molecule. Do inner part of atoms touch each other? But quantum mechanics just tells us the other way around. Any two electrons, or atoms, or big bodies are in touch with each other as their wave functions extend in space everywhere in the universe. Say, all things are in contact with each other according to quantum mechanics.
If you are after what really happens you have to go to the dynamics at the atomic scale. If you are just looking how the bulk behaves use either one kinetic or thermodynamic approach, whichever suits to what you are looking for.
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