What makes a molecule thermodynamically stable

Thermodynamically stable. Post by » Sun Feb 23, pm I understand that kinetics deals with the rate of a reaction and thermodynamics deals with whether the rxn is forward or backwards.

Re: Kinetically stable vs. Post by JD » Mon Feb 24, am Kinetic stability basically occurs when the reactants react really slowly. The slower the reaction occurs, the greater the kinetic stability. If you say, "This reaction is kinetically stable," then that implies that the reaction occurs very slowly. Thermodynamic stability depends on whether or not the reaction is spontaneous. A thermodynamically stable reaction is one that basically does not react. As a result, it is independent of the pathway between reactants and products.

I also understand local and global minima, as discussed in this answer which references a single set of atoms that undergoes transformation. The problem arises when comparing compounds with different sets of atoms. Thermodynamics usually is pretty well defined. Thermodynamic stability, however, is a bit of a mystery to me. Similarly, a diamond is not forever which may not please De Beers and ladies. It is thermodynamically unstable with respect to conversion to graphite.

Also, thermodynamic stability is a relative term which is often contrasted with reactivity or kinetic stability. Diamond is kinetically stable at room temperature for the same process lucky ladies can smile again. Some background information is necessary to make sense of this. I hope you will find the following helpful! Phenomenologically, thermodynamic stability is the absence of visible change. This is the 'original' definition, employed by experimentalists during the 18th and 19th centuries.

If repeated observations of your system - such as measurements of its temperature, pressure, density, colour, etc - don't indicate any change, you can tentatively regard it as stable. Why tentatively? Because as you've alluded to, some changes can be tortuously slow, so unstable systems can appear stable because the intervals between each observation are fleeting by comparison with the system's rate of change. Such states are called 'metastable'.

The existence of metastable states severely limits the scope of this observational-based definition of stability. A more fundamental definition, that can distinguish between truly stable and merely metastable states is clearly desirable.

This alternative, quantitative approach, involves measuring the energy changes that accompany different chemical reactions. This is a tricky process, because the differnet forms of energy transfer accompanying any reaction can be are numerous: heat thermal conduction ; work exertion of a force or pressure ; current transfer of charge across an electrical potential ; to name the most common ones. The basis of this approach is that chemical compounds store energy in their bonds, so by tabulating the energy changes associated with many different reactions, their capacities for storing energy can be calculated.

But remember! The defining property of energy is that it is conserved! A table of bond energies such as that described above cannot by itself function as an indicator of stability. A final step is needed, which is to identify a particular form of energy that is minimized by all chemical reactions, and which will therefore be amenable to the kind of 'potential well' analysis described in the linked answer by Thomij.

This form, commonly called 'Gibbs energy', is the energy associated with a change in entropy. It's the increase of entropy that is the true driver of spontaneous processes. Accordingly, the condition of maximum stability for a chemical system is defined by the maximization of its entropy. Entropy is a measure of how the energy in a system is distributed among it's constituent particles.

More statistically probable distributions have higher entropy. The most probable distribution has the highest entropy. Entropy is often described as a measure of disorder, although I personally find this exposition misleading.

Entropy is a subtle and unnerving concept, which whole books have been written about, and which I've taken literally years to make peace with. Transfers of energy always accompany some other change, such as an increase in volume, or a flow of current, or a transfer of mass. Indeed, the 'forms' of energy familiar from high school are defined by the nature of their accompanying change e.

An increase in entropy is simply another admittedly more obscure example of this. But you can think of entropy as a property somewhat analogous to volume, in the sense of being a feature of a system that can be changed by the application of a particular form of energy.

In the case of entropy, the corresponding 'form' of energy is heat, rather than work. As alluded to here, there is a very close connection between temperature and entropy; in fact, a definition of temperature is the limiting ratio between the heat supplied to a system and the change in entropy that results. That hump is a measure of how difficult it is to get the reaction to go in any reasonable amount of time. For diamond, the hump for the conversion into graphite is high. Even though the reaction should go thermodynamically , it does not because it is kinetically unfavorable.

Why is the hump so high? Breaking bonds always requires the input of energy, and there are a lot of carbon-carbon bonds in diamond. This is the difference between thermodynamics and kinetics. Thermodynamics can tell you only that a reaction should go because the products are more stable have a lower free energy than the reactants. Another way of saying this is that the reaction has a negative free energy change: D G is negative and therefor the reaction is spontaneous.

Yet another way of saying this is to say that the reaction has a large equilibrium constant, signifying that if nature could ever attain equilibrium, there would be many more products present than there are reactants. Kinetics, on the other hand, can tell you how fast the reaction will go but doesn't tell you anything about the final state of things once it gets there.

Consider the following reaction sequence:. The equilibrium constant, capital K, is a thermodynamic quantity. As such, it depends only on the overall reaction.

What the individual reaction steps that changed A and D into B and E were doesn't matter in the slightest. Well, that's the one that should thermodynamically predominate at equilibrium.

Notice that the reaction intermediate C doesn't even enter into the equation for K. For the purposes of thermodynamic analysis, the individual steps above the overall reaction might as well not even be there. One side note about K is that it depends on how the overall reaction equation is written. If the stoichiometric coefficients were multiplied by two, then the value of K for the original overall reaction would have to be squared in order to be appropriate for the new overall reaction.

Thus, in order to determine K for a given reaction, you must be aware of exactly which stoichiometric coefficients you are using. Assuming the experiment is reproducable in the first place, the number that the colleague gets will be the square root of the number that the original chemist got.

A second side note about K is that pure solids and pure liquids don't get included in it. Their concentrations are so big that we can just take them as being constant throughout the reaction. Thirdly, people often analyze perturbations to equilibrium in terms of Le Chatelier's principle, which says that a system will shift to counter any change you try to make. For example, if a reaction is exothermic, that means that it gives off heat. If you raise the temperature, you are effectively adding a product to the reaction, which will cause it to shift back to the reactant side.

Be careful about this though because temperature can change equilibrium constants. Kinetics, on the other hand, does not depend in the slightest on what the situation looks like at equilibrium.

The rate of the reaction has no dependence on the overall reaction equation but instead depends on the reaction mechanism, the elementary steps. This was the part of the reaction sequence that we ignored for thermodynamics. The molecules on the left of each elementary step must collide in order to react so that the products on the right are formed.

Notice that in the first step of the reaction sequence above, the reactant A doesn't have to collide with anything. This is what is called a "unimolecular," "first order" elementary step because only one atom is involved. In the second step of the reaction sequence, C and D do have to collide in order to produce E.

This is what is called a "bimolecular" step because two atoms have to come together for the reaction to occur. The rate of an elementary step depends on the concentration of species available to react. For example, in the second step, if there are many molecules of C and D around, then the likelihood of a molecule of C colliding with a molecule of D with sufficient energy and the right orientation to make the elementary step go is high. Therefore, the rate of the elementary step is proportional to the concentrations of the reactant molecules.

Here are expressions for the rates of the two elementary steps for the reaction sequence above:. Notice that if for instance one of the elementary steps were to involve two molecules of C colliding, then the rate of that step would be proportional to [C] 2.

Elementary steps of higher molecularity termolecular and on up are very rare because in any real scenario, it is unlikely that three molecules would hit each other in exactly the right way and with exactly enough energy for the step to happen. The rate constant k is a quantity that students love to confuse with the equilibrium constant K. K tells you the ratio of products to reactants at equilibrium while k tells you the rate of an elementary step in the reaction mechanism!

Although it is most usual to find little k experimentally, it can also be found from the Arrhenius equation,. The Arrhenius equation does not tell you the rate of the reaction; it tells you the rate constant for an elementary step of the reaction.

The variable Ea is the activation energy for the step, or the height of the hump on the reaction diagram at the beginning of the section. The constant R is our old friend the gas constant, and T is the temperature at which the elementary step is performed. The large sensitivity of k to T is the reason that it is extremely difficult experimentally to find rate constants.

Most elementary steps either give off or take up heat, and the resulting temperature change changes the rate of the elementary step itself. Thus, the practical utility of the Arrhenius equation is limited. The constant A is the "Arrhenius factor. When you take Chem 33, you will learn that for some reactions classified as "SN 2 " the collision must involve one molecule putting electron density into an antibonding orbital on another molecule.

This is a very precise place to have to put electron density! Therefore, only a small fraction of collisions result in reaction. The Arrhenius factor is also called an "entropic factor" to stress that it accounts for how random collisions can be if they are to result in a reaction.

From experimental data, it is often possible to find the rate law for a reaction.