thermodynamic stability vs. kinetic

[r1dermon]

to my best knowledge, thermodynamic stability is a gauge of how readily a compound will reduce to form a product...(correct me if im wrong)

 

kinetic stability on the other hand is a gauge of how much input kinetic energy is required in order to initiate a reaction...(correct me if im wrong)

 

with this in mind, a standard pyrotechnic composition would be extremely thermodynamically unstable, meaning, it reacts vigorously to form products upon reaction...however, since most pyrotechnic compositions require an outside energy source to be initiated, i presume, pyrotechnic compositions would be extremely (or mostly) kinetically stable compounds.

 

with that in mind, to the best of my knowledge, flash powder requires substantially higher temperature than black powder to initiate a reaction. of course, the amount of actual kinetic energy needed to be transferred (i presume) would fully depend on the size of the particles of the reactants. based solely on the higher temperature required for reaction, would that mean that flash powder is actually more kinetically stable than black powder? are there other factors at play which i have not considered?

 

if this is not posted in the correct forum, forgive me...feel free to move it. i wasnt sure if i should post it here, or in the physics forum.

 

thanks.

[hypervalent_iodine]

Your definitions are a little off. Firstly, this isn't specifically reductions, it applies to any reaction. The thermodynamic product of a reaction is the product that is the most stable. For instance, if I wanted to add something across a double bond, the thermodynamic product would be the one where the thing I'm adding adds to the most substituted end. The kinetic product is simply the one that forms the fastest. So in my simplistic analogy above, the kinetic product would be where the substituent adds to the least hindered and therefore least substituted end. You can control which you get as a major product by lowering the temperature (for the kinetic product) or raising it (for the thermodynamic product).

[spin-1/2-nuclei]

Hello,

 

This will be a long post:

 

the kinetic energy of a particle describes the work required to accelerate/decelerate an object from rest/motion to it's current velocity.

 

You're not that off tract - i.e. there is a relationship between temperature and kinetics and it comes in the form of particle/molecular motion and entropy. In chemistry the temperature of a molecule is the measure of the speed at which the particles/molecules are moving - thus it is a measure of their kinetic energy. Increasing temperature increases molecular movement, therefore, increasing temperature increases kinetic energy.

 

In chemical reactions:

The time it takes a reaction to occur is the measure of it's kinetic stability. So the faster it reacts the less kinetically stable it is. Thus products that are kinetically stable are sometimes incorrectly said to be unreactive when in reality they need energy to increase the rate at which they react. Moreover, people often confuse kinetic and thermodynamic stability. In short, the kinetics control both the rate and the pathway of the reaction, the thermodynamics focus on the energy differences between the products and the reactants.

 

In thermodynamics you have

- Enthalpy

 

If a reaction takes in heat (i.e. if heat is required to move the reaction forward) - it is said to be endothermic.

If the reaction gives off heat (i.e. if heat is produced during the reaction) - it is said to be exothermic.

 

Whether or not a reaction will give off heat or require heat is related to the bond dissociation energy. This can be quantified by the enthalpy (H) of reactions.

The deltaH = H-products - H-reactants.

So, to put endo/exothermic reactions another way..

 

If you have a positive deltaH then you have an endothermic reaction.

If you have a negative deltaH then you have an exothermic reaction.

 

Moreover,

if the products have a greater total bond energy than the reactants the reaction is exothermic

if the products have a smaller total bond energy than the reactants then the reaction is endothermic

*note, that bond energy is NOT an innate energy source for molecules - i.e. - it is not part of the molecules potential energy, but rather has to be introduced into the system from a third party source.

 

Activation Energy - is the energy barrier that the reactants have to overcome to begin the process of conversion to products. It's important to note that exothermic reactions - are just as susceptible to activation energy as endothermic reactions - i.e. most exothermic reactions do not proceed spontaneously.

 

and this is how heat is related to kinetics in a chemical reaction: The energy (in the form of heat) - activation enthalpy deltaH(double dagger) that is required to bring the reactants to the activation enthalpy - is for the most part inversely proportional to the reaction rate and is highly dependent on the concentrations of the reactants.

 

Thus, as far as stability goes: thermodynamic stability can be described as the enthalpy or potential energy (this is an over simplification but works at the qualitative level - for more info look at the Gibbs free energy function - deltaG) when compared to some standard. For exothermic reactions the products are thermodynamically more stable than the reactants. For endothermic reactions the reactants are more thermodynamically stable than the products.

 

In Gibbs free energy, entropy (deltaS) is incorporated and you have what are called endergonic and exergonic reactions.

Endergonic reactions have a positive deltaG and require external input from the environment into the system

 

in contrast

 

exergonic reactions have a negative deltaG and often occur spontaneously.

 

In cases where deltaS (entropy change) is small deltaH = deltaG, but in cases where entropy change is large that is not what happens.

Looking at the equation below, you can see that the significance of deltaS increases with increasing temperature.

 

deltaG = deltaH -TdeltaS

 

which is another way of saying: free energy = enthalpy - (temperature)x(entropy)

 

there is also a free energy of activation - which incorporates entropy as well, and explains why NOT ALL exergonic reactions are spontaneous.

 

Thermodynamically speaking - entropy relates to the order/disorder of a system. Mathematically, you can think of it simplistically as a description of all the energetically relevant degrees of freedom available for the system. In atoms and molecules the more molecular movement possible (bond rotations, etc) the greater the entropy, the same is true for the mass.. the greater the mass the greater the entropy.

 

Since I already stated earlier that kinetics deals with the movement of molecules which is just another way to define the temperature you can see now how kinetics relate to temperature and chemical stability.

 

So why then does kinetic stability decrease with increasing temperature? Well this becomes more apparent when we relate temperature to entropy. We can do so via

T^-1 = (d/d(E))x(S(E)).. you can see that when temperature increases the entropy of the system also increases.

 

So, if you look at kinetic stability - the molecules that are the MOST kinetically stable - react very slowly and in some cases do not react at all. So increasing the kinetic energy of a system, increases the temperature, which in turn increases the entropy, and makes kinetic control of the reaction less likely as temperature increases.

 

For example. If we have reactants A&B that are kinetically stable, but are thermodynamically unstable (i.e. once they react they will react exothermically) depending on the reaction conditions (thermodynamic vs kinetic) control you will get two different results. The activation energy for the reaction that leads to the thermodynamic product will be higher than the activation energy for the reaction that leads to the kinetic product.

 

So in this way it is somewhat confusing to say that the reaction temperature is lowered to provide the kinetic product. That reaction temperature typically only corresponds to the thermodynamic product, which in fact has a higher energy of activation, so from the perspective of the kinetic product the temperature has not been lowered, and has in fact been raised to a point that is sufficient to over come the kinetically stable molecule while at the same time not exceeding the energy required to reach the activation energy for the thermodynamic product.

 

It is also confusing to speak about kinetic energy when speaking about kinetic stability, because every activation energy for every reaction is relevant to the kinetic energy of the reactants, that is to say - all chemical reactions have activation energies and all molecules have kinetic energy.

 

So if you are working with reaction conditions that are specified for the thermodynamic product and you want the kinetic product you will lower the temperature.

If, however, you are starting from scratch with your own procedure and you want to obtain the kinetic product from kinetically stable reactants you will have to raise the temperature to increase the kinetic energy of the reactants.

 

Hope this was helpful,

Cheers

Edited August 18, 2011 by spin-1/2-nuclei