[MrDe]

hello all..
I am here again with another question about equilibrium:

Say we have a system with some amounts of reactants and products so that the system isn't at equilibrium state, then what will happen if we make changes (volume, amounts, temperature, ... etc) to it from the beginning? and can we make such a change to create equilibrium immediately?

What I know is that when we make changes a system at equilibrium we use Le Chatelier rule, but it doesn't apply on this case..
I have no idea about what shall be done in that case..

regards, and thanks in advance.

[XGen]

I am not sure if I am interpreting your question correctly, but it is certainly possible to change the conditions of a chemical "equilibrium" before it has already reached equilibrium. Because this is possible, it should also be possible to change the conditions to equilibrium conditions.

However, I am not an expert, so take my advice with a grain of salt.

[fledarmus]

Reactions have two rates, the rate of the forward reaction and the rate of the reverse reaction. (Although if the rate of the forward reaction is much faster than the rate of the reverse reaction, the reverse reaction is negligible and the reaction is said to be irreversible). The rate of both reactions is depending on the concentration of the starting materials for that reaction; so if you have A + B   C + D, the rate of the forward reaction will depend on the concentrations of A and B, and the rate of the reverse reaction will depend on the concentrations of C and D. Equilibrium is the point where the forward reaction rate matches the reverse reaction rate.

So it doesn't matter whether your reaction is currently at equilibrium or not, adding more A or B will speed up the forward reaction, and adding more C or D will speed up the reverse reaction. They will also, as a consequence, change the equilibrium concentrations of the components.

[MrDe]

I am not sure if I am interpreting your question correctly, but it is certainly possible to change the conditions of a chemical "equilibrium" before it has already reached equilibrium. Because this is possible, it should also be possible to change the conditions to equilibrium conditions.

However, I am not an expert, so take my advice with a grain of salt.

thanks much for this. I wanted to know how these changes will affect a reaction not at equilibrium..

So it doesn't matter whether your reaction is currently at equilibrium or not, adding more A or B will speed up the forward reaction, and adding more C or D will speed up the reverse reaction. They will also, as a consequence, change the equilibrium concentrations of the components.

Thanks, extremely much thanks!!
This is the very piece of information that I wanted to know.
so here if we increase the concentration of A, that will only increase the forward reaction rate no matter what it was before. The backward rate will not change, but it will change as reactions goes on. (is that right?)
I thank you again: this info will help me with my other post earlier.

[juanrga]

What I know is that when we make changes a system at equilibrium we use Le Chatelier rule, but it doesn't apply on this case..

The rule applies to non-equilibrium and explains what a perturbed system will do to achieve an equilibrium state again.

[MrDe]

What I know is that when we make changes a system at equilibrium we use Le Chatelier rule, but it doesn't apply on this case..

The rule applies to non-equilibrium and explains what a perturbed system will do to achieve an equilibrium state again.

Le Chatelier's principle says:
"If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or partial pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established."

that's unless I misunderstood the rule or your post.

an example on what I was asking for: if the fraction between products and reactants is more than K and we change temperature or the vessel volume for example, what will happen?

[juanrga]

What I know is that when we make changes a system at equilibrium we use Le Chatelier rule, but it doesn't apply on this case..

The rule applies to non-equilibrium and explains what a perturbed system will do to achieve an equilibrium state again.

Le Chatelier's principle says:
"If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or partial pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established."

that's unless I misunderstood the rule or your post.

an example on what I was asking for: if the fraction between products and reactants is more than K and we change temperature or the vessel volume for example, what will happen?

You give exactly the Wikipedia version. Once again Wikipedia must be taken with suspicion [1].

First, the Le Chatelier's principle is not a principle but a theorem: the Le Chatelier & Braun theorem of moderation. This theorem can be derived from the thermodynamic stability theory based in the production of entropy (sometimes named the Prigogine-Defay method).

Second, if the system is at equilibrium, Le Chatelier & Braun says nothing. It is only when the system is perturbed and shifts away from equilibrium that Le Chatelier & Braun says you what the system will do to achieve again an equilibrium. Example consider the equilibrium

A + B C

Le Chatelier & Braun says nothing. Now add some amount of A. The system is not at equilibrium and Le Chatelier & Braun says that

A + B C

up to that a new equilibrium state is reached.

Third, contrary to Wikipedia, Le Chatelier & Braun does not apply to changes in volume. From the section C. The Le Chatelier-Braun theorem of moderation [2]:

But these theorems of restraint or of moderation cannot be applied in the same manner when we use other variables. If we decrease the volume of a system, the reaction produced does not tend to moderate this decrease.

Fourth, "if the fraction between products and reactants is more than K and we change temperature" what will happen depends. Is the concentration away from equilibrium because of some perturbation or because the system is in a stationary state maintained by flows? If it is the first case,  Le Chatelier & Braun applies to the non-equilibrium state as always, it does not matter if non-equilibrium was achieved due to variation in concentration, temperature or both at once.

The general procedure to know what a chemical reaction will do against perturbation is to apply non-equilibrium thermodynamic theory.

[1] The academic level of many Wikipedia articles is the reason which I started the reference work presented in the Educative forum.

[2] 2A Equilibrium, Stability, and Displacements, 1971: In Physical Chemistry, An Advanced Treatise Volume I/Thermodynamics; Academic Press, Inc.; Wilhelm Jost (Editor). Sanfeld, A.

[fledarmus]

Le Chatelier's principle is a consequence of the changes in rates between the forward and reverse reaction. Basically, what it points out is that if you take a system in which the forward reaction rate is currently the same as the reverse reaction rate (a system at equilibrium) and perturb the equilibrium so that is not the case, the concentrations will change until the rates are once again balanced and the system is at equilibrium. If you add more of a starting material, the rate of the forward reaction will increase - that is not a consequence of Le Chatelier's principle, and does not require the system to be at equilibrium. However, if you take a system that is at equilibrium, and add some more starting material, the equilibrium concentrations of reactants and products will shift, as a direct consequence of increasing the rate of the forward reaction. The concentrations will shift until the rate of the reverse reaction once again balances the rate of the forward reaction and the system is once more at equilibrium.

Le Chatelier's principle does apply to changes in volume, but you have to know what the change in volume will do to the rates of forward and reverse reactions in order to apply it. For example, if you take reaction where two different gaseous starting materials are reacting in a 1:1 ratio to form a gaseous product ( A(g) + B(g)  C(g)) and you increase the volume in which the reaction is taking place without changing the total amounts of A,B, and C, you will reduce the concentration of all three components. Since the forward reaction depends on the concentrations of both A and B while the reverse reaction depends only on the concentration of C, the forward reaction will be slowed more than the reverse reaction, and the equilibrium concentrations of the components will shift in the direction of more starting materials and less product.

[MrDe]

Thank you all..

Would you please check my answers to my earlier post it is about the same problem?

Mr. Juanrga, I used Wikipedia version because I couldn't translate the principle (or theorem) from my book and because it looks similar to it. I didn't know that Wikipedia doesn't have much trusted academic level.
As for the volume point, Fledarmus has explained that. I may should have used overall pressure instead of volume.
The example I gave, I don't think It was much good, but I got your point.
At the end, it is the matter of reaction rate, regardless of the state of reaction before the change.
 

[juanrga]

Thank you all..

Would you please check my answers to my earlier post it is about the same problem?

Mr. Juanrga, I used Wikipedia version because I couldn't translate the principle (or theorem) from my book and because it looks similar to it. I didn't know that Wikipedia doesn't have much trusted academic level.
As for the volume point, Fledarmus has explained that. I may should have used overall pressure instead of volume.
The example I gave, I don't think It was much good, but I got your point.
At the end, it is the matter of reaction rate, regardless of the state of reaction before the change.
 

Le Chatelier & Braun theorem of moderation in essence says that when a system initially at equilibrium is perturbed up to some non-equilibrium state, it tends to evolve towards a new equilibrium system that moderates the change done: For instance, if you heat a reaction it tends to absorb that heat, if you increase the concentration of a reactant, the reaction tends to consume that reactant, and so on.

As stated in the above reference [2] this theorem does not apply to volume changes, because if we increase the volume of a system, the reaction produced does not tend to moderate this increase. The equilibrium can change, but not in the way required by the moderation theorem.