[sameeralord]

Hello guys,

Let's say the concentration of a reactant was increased so according to Le chatelier principle the the system would change equilibrium postition to  decrease the concentration . So why is that in these graphs the concentration never reaches the original value. Why is slighly above the intial value in this case. Any help would be appreciated. Thanks

This is a different question

While we are it I just want to ask another question. When working with equilibrium questions we use the table

Initial
Equilibrium
Final

With reactant we do intial-final to get E and in product we add them.I know how to do these tables but I don't understand how this give equilibrium values. I'm just blindly applying it. I also want to know why we work why we substract in reactants and add in products. Your help in any of these questions would be much appreciated!!. Thank you

[Astrokel]

hey sam!

1) Le chatelier's principle states that the system will counteract the change so as to MINIMIZE the effects imposed. Systems are not ideal or perfect, you can only minimize and not fully counteract the change.

And also, let's say the system of N2 + 3H2 <---> 2NH3

when concentration of H2 increases, the graph of H2 will show an upward vertical line, however it does not decreases to reach its original value, because the N2 might be in limiting.

2) It's called the ICE table, check this out http://en.wikipedia.org/wiki/ICE_table

What you are using in the ICE table is actually in accord to le chatelier's. The 'C' which is the change, is actually when the system counteracting the change so as to minimize the effects imposed, which leds to 'E', the final equilibrium values.

[sameeralord]

hey sam!

1) Le chatelier's principle states that the system will counteract the change so as to MINIMIZE the effects imposed. Systems are not ideal or perfect, you can only minimize and not fully counteract the change.

And also, let's say the system of N2 + 3H2 <---> 2NH3

when concentration of H2 increases, the graph of H2 will show an upward vertical line, however it does not decreases to reach its original value, because the N2 might be in limiting.

2) It's called the ICE table, check this out http://en.wikipedia.org/wiki/ICE_table

What you are using in the ICE table is actually in accord to le chatelier's. The 'C' which is the change, is actually when the system counteracting the change so as to minimize the effects imposed, which leds to 'E', the final equilibrium values.

Thanks a lot for the help Astrokel . It was very helpful.snack I understood your limiting example well. So that's why in the definitions of my work book says the system partially opposes the change. With the second question we haven't learned a C in our school

What we have done is

Initial
Equilibrium
Reacted

Rest of the steps we do the same. So how is that reacted and Change the same. I thought change implies Final-intial or is it just the new concentration?

[Astrokel]

What we have done is

Initial
Equilibrium
Reacted

Rest of the steps we do the same. So how is that reacted and Change the same. I thought change implies Final-intial or is it just the new concentration?

Well, i guess reacted means the same thing as change. It's true that change implies final-initial or rather equilibrium-initial, but you need to know why there is this change. The change is there because when the reaction proceeds to establish an equilibrium from its intial reactants, there is a certain concentration of reactants that is being reacted and change to the product. We do not know this certain concentration because its a reversible reaction and not all reactants are converted to the products.


N2 + 3H2 <---> 2NH3

Let's say your intial [N2] = 0.1 and [H2] = 0.3
                
                N2   +   3H2 <--->  2NH3
[Initial]       0.1        0.3              0     
[Change]    -x          -3x             +2x 
[Equilibrium] 0.1-x    0.3-3x          2x  

Let's say the 'certain concentration' of N2 being reacted is x, therefore it will decrease by x, and H2 by 3x(due to stoichiometric ratio) to form 2x of NH3.

This affects the equilibrium concentrations, therefore its important to account the change.

[sameeralord]

What we have done is

Initial
Equilibrium
Reacted

Rest of the steps we do the same. So how is that reacted and Change the same. I thought change implies Final-intial or is it just the new concentration?

Well, i guess reacted means the same thing as change. It's true that change implies final-initial or rather equilibrium-initial, but you need to know why there is this change. The change is there because when the reaction proceeds to establish an equilibrium from its intial reactants, there is a certain concentration of reactants that is being reacted and change to the product. We do not know this certain concentration because its a reversible reaction and not all reactants are converted to the products.


N2 + 3H2 <---> 2NH3

Let's say your intial [N2] = 0.1 and [H2] = 0.3
                 
                N2   +   3H2 <--->  2NH3
[Initial]       0.1        0.3              0     
[Change]    -x          -3x             +2x 
[Equilibrium] 0.1-x    0.3-3x          2x   

Let's say the 'certain concentration' of N2 being reacted is x, therefore it will decrease by x, and H2 by 3x(due to stoichiometric ratio) to form 2x of NH3.

This affects the equilibrium concentrations, therefore its important to account the change.

I actually got it . Thanks heaps!!

It does make sense. If we had something initially and if that is a reactant it would get used up making product. So the concentration at equilibrium is intial substract reacted .  Thanks again

But if you don't mind another question just popped

I'm just wandering how come that something is always in equilibrium
after initial-reacted. I thought different reactions reach equilibrium at different rates. So let's say there is o intial concentration. Does that mean all reactions are at equilibrium when they have fully reacted according to their mole ratios. Does mole ratios show the moles of reactant and products at equilibrium. I'm just wandering.

[Astrokel]

I'm just wandering how come that something is always in equilibrium
after initial-reacted. I thought different reactions reach equilibrium at different rates. So let's say there is o intial concentration. Does that mean all reactions are at equilibrium when they have fully reacted according to their mole ratios. Does mole ratios show the moles of reactant and products at equilibrium. I'm just wandering.

Dynamic equilibrium is reached only when the rate of forward reaction is equal to the rate of backward reaction.

If reactants fully reacted according to their stoichiometric ratio, then the system will not be in equilibrium because its a one-way reaction.

[Equilibrium] 0.1-x    0.3-3x          2x 

Is the mole ratio the same at equilibrium?

 
 
 

[sameeralord]

[Equilibrium] 0.1-x    0.3-3x          2x 

Is the mole ratio the same at equilibrium?

Oh yeah I got it . Thank you astrokel . The difference with this equilibrium and a normal question involing intial values is that in that one would fully react (limiting) and one would be in excess. Also to have equiliborum there should be a backward reaction so cn't act in stoihiometric ratios. Thanks for the help. Really appreciate it