The Student Room Group

Le Chatelier's Principal

I know it's a fairly simple concept but whenever I attempt to use it in an exam question I always predict the wrong side of the equilibrium to shift to. Can anyone give any tips? The main sorts of questions I'm having problems with are those which ask you to predict a change in concentration, eg:

Some solid sodium chloride is added to a solution of chlorine in water. Use the chatelier's principal to predict what will happen to the concentration of Cl2 (aq).

I know the answer to this question, but I just don't know how to use the Chatelier's principal to get to it.

Help much appreciated,

Alex
Reply 1
How would you answer it now?
whistlecru
I know it's a fairly simple concept but whenever I attempt to use it in an exam question I always predict the wrong side of the equilibrium to shift to. Can anyone give any tips? The main sorts of questions I'm having problems with are those which ask you to predict a change in concentration, eg:

Some solid sodium chloride is added to a solution of chlorine in water. Use the chatelier's principal to predict what will happen to the concentration of Cl2 (aq).

I know the answer to this question, but I just don't know how to use the Chatelier's principal to get to it.

Help much appreciated,

Alex


Hmm

Okay, the Le Chatelier Principle states :
"If a system in a state of dynamic equilibrium is disturbed, the equilibrium position shifts in the direction which reduces the disturbance and restores the equilibrium (i.e. oppose the change)."

Now, take for example

A + B --> C + D

If the concentration of A is increased, the 'state of dynamic eqbm.' is disturbed. So effectively, more reactants is produced - the thing that react together to give the products.
So, to oppose this increase in reactants, the system increases the Yield (or Amount) of Products. This would, in effect, oppose the increase in reactants and would allow the system to return back to equilibrium. So, to increase the yield of products, the equilibrium position shifts to the RIGHT.

Now, if we apply Le Chatelier's principle, we see that it is perfectly valid here.
By producing more products we are 'reducing the disturbance' caused to the system by the increase of concentration of reactants. So, to reduce the disturbance, the equilibrium position shifts to the RIGHT (i.e. to the Products side).

I hope you got what I meant to say :biggrin:

However I didnt really understand how Le Chatelier's Principle could be applied to your question "Use the chatelier's principal to predict what will happen to the concentration of Cl2 ". As more and more products form, it becomes obvious the concentration of the Cl2 decreases .

I hope the rest helped though :smile:

Rohit
Well, I've seen the markscheme so I know that the equilibrium shifts to the left. However, when I answered it myself I said that the concentration will decrease because the equilibrium shifts to the right, increasing the rate of the forward reaction. I'm sure this is wrong but I don't really know how to get to the right answer.
Reply 4
I try and remember it by thinking that the equilibrium is a bit perverse. So if you put one thing into a reaction, its going to move to get rid of everything that you've put in it.
Is that the whole question?

Some solid sodium chloride is added to a solution of chlorine in water. Use the chatelier's principal to predict what will happen to the concentration of Cl2 (aq).

Reply 5
okay...im going to use my own example if that is fine?

SIMPLE ESTERFICIATION!

ethanol + ethanoic acid <----> ethyl ethanoate + water

mole ratio of everything is 1.1.1.1
everything is a liquid also

ok, lets say the conc of ethanol is increased. (this is how i wouild answer thew question)

'le chateliers principal says that when the concentration of reactants is increased, it will shift the position of equilibrium to oppose this change. therefore, the position of equilibrium will shift in the direction of the products, therefore increasing the concentration of ethyl ethanoate and water'
Reply 6
a+b->c. Just remember if concentration of reactants is increased, the position of the equilibrium moves to the right. If the product conc is increased it moves to the left. Just remember, whatever you increase in conc, the equilibrium would move to the oppsite direction.
This is what my bloody assignment is on now, its so annoying, i understand that principle but when you get a question like in an exam, i get so confused lol. hmmm..... just looked at my assignment an the above poster might of just helped me woohoo :smile:
Reply 8
Yeah, basically Le Chatelier's principle means that if you change something in an equilibrium, the system will move so as to restore it to the original conditions. So if you you have a reaction where the forward one is exothermic (say the Haber process, N2 + 3H2 --> 2NH3), then if you increase the temperature, the equilibrium will shift to the left (because the equilibrium favours the backward endothermic reaction, in order to absorb the extra heat produced). Similarly, in the above example if you increase the pressure, the equilibrium will shift to the right because it favours the reaction that reduces the pressure, ie the reaction that produces less gas molecules. Hope that made sense.
Reply 9
It did :smile: OP - it it absolutly VITTAL that you understand this concept, particulary that equilibrium is a state of dynamic change. Much of chemistry is based around this concept.
Good :p: Just remember, the equilibrium wants to put everything back - so if you increase the concentration of reactants, forward reaction favoured to reduce the reactants again. If you increase the temperature, the endothermic reaction is favoured to absorb excess heat. If you increase the pressure, the reaction that produces less molecules is favoured. And catalysts does NOT change the position of equilibrium but only how quickly it can be attained.

And it's Le Chatelier's Principle :p:
Reply 11
.....because they equally increase the rate of reaction on both sides of the Eq, by providing a route of lower activation energy, or securing molecules in a favoured position...
Reply 12
http://www.chemguide.co.uk/physical/equilibria/lechatelier.html

Try that, helped me when I got confused during AS.
Help me,

We did a practical heres one result an the question which i need to use on lots of results.
Cu(H2o)62+(aq) (blue) + 4CH(aq) > CUCL42(aq) + 6H2o(i)
(Copper sulphate) (Hydrochloric acid) green yellow.
The bits underneath the equation explain what it is.
Reaction 1 - place 2cm3 of copper sulphate solution into a boiling tube and add a few cm3 of concentrated hydrochloric acid untill the solution changes colour.

I did this practical and this is what happened: copper sulphate was blue, once a few drops of hydrochloric acid was added it changed to green.

The question is explain your observations using le chateliers principle.
I was just gonna put le chateliers principle means their will be an oppose to the change in the system so the equilibrium moves to the left. I think this because if it changed green it needs a little more blue again to balance it back out, or am i completely wrong. someone please help an keep it simple. thanks
anyone? please. :mute:
There is an equilibrium:

[Cu(H2O)6]2+ + 4Cl- <==> [CuCl4]2- + 6H2O

When you add conc. HCl you are adding EXCESS Cl- ions. According the LeChat this adding to the LHS pushes the equilibrium to the RHS to remove the excess Cl. ions and it goes green.
Add water and you are adding to the RHS pushing the equilibrium to the LHS to remove the excess water. It then goes blue.
eg. 2A + B ----> C + D forward reaction here is exothermic.

if TEMPERATURE increased the equlibrium shift to the left, because increase in temp favours the endothermic route, and in the example above the forward reaction is exothermic.

If PRESSURE (only for gases) increased the equlibrium shifts to right because less number of moles on the right (2 moles) than left (3 moles)

If CONCENTRATION OF A or B is increased the equillibrium shifts to right to minimize this increase. on the other hand if CONCENTRATION of C or D is increased the equilibrium shifts to left.

Addition of CATALYST affects the forward and reverse reaction equally therefore has no effect on the postiton of the equlibrium.

this is the way to handle exam q's on equilibria. make sure to look at the equation and any delta H values they have given.

Hope this helps as its really logical when you understand it all! :smile: