equilibrium

[iwilliwill]

i have some quesiotns about equlibrium..i have a test coming up

when an equlibrium system shifts to the left..what is it saying about the forward and reverse reaction rate?

 

reverse reaction is faster since it is shifting to the left?

 

or forward reaction is faster so it has to shift to the left to reestablish equlibrium?

 

maybe im not very clear about the concept...all i know is at equlibrium forward and reverse reactions occur at equal rate..

[prion]

maybe you are getting a bit confused by just focussing on the rates individually. You always have to think about the whole system otherwise it will make no sense. Firsty, equilibrium doesn't mean the forward and reverse rates are the same, or that the products and reactants are the same concentration. It just means that the whole thing has settled down so that you always have the same amount of everything. Even with a totally different forward and reverse rate, it will eventually settle down so you have more of one thing and less of the other thing (assuming the reaction is reversible). This can be hard to get your head round.

 

When you shift an equilibrium, e.g. by changing the temperature or pH, it is the molecules at each end that are affected, and that is why you see a change in rates. You need to think about what is happening to those molecules when there is a particular shift. For example, if you have A<->B and you change the temperature, maybe it destabilises A, so it is more likely to form B (i.e. forward reaction rate goes up). But maybe it destabilises B even more so it is much more likely to go back to A (reverse reaction goes up more). In this case, the equilibrium shifts to the left (towards A), but both rates have gone up.

 

So what matters is not what happens to each individual rate, but what happens to the ratio, i.e. which one is faster. If the forward rate is 10 and the reverse rate is 100, then if you divide the forward rate by the reverse rate (10/100) you get 0.1, this is the equilibrium constant. If the equilibrium constant is more than 1, then it tells you the forward rate is the fastest and you get more B. If it's less than 1 then the reverse rate is fastest and you get more A. If it's 1 then the rates are the same and there's the same amount of A and B. You could have the same equilibrium constant with totally different rates, e.g. 100/1000 or 1/10.

 

When the individual rates matter is if you want to know how fast the system will settle down to a new equilibrium. Fast rates means fast change in equilibrium, i.e. you will suddenly have a load more of A, rather than gradually getting more A.

 

sorry that turned into a bit of an essay!