Equilibrium

Chemical Equilibrium Many reactions are reversible Ex) 2NO2(g) <--> N2O4(g) Not all rxns are A rxn is not reversible if: - a precipitate forms - a gas forms and then escapes - the products are much more stable than the reactants

If a rxn is reversible, the system may reach chemical equilibrium. Equilibrium is a balance between 2 opposite processes. Chemical equilibrium: when the forward and reverse reactions are happening at the same rate. Does NOT mean that [reactants] = [products]

How do we get to the point where fwd. and rev How do we get to the point where fwd. and rev. reactions are happening at the same rate? The following is to think about, rather than write. Imagine that you are one of 500 people waiting to enter a mall on a busy shopping day. This mall has two levels, but all of the entrances are on the first level. How would you describe the number of people on the first floor compared to the second floor when the doors first open? All 500 would be on the 1st floor, with none on the 2nd. After 10 minutes, how would this comparison change? (Assume we’re just talking about the original 500 people.) Many people would have taken the escalator to the 2nd floor, bringing the numbers closer to even. At this point there would probably also be more people per minute riding the up-escalator than the down- escalator. This is simply because there aren’t that many people on the 2nd floor yet, so not many people can come back down to the first floor. What about after one hour? The populations on each floor still may not be even (depending on how popular the stores are on the 1st floor vs. the 2nd). We should, however, see a similar number of people riding the up-escalator vs. the down-escalator. This is because we’ve allowed enough time for the 2nd floor to become populated, and some of these people will probably be returning to the 1st floor. At some point, if the same number of people per minute are going up as are coming down, we’ve reached equilibrium. Notice that it does not require the populations on each floor to be equal. It only requires that the two opposite processes be happening at the same rate (people traveling up vs. down).

Ok – go back to writing!! Ex) 2NO2(g) <--> N2O4(g) We always begin our reaction with 100% reactants (NO2) and 0% products (N2O4) At first the only thing that can happen is for the NO2 to get used up and the N2O4 to be made. The concentrations of of both change rapidly, because only one process is happening (the fwd. rxn.)

As more time passes… The two concentrations level off. Why As more time passes… The two concentrations level off. Why? (Think about the mall example)