Themes > Science > Chemistry > Miscellenous > Help file Index > Equilibrium > Equilibrium


In the study of kinetics, we looked at how fast a reaction occurs. One notable result is that as the concentration of a species increases, in general the rate of the reaction containing that species increases.

In general, chemical reactions run "both ways"- both the forward and reverse reactions have some rate. For a generic reaction

aA + bB < - > cC + dD
if we start with lots of A and B and no C or D, the reaction will go to the right. As the concentrations of A and B decrease, the rate of the forward reaction will slow down. Similarly, as the concentrations of C and D increase, the rate of the reverse reaction will increase. Eventually, these rates will be equal and the concentrations of the various species will not change. This is the equilibrium state for that reaction.

An example is the simple gas phase reaction

N2O4(g) < - > 2NO2(g)
If we start with a 1 M concentration of N2O4 and no NO2 and measure the concentrations of the two species as time passes, we find that the concentrations look like this

equilibrium approach

Note that after about 60 seconds, the concentrations of N2O4 and NO2 don't change anymore. The system has reached equilibrium, where the forward and reverse rates are equal, and the concentrations will stay equal until the equilibrium is disturbed.


Information provided by: http://learn.chem.vt.edu