Untitled - Kelly Walsh High School

Chemical Equilibria 213
Quick Tip
For the reaction: I 2(g) K 2 I(g) at a certain temperature, the value of K is 3.76
10 5 . If the initial I 2 concentration is 0.500 M, what will be the equilibrium concentrations
of I 2(g) and I(g)?
A generic K usually means K c. Your instructor may specify you to treat a generic
K in some other way.
The first step in this, and most equilibria problems, you should begin by writing
the equilibrium constant expression. In this case, the expression is:
K 3.76 105 [I2 ]
In this problem, we know the initial concentration of iodine and we want to find
a new concentration at equilibrium. Looking for a new concentration is a very
strong hint to create a reaction table. To begin a reaction table, you need to list
each substance present in the reaction at the head of a column. Below each
heading, you should enter the initial value for each substance. In this case, our
table will begin as:
I2 I
Initial 0.500 0
The problem only specified an initial I 2 concentration. Any unspecified initial
value appears as a zero in the table.
The next line in the table will include information on how the initial values will
change. A zero value in the table indicates how the change will occur. The equilibrium
concentration of any substance can never be zero. Since the iodine atom
concentration is initially equal to zero, we must add some quantity to this. The
source of these iodine atoms must be the iodine molecules. This will result in a
decrease in the concentration of iodine molecules. If we assume that the change
in the molarity of iodine molecules is x, then, based on the reaction stoichiometry,
the iodine atom concentration change will be 2x. This information begins
the next line in our table:
I 2
Initial 0.500
I
0
Change x 2 x
[I] 2