General Approach to Solving Chemical Equilibria Problems
The general form of a chemical reaction is:
aA
+
bB
Ù
cC
+dD
where A and B are reactants in the forward direction and C and D are products in the forward direction.
The lower case letters are the
stoichiometric coefficients for the balanced equation.
The general form of the equilibrium constant equation is then
K
eq
= [C]
c
[D]
d
/[A]
a
[B]
b
Problems involving chemical equilibria can be placed into a matrix format with two kinds of concentrations identified: the initial or
nonequilibrium concentration
C
x
and the equilibrium concentration
[X
].
(Note the bracket versus the capital C.
It is the equilibrium
concentrations,
[X
],
that are used to calculate K
eq
.
A
B
C
D
initial
C
A
C
B
C
C
C
D
change
C
A
[A]
C
A
[A]
[C] C
C
[D]  C
D
equilibrium
[A]
[B]
[C]
[D]
There are a variety of equilibrium problems that can be solved using the construction above.
In some cases the problems are solved
directly, with one unknown per equation.
Others problems require significant algebraic manipulation.
All of them are easy if you can
identify the type of information used and place it in the matrix.
Examples:
Model Equilibrium System
As the chemical system for all the problems on this work sheet, we will use
2NH
3
Ù
3H
2
+ N
2
K
eq
= 3.8 = [H
2
]
3
[N
2
]/[NH
3
]
2
Problem 1.
Calculating K
eq
from equilibrium concentrations.
In these problems it is necessary to identify all the bottom row equilibrium concentrations (the ones in [X]) so that K
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 Spring '07
 Holcombe
 Chemistry, Equilibrium, Reaction, Keq, initial change

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