| Contents for this page | Related topics |  |
|---|---|---|
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Reversible reactions Dynamic equilibrium The equilibrium constant Additional questions |
Reaction rates Catalysis Le Chatelier's principle |
 Data Glossary |
| Learning Outcomes | ||
| After studying this section, you will (a) understand what is meant by a dynamic equilibrium, (b) understand the meaning of the equilibrium constant, Kc, and (c) know how to formulate the expressions by means of which it may be calculated, and (d), be able to perform calculations involving the equilibrium constant. | ||
Many chemical reactions give rise to products which are capable of reacting in such a way as to give back the original reactants:
For example, at 20 ºC, two molecules of the gas NO2 can react to form one molecule of another gas, N2O4. But this gas in turn is not very stable, and can dissociate at 20 ºC into two molecules of NO2.
We say that this reaction is REVERSIBLE, and indicate this by the symbol 
.
Reversible reactions give rise to a state of DYNAMIC EQUILIBRIUM, where the products of the reaction are converted back into the reactants from which they were derived as fast as they are formed. In other words, at equilibrium, the rates of the forward and reverse reactions are exactly the same:
When a reversible reaction has established a dynamic equilibrium, things just don't stand still! There are countless numbers of A and B molecules being converted to C and D, and vice versa.
The MOST IMPORTANT thing to bear in mind is that when equilibrium has been reached, the CONCENTRATIONS of A, B, C and D do not change with time - they have reached a CONSTANT value.
| In the animation you are about to see here on the right, watch how the concentrations of A and B change with time. Initially, we start with A at some fixed concentration [A]0, while the concentration of B, [B]0 = 0. As time progresses, the concentration of A decreases, while that of B increases by the same amount. At some time, the concentrations of both A and B remain unchanged. Equilibrium has been reached! (Start the animation.) |  |
Consider a general case of a reversible reaction
It is a fact that when equilibrium has been established, the concentrations of A, B, C and D are constant. (We write [A] for the concentration of A, [B] for the concentration of B, and so on).
Under a given set of conditions, such as a constant temperature, the concentrations of the reactants and products are linked by the expression
where Kc is the EQUILIBRIUM CONSTANT.
The equilibrium constant Kc has a value which will be fixed for each reversible reaction, under a particular set of conditions (notably temperature).
It is most important to note that the expression
describing the equilibrium constant Kc is valid only at equilibrium. The concentrations in the expression for Kc must be those concentrations actually found at equilibrium.
The implication of the equilibrium constant is that no matter what the initial concentrations of reactants and/or products might be, the reaction will take place in such a manner that at equilibrium, the concentration values will have adjusted so that [C][D]/[A][B] will equal the appropriate value of Kc.
For the generally stated reversible reaction
the equilibrium constant is given by
where a is the number of molecules of A taking part in the reaction, b the number of molecules of B, and so on.
| As an example take the reaction |  |
| The equilibrium constant is |  |
(Easy)
(More difficult)