|
|||||||
|
|
|
|||||
|
|
|||||||
   |
|||
|
CHEMISTRY TOPIC: CHEMICAL EQUILIBRIUM By Kingsley Idiagbor, B.Sc.(Hon), PGDCs, NCE, MNSM Equilibrium is a state where there is no change in a property within a given time. This could be physical or chemical. Physical equilibrium is static once attained. Take for instance a sea-saw with a fat person at one end and a slimmer one at the other. For the sea-saw to be at equilibrium or be stable, the fat person have to shift further towards the fulcrum or the pivot. As soon as the state of equilibrium is attained, except it is disturbed by changing one of the persons or by changing their position, the state remains stable for a very long time. However for chemical equilibrium, it is a different kettle of fish altogether. It is a dynamic state - that is to say - it is always hanging. For example, if one is to seat close to a traffic runway and observe or even count the number of cars driving on the right lanes and those on the left lanes. At a time, the number of cars on the right lane would equal those on the left lanes. We might call this situation, Equilibrium. No sooner has this state of equilibrium been attained, than it is again lost. This is because, a car for example soon joins those on the right lane (or the left lane). Chemical equlibrium is like this. It is ever dynamic. Chemical equilibrium state is usually associated with reversible reactions. By the way, reversible reaction is a chemical reaction that is capable of proceeding in either direction. Consider the hypothetical reaction:  A and B reacts to form C and D as products in what may be regarded as forward reaction. As the concentration of A and B reduces with the ongoing reaction, that of C and D appreciates. Soon, enough C and D have been formed to act as reactants and in a backward reaction form A and B as products. Consequently, there is a reversal of roles in reversible reaction. It will not therefore be quite correct to call any side of the reversible reaction reactant or product. They (i.e. A, B, C, and D) are better referred to as reacting species. A stage is reached when the rate of the forward reaction, r1 equals the rate of backward reaction, r2. Thus, r1 = r2 = 0 This stage is the Equilibrium. OPEN SYSTEM This is a reacting unit such as a vessel, container, or some other kind of set-up that has no cover, lid or zero-confinement. Consider the earlier example of reversible reaction between A and B to form C and D and vice versa. Suppose one of the reacting species, say D, is gaseous and the reaction is taking place in an open system, the reaction would proceed to completion. By this we mean that there may be no sufficient D to react with C to form A and B. Equilibrium is very hard to reach, therefore, in an open system. CLOSED SYSTEM In a closed system, all the reacting species are confined within a given space such as in a kettle with a lid containing boiling water. In such situation, the rate at which the water is turning to steam may equal the rate of condensation of the steam back to water. This is due to the fact that the reacting species are all confined within the kettle space. It is obvious to see therefore that equilibrium is much easily attainable in closed system than in the open system. THE LAW OF MASS ACTION AND EQUILIBRIUM CONSTANT  Prior to the 19th century, it was assumed that the rate of reaction in rapid reaction permitted them to go on to completion. It was soon realized however that the degree of completeness of a reaction is determined by its equilibrium constant. This concept was introduced and explained by the Norwegian chemists, Cato Maxillian Guldberg and Peter Waage in 1864. Working together, they arrived at the conclusion that the rate of
reaction is proportional to the active masses of the reactants. This is
better known as the law of mass action. From the mathematical elucidation of this law of mass action, equilibrium constant, K, is obtained:  
Copyright © Kingsley Idiagbor | 5 September 2005- All rights reserved. No part of this article may be reproduced, transmitted or stored in electronic retrieval system without first obtaining permission from the owner of the copyright. Violators of this copyright notice are liable to civil action under the applicable Federal Laws |
Print this article