Concept's of Chemical Equilibrium

(C) - Copyright, 2000 F.W. Boyle, Jr., Ph.D.

Chemicals known as reactants undergo change, called a "reaction", and form products which are chemicals that are different from the starting materials. Studies of these reactions have driven the chemist to determine the relationships between these products and reactants. One of the methods developed by chemists stems from the fact that all chemical reactions are reversible and reach a point where the rate of formation of products equals the rate of formation of reactants.

A simple reaction to demonstrate reversibility is that of the carbonation of water. Water condenses in the atmosphere where CO2(g) exists. Some of this CO2(g) dissolves in the water. Once in solution the CO2(g) reacts with water, H2O, to produce carbonic acid, H2CO3(aq). The amount of CO2(g) dissolving depends directly on the pressure of CO2(g) in the atmosphere. The chemical reaction for CO2(g) and water is shown as (1) below. Bottling companies take advantage of this chemistry to produce carbonated drinks.

CO2(g) + H2O(l) <---> H2CO3(aq) (1)

Since water is available in large quantities both in the atmosphere and in a carbonated drink, the amount of water is large when compared to the concentration of CO2(g) or H2CO3(aq). It's concentration in this system is considered constant. Thus the amount of carbonic acid produced is directly proportional to the amount of CO2(g). Through the use of numerous experiments, chemist develop a mathematical relationship called a "K" which is a mathematical constant. Each particular reaction has a constant. The magnitude (size) of the constant tells the balance of the reaction. This balance is a proportion, sometimes called a ratio. The constant, K, is found by calculating the product (multiplication type) of the amounts (either concentrations or pressures) of each chemical that are produced (products) and dividing by the product (multiplication kind of product) of the amounts (either concentrations or pressures) of the chemicals that were reacted (reactants).

Of interest to the students is the fact a given reaction, the proportion found by this methods is always constant when equilibrium is reached. That is to say, no matter where one starts with concentration and/or pressures the final relationship of the product of the amounts of the products divided by the product of the amounts of the reactants will always, always be the same.

Equilibrium is defined as the point where the forward and reverse reactions occur at the same rate. To an observer, the reaction appears to have halted since the observer is viewing at a macroscopic level. At the molecular level, each reaction which makes more products is countered by a reverse reaction (breakdown) of some products into the original reactants. Because there is a balance (equilibrium), the concentration or pressures are no longer changing and a static or constant relationship occurs.

The K for reaction (1) is:


                       [H2CO3(aq)]
 
                 K =  -----------
 
                      PCO2[H2O(l)]
The [H2O(l)], color used for clarity only and square brackets, [ ], are used to symbolize concentration, is constant and is not used in calculating a K for this reaction. If water were in limited amounts, one would include its concentration in the calculation. The K for that reaction would be different than the one given here.

One might note that since [H2O(l)] is constant it can be moved to the lefthand side of the equation and included in the constant for the CO2(g)/H2CO3(aq) equilibrium like so:

                         [H2CO3(aq)]
 
             K.[H2O(l)] = ---------
 
                            PCO2
And we can replace the K.[H2O(l)] with another K where K represents the new constant. I am using colors only to indicate that the Ks are different.

                     [H2CO3(aq)]
 
                 K = -----------
 
                        PCO2
For reaction (1), the above constant has a value of 3.47 x 10-2. The units of this constant will be molarity over atmosphere. Some K's have no units while others have mixed units. This is wholly dependent upon the reaction. It is the magnitude (size) of the K that is important, not the units.

Note that the magnitude of K is small, 0.0347, which indicates the reaction equilibrium lies (or favors) the CO2(g) side of the reaction. One needs a high pressure of CO2(g) to produce much H2CO3(aq).

In a bottled drink, the CO2(g) pressure was artificially high when the bottle was sealed. Thus the reaction equilibrium is forced to favor the products. When the consumer opens the bottle, CO2(g) is released from solution. This loss of CO2(g) is an example of what is known as Le Chatelier's Principle .

Le Chatelier's Principle states that a reaction at equilibrium will shift to reattain equilibirum when a change is made to one of the components of the reaction. In the case of the bottle, the PCO2(g) is high in the small volume of space above the drink. This pressure change is noted by the "pffft" that occurs when a bottle (or can) of carbonated drink is opened. Once the drink is open the PCO2(g) drops to atmospheric levels and the CO2 dissolved in the water begins to flow back out. The H2CO3(aq) begins to form CO2(g) and H2O(l). Reaction (1) is running in reverse.

There is a rate effect which is why the soda doesn't go "flat " immediately but the equilibrium constant above shows that ratio of the concentration of H2CO3(aq) to pressure or CO2(g) will be 0.0347 M/atm when equilibrium is reestablished. M is the symbol for molarity.

See my Ksp paper for solubility product discussion .