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Chemical Equilibrium Problems

The chemical equilibrium problems is the situation in which the chemical activities or concentration of the reactants and products do not vary for a moment. Typically, this type of reactions fallout when the forward chemical process proceeds at the same rate as their reverse reaction. The resultants rates of the forward and reverse reactions are in general not zero but, being equal; there are no net changes in any of the reactant or product concentrations. This process is called dynamic equilibrium.


Significance of chemical equilibrium problems


The laws of mass achievement states that the motivating energy of a chemical reaction is proportional to the active masses of the reacting substances. Assume that the dynamic force determine the reaction rate, the law may be stated as follows: The rate at which a substance reacts is proportional to its active mass and the rate of the chemical reaction is directly proportional to the product of the active masses of the reacting substance.

Consider a general reversible chemical reaction

aA + bB ↔ mM + nN

According to the law of mass action, assuming that active masses are equivalent to molar concentrations,

The rate of the forward reaction, rf α [A]a [B]b = Kf[A]a [B]b

The rate of the reverse reaction, rr α [M]m [N]n = Kr [M]m [N]n

Where Kf and Kr are irreversible constants and square brackets symbolize the molar concentrations of the entities enclosed. At equilibrium, the speed of the forward reaction is equal to the velocity of the reverse reaction, that is, Kf[A]a [B]b = Kr[M]m [N]n

Kf /Kr = Keq = [M]m [N]n / [A]a [B]b

The constant Keq is called the equilibrium constant of the reaction.

Factors that alter the state of chemical equilibrium problems:


Le Chatelier's principle


There are three main factors which alter the state of chemical equilibrium problems. These are: concentration, temperature and pressure.

The addition of a catalyst has no effect. The function of a catalyst, as already stated, is merely to hasten the approach of equilibrium. It does so by speeding up the forward as well as the backward reaction. Le Chatelier's, a noted French chemist, studied the effect of concentration, temperature and pressure on a large number of chemical equilibrium. He summed up his conclusion in the form of a generalization known as Le Chatelier's principle, which states as follows:

If equilibrium is subjected to a stress, the equilibrium shifts in such a way as to reduce the stress.

According to this principle if a system at equilibrium is subjected to a change of concentration, pressure or temperature, the equilibrium shifts in the direction that tends to undo the effect of the change.


Solve this problems


Example 1:

Find out:

What should be the pH of the solution when 0.100 L of 0.500 M sodium hydroxide NaOH is added to 0.150 L of 0.400 M hydrochloric acid HCl?


Example 2

Find out

To what volume be supposed to a solution have 0.050 mol of each monosaccharide (formation of disaccharide sugar from monosaccharide) be diluted in order to bring about 5% conversions to sucrose phosphate?