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Introduction

In stoichiometric calculations, we assume that reactions run to completion. However, when a chemical reaction is carried out in a closed vessel, the system achieves equilibrium. Equilibrium occurs when there is a constant ratio between the concentration of the reactants and the products. Different reactions have different equilibria. Some may appear to be completely products, however, all reactions have some reactants present. A reaction may look "finished" when equilibrium is reached, but actually the forward and reverse reactions continue to happen at the same rate. A reverse reaction is when the written reaction goes from right to left instead of the forward reaction which proceeds from left to right. This is why equilibrium is also referred to as "steady state".

In this chapter we are going to study how equilibrium expression is derived for difficulty soluble salts and its application in qualitative analysis.

Chemical reactions in which reactants are almost converted into products are well known. Such reactions are called irreversible reactions. A few such reactions are given below.
  1. Neutralization reaction of an acid with a base. Taking an example of HCl and NaOH, we have HCl + NaOH NaCl + H2O
    The neutralization can be simply written as
    H+ + OH- H2O
  2. Precipitation reaction of a cation with an anion. Taking an example of AgNO3 and KCl, we haveAgNO3 + KCl AgCl + KNO3 or simply written as Ag+ + Cl- AgCl
  3. Redox reaction of an ion with appropriate oxidizing agent. Taking an example of FeSO4 and KMnO4 in acidic medium, we have
    2KMnO4 + 10FeSO4 + 8H2SO4 2MnSO4 + K2SO4 + 5Fe2(SO4)3 + 8H2O
    or simply written as
Besides the above category, many reactions are known in which reactants are not completely converted into products. In fact, the reaction proceeds only to a certain extent and the resulting mixture contains both reactants and products in equilibrium with each other. At the final stage, when the properties of the system remain constant with time, the reaction is said to be at equilibrium. A few such reactions are given below. 
H2(g) + I2(g)2HI(g)
N2(g) + 3H2(g)2NH3(g)
N2O4(g) 2NO2(g)

At equilibrium, the processes of converting reactant(s) into product(s) and vice versa do not cease but both of them occur simultaneously with equal rates, i.e. the rate of conversion of reactant(s) into product(s) becomes equal to the rate of conversion of product(s) into reactant(s). In other words, the reactant(s) and products(s) are formed as quickly as they are destroyed; consequently, their compositions remain constant. Thus, at equilibrium, the system involves dynamic equilibrium and not static equilibrium. In the latter, all processes taking place in system are supposed to cease.

I: Equilibrium in physical and chemical processes.
An equilibrium stage can be reached starting from either reactant(s) or product(s) or both. For example, the equilibrium stage of
N2O4(g)2NO2(g)

can be achieved starting from either reactant (any amount of N2O4 can be taken) or product (any amount of NO2 can be taken) or both (any arbitrary amounts of N2O4 and NO2 can be taken). Irrespective of how the equilibrium is achieved, at equilibrium the rate of formation of NO2 from N2O4 and the rate of formation of N2O4 from NO2 are equal. Initially, if we start from N2O4 the rate of formation of NO2 is fast. It then steadily decreases with time till it becomes constant at the equilibrium stage. On the other hand, the rate of formation of N2O4 from the obtained NO2 steadily increases till it also becomes constant at the equilibrium stage (Fig.a). Similarly, if we start from NO2, then the rate of consumption of NO2 to give N2O4 is fast to start with and steadily decreases with time till it becomes constant at the equilibrium stage. On the other hand, the rate of formation of N2O4 from the obtained NO2 steadily increases till it becomes constant at the equilibrium stage (Fig.b). Since the equilibrium position can be reached in either direction (reactants
equilibrium products), such reactions are called reversible reactions.




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