Characterstics of chemical equilibrium

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ATTAINED IN CLOSED SYSTEMA closed system is that which neither gains nor loses any matter with the surroundings. Thus in a closed system the amount of matter remains constant. Equilibrium state is achieved when a reversible reaction is carried out in a closed container. This is because, for a reversible reaction to happen, the products should be converted back to reactants. So, a closed container will ensure that none of the products escape.

EQUAL RATES OF REACTIONSAt equilibrium, the rate of forward reaction is equal to the rate of backward reaction. That is, the rate of change of concentration of reactants is equal to the rate of change of concentration of products .i.e. R1=R2

CONSTANT COMPOSITIONAt equilibrium ,the rate of forward reaction is equal to the rate of backward reaction. This means that after equilibrium is reached, the concentration of reactants converted to products is equal to the concentration of products converted to reactants. For example, if 10 Moles/Lt of reactants is converted to products ,then 10 Moles/Lt of products convert to reactants. Thus , the concentration of reactants and products , the composition of mixture remains constant at equilibrium.

DYNAMIC IN NATUREAt equilibrium, the concentrations of reactants and products may or may nit be equal, but they remain unchanged with time. This, however, does not mean that the reaction has stopped at equilibrium. In fact, at this stage the reaction proceeds in such a way that the rate of formation of products from reactants is equal to the rate of formation of the reactants from products. Such a state of equilibrium is termed as dynamic equilibrium.

The three possible situations at the equilibrium stage are shown in the figures:

(a) When the concentrations of reactants and products are equal:

(b) When a very small amount of product is formed i.e. the concentration of unchanged reactant is large compared to the concentration of product:

(c) When the concentration of the product is much higher than that of the reactants:

DIRECTIONS OF ATTAINMENT OF CHEMICAL EQUILIBRIUMThe reactants form products until the state of equilibrium is reached , and the products form reactants until the state of equilibrium is reached. Thus, chemical equilibrium can be attained from either end of a reversible reaction. This can be demonstrated with the help of a simple experiment involving the chemical equilibrium.

EXPERIMENTTake two sealed glass bulbs A and B filled with the same sample of NO vapours. At room temperature both the bulbs have a light reddish-brown colour. Place the bulb A in ice(0oC) and B in boiling water(100oC).

The bulb A (which is placed in ice) is colorless due to the formation of N2O4, while the bulb B (which is placed in boiling water) has become deep reddish brown due to the formation of NO2. Now shift both the bulbs to a trough containing ordinary water at the room temperature. After sometime, the gas in both A and B becomes equally brown , i.e. ,deeper than that at 0o C, and lighter than that at 100oC.This shows that , the chemical equilibrium, can be attained from either end of the chemical reaction.

DISTURBANCE IN EQUILIBRIUMThe system in equilibrium state will continue to be in equilibrium , as long as the external conditions are not disturbed. That is, at equilibrium, the macroscopic properties of the system such as concentration, temperature, pressure, colour do not change with time.

RECOGNITION OF EQUILIBRIUM STATEThe attainment of equilibrium state is recognized by the constancy of some properties of the system such as density, pressure, concentration or colour intensity of the constituents. The attainment of equilibrium state can be illustrated by the following example: When limestone(CaCO3) is heated in a closed vessel at 800oC, it decomposes to give quick lime (CaO) and Carbon-dioxide(CO2). Due to the formation of gaseous product, pressure is developed in the vessel. This pressure can be recorded with the help of a Manometer.

As the decomposition of limestone continues, the pressure becomes constant at constant temperature. Thus the attainment of equilibrium is recognized.