Chemical Equilibrium:
The state of equilibrium set up a chemical reaction is called chemical equilibrium.
Decomposition of Calcium carbonate:
Consider that calcium carbonate is heated in a closed vessel at 1073 K. The vessel is fitted with a manometer.  Due to the formation of CO2, manometer records a rise in pressure. After some time, the pressure (measurable property) becomes constant although some CaCO3 is still present in the vessel. This shows that the amount of CO2 formed has become constant.  Thus, “the state at which the measurable properties of the system such as concentration, pressure, density, colour etc. Do not undergo any further noticeable change is said to be in a state of equilibrium.
At the state of equilibrium; the Rate of forward reaction =the rate of the backward reaction
(I) To start with, the conc. Of reactants is large. Thus, only forward reaction takes place.
(II) With the passage of time, the conc. of reactants goes on decreasing and so rate of forward reaction decreases. As more and more products are formed the rate of backward reaction increases.
(III) Ultimately a stage reaches when the rate of forward reaction equals the rate of backward reaction. This is the state of equilibrium.
In this example, the concentrations of CaCO3, CaO and CO2 become fixed or constant at the state of equilibrium. Since CO2 is gaseous species involved, the constancy in pressure indicates the state of equilibrium.
Combination of hydrogen and iodine:  Let H2 and I2 be enclosed in a vessel at 4500C. The colour is deep purple to start with. But with the passage of time, the intensity of the colour decreases and no further change in colour takes place. At this stage: The reaction attains the state of equilibrium.
Characteristics of Chemical equilibrium
The characteristics of chemical equilibrium are:
(I) Constancy in properties: At the state of equilibrium, the observable properties such as pressure, concentration, colour etc. Become constant. These properties remain constant till the conditions are not changed.
(II) Chemical equilibrium is dynamic in nature: The constancy in properties gives an impression that the reaction has stopped. But it is not true. Actually both the forward and the backward reactions continue even after the state of equilibrium has reached. Here the rates of the two opposing processes become equal. Since both the opposing processes continue to take place even after the attainment of equilibrium, the equilibrium is said to be dynamic.
(III)(a) The state of equilibrium can be approached from either side. If H2(g) and I2(s) are heated in a closed vessel at 450 0C, 78% of HI is formed at equilibrium.
Also if we state the reaction by taking HI in the closed vessel at 4500C, then 78% of HI is again left at equilibrium.
(b) Consider the following equilibrium in a long necked flask.
At 00C, N2O4 is stable and exists as pure N2O4 and is colourless.
At 1000C, it decomposes completely to NO2 which is dark brown colour. If the two flasks containing N2O4 at 00C and NO2 at 1000C are placed in water at 250C, then colour in the two flasks start changing and become identical after sometime. The constancy in colour indicates that the equilibrium has been attained in both cases and both the flasks contain mixture of N2O4 and NO2 of the same composition
(iv) A system of chemical reaction can attain a state of equilibrium only in a closed vessel if one or more reacting species are in the gaseous form.
(v) The presence of catalyst increases, the rate of both the opposing reactions and thus equilibrium is reached sooner.








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