Dynamic Equilibrium Dynamic Equilibrium A dynamic equilibrium exists once a reversible reaction ceases to change its ratio of reactants/products, but substances move between the chemicals at an equal rate, meaning there is no net change. It is a particular example of a system in a steady state. In thermodynamics a closed system is in thermodynamic equilibrium when reactions occur at such rates that the composition of the mixture does not change with time. Reactions do in fact occur, sometimes vigorously, but to such an extent that changes in composition cannot be observed. Equilibrium constants can be expressed in terms of the rate constants for elementary reactions. A reaction which can go in the forward and backward direction simultaneously is called a reversible reaction. The state of a reversible reaction is achieved when the two opposing reactions occur at the same rate and the concentrations of reactants and products do not change with time and is found to be in chemical equilibrium. The state of equilibrium is a state in which the measurable properties of the system would not undergo any observable change under a given set of conditions.
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<<--- All these observable properties of a system become constant at the state of equilibrium. <<--- This observation might be presumed as that the reaction has stopped completely. <<--- At equilibrium the rate of forward reaction becomes equal to the rate of backward reaction. <<--- At equilibrium, there is no net change in the concentration of the molecules of the system. <<--- eventually, the equilibrium state is achieved and is a kind of dynamic balance between the forward and backward reaction taking place in between the reacting molecules. <<--- This phenomenon could be shown by reacting hydrogen and bromide to form hydrogen bromide. With the progress in reaction, the concentration of H2 and Br2 decreases and the rate of forward reaction invariably slows down. <<--- On the other hand, concentration of hydrogen bromide keeps on increasing, resulting which, the rate of backward reaction also keeps on increasing. <<--- The rate of backward reaction equals to the rate of forward reaction after sometime and the overall equilibrium of the system is achieved. <<--- These variations of the forward reaction and backward reaction rates are depicted in the graph. <<--- Thereafter, no change in the concentration is allowed, provided, the temperature of the reaction mixture is also kept constant.
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<<--- In most of the equilibrium states the reaction usually do not stop and at same time the system acquires the constant recordable properties. <<--- This is due to the equal rates of forward and backward reactions resulting in the equilibrium dynamism. The equilibrium reaction is represented as: H2(g) + Br2 (g) <==> 2HBr (g) <<--- The equilibrium achieved in physical processes such as dissolution of salt in water, or evaporation of water etc. are categorized as physical equilibrium, whereas, the equilibrium achieved in chemical processes such as reaction between H2 and Br2 is categorized under chemical equilibrium. Dynamic Equilibrium Examples Recrystallization :- Recrystallization is a process wherein the solute particles from a solution are processed to crystallize and come out of the solution. Solute Crystal â&#x2021;&#x201D; Solute dissolved The rate at which the particles leave the solid will be much greater that the rate at which they recrystallize in the beginning. After a lapse of time, the rate of dissolution of solid in the solution and rate of recrystallization becomes constant. At this stage, a state of equilibrium prevails between the molecules of solute in the solvent and recrystallized solute. This state, where, there will not be any further change in amount of solute in the solution and the recrystallized solute is said to be dynamic state or the system, is said to have reached DYNAMIC EQUILIBRIUM.
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