CBSE Class XIth Chemistry Chemical Equilibrium -Shruti Mehra
Chemical Equilibrium Chemical reactions also attain a state of equilibrium. These reactions can occur both in forward and backward directions. When the rates of the forward and reverse reactions become equal, the concentrations of the reactants and the products remain constant. This is the stage of chemical equilibrium. This equilibrium is dynamic in nature as it consists of a forward reaction in which the reactants give product(s) and reverse reaction in which product(s) gives the original reactants.
Homogenuous Equilibrium In a homogeneous system, all the reactants and products are in the same phase. For example, in the gaseous reaction, N2 (g) +3H2(g) 2NH3(g), reactants and products are in the homogeneous phase. Similarly, for the reactions, CH3COOC2H^(aq) + H20 () CH COOH (aq)+C2H,OH (aq) and, Fe3+ (aq)+ SCN-(aq) Fe(SCN)2+(aq) all the reactants and products are in homogeneous solution phase. We shall now consider equilibrium constant for some homogeneous reactions.
Heterogeneous Equilibrium Equilibrium in a system having more than one phase is called heterogeneous equilibrium. The equilibrium between water vapour and liquid water in a closed container is an example of heterogeneous equilibrium H20() H20(g) In this example, there is a gas phase and a liquid phase. In the same way, equilibrium between a solid and its saturated solution, Ca(OH)2(s) (aq) Ca2+ (aq) 20H(aq) is a heterogeneous equilibrium
Let us consider a general case of a reversible reaction, A+BCD With passage of time, there is accumulation of the products C and D and depletion of the reactants A and B .This leads to a decrease in the rate of forward reaction and an increase in he rate of the reverse reaction. Eventually, the two reactions occur at the same rate and the system reaches a state of Equilibrium. Similarly, the reaction can reach the state of equilibrium even if we start with only C and D; that is, no A and B being present initially, as the equilibrium can be reached from either direction.
Attainment of Chemical Equilibrium Time &quiloi ->equitoium
+ Equilibrium can be attained from both sides, whether we start reaction by taking, H2(g) and N2(g) and get NH3(g) or by taking NH3(g) and decomposing it into N2(g) and H2(g) N2(g)3H2(g) 2NH3(g) 2NH3(g)N2(g) + 3H2(g) Synthesis of ammonia by Haber's process H2 Cxeactent) N2 Csreac hant) adue) Time
+ Let us consider the reaction, H2(g) 2(g) 2HI(g) If we start with equal initial concentration of H2 and 12, the reaction proceeds in the forward direction and the concentration of H2 and 2 decreases while that of HI increases, until all of these become constant at equilibrium I Hl
+ Based on the extent to which the reactions proceed to reach the tate of chemical equilibrium, these may be classified in three groups negligible concentrations of the reactants are left. formed and most of the reactants remain unchanged at + () The reactions that proceed nearly to completion and only + (i) The reactions in which only small amounts of products are equilibrium stage. + (ii)The reactions in which the concentrations of the reactants and products are comparable, when the system is in equilibrium. The extent of a reaction in equilibrium varies with the experimental conditions such as concentrations of reactants, temperature, etc.
A mixture of reactants and products in the equilibrium state is called an equilibrium mixture + What is the relationship between the concentrations of reactants and products in an equilibrium mixture? + How can we determine equilibrium concentrations from initial concentrations?
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