Title CHEMICAL EQUILIBRIUM.pdf ✅

 Digital www.allendigital.in [ 105 ] Introduction : Chemical reaction: Symbolic representation of any chemical change in terms of reactants and products is called a chemical reaction. (a) On the basis of heat: (i) Endothermic reaction: The reaction in which heat is absorbed is called as an endothermic reaction. H = HP – HR = +ve i.e. (i) Stability of reactant > Stability of product (because more heat is required to break the bonds of reactant). (ii) The product formed in the endothermic reaction is called as an endothermic compound. (iii)If more heat is absorbed then the product formed in the reaction will be less stable or the reactant is more stable. Representation of an endothermic reaction : A + B + x Cal → C + D (endo) A + B → C + D – x Cal (endo) A + B → C + D, H = + x Cal (endo) Examples: (I) Dissociation reactions (mostly) (II) Fusion (III) Vaporisation (IV) Sublimation (V) Photosynthesis In photosynthesis: 6CO2 + 6H2O → C6H12O6 + 6O2 ; H = +ve Types of Chemical Reaction (a) On the Basis of Heat (b) On the basis of direction (i) Exothermic Reaction (ii) Endothermic Reaction (i) Reversible Reaction (ii) Irreversible Reaction H = +ve H > H P R 03 Chemical Equilibrium
NEET : Chemistry [ 106 ] www.allendigital.in  Digital (ii) Exothermic reaction: The reaction in which heat is evolved is called as an exothermic reaction. H = –ve H = HP – HR = –ve i.e. HP < HR (i) Stability of reactant < Stability of product. (Because less heat is required to break the bonds of reactant). (ii) The product formed in the exothermic reaction is called as an exothermic compound. (iii)If more heat is released then the product formed in the reaction will be more stable or the reactant is less stable. Representation of exothermic reaction. A + B → C + D + x Cal (exo) A + B – x Cal → C + D (exo) A + B → C + D, H = – x Cal (exo) Examples: (I) Combustion reactions (II) Neutralisation reactions (III) Respiration reaction eg. C6H12O6 + 6O2 → 6CO2 + 6H2O (IV) Formation reactions Exceptions of formation reactions : (endothermic) (b) On the basis of direction Reversible reaction Irreversible reaction (i) Chemical reaction in which products can be converted back into reactants. N2 + 3H2 2NH3 3Fe + 4H2O Fe3O4 + 4H2 H2 + I2 2HI (i) Chemical reaction in which products cannot be converted back into reactants. AgNO3 + NaCl → AgCl  + NaNO3 NaCl + H2SO4 → NaHSO4 + HCl Zn + H2SO4 → ZnSO4 + H2  (ii) Proceed in forward as well as in backward direction. (ii) Proceed only in one direction (forward direction). (iii) Generally possible in closed container (iii) Generally possible in open container (iv) They can attain equilibrium (iv) They do not attain equilibrium (v) Reactants are never completely converted into products (v) Reactants are nearly completely converted into products. 2 2 2 3 2 2 2 2 2 4 2 2 2 2 2 C + 2S CS 1 3 N Cl NCl 2 2 H = +ve N O NO/N O / NO / N O O F OF / O F   ⎯⎯→     + ⎯⎯→        + ⎯⎯→     + ⎯⎯→ endo exo (generally)
Chemical Equilibrium  Digital www.allendigital.in [ 107 ] Equilibrium and Chemical Process Chemical Equilibrium The most important characteristic property of a reversible reaction is that it always attains a state of chemical equilibrium. Consider a general reversible reaction in a closed vessel where rf = rate of forward reaction rb = rate of backward reaction Initially reaction begins in the forward direction but as the concentration of products increase reaction also begins to occur in the backward direction. At a certain stage, rate of forward reaction becomes equal to the rate of backward reaction known as the equilibrium state. At equilibrium state Rate of forward reaction (rf) = Rate of backward reaction (rb)  The concentration of the reactants and products does not change with time.  At this stage, number of moles of substances produced per second in the forward reaction is equal to the number of moles of substances which disappear per second in the backward reaction. Characteristics of equilibrium: (a) Chemical equilibrium is dynamic in nature i.e. the reaction although appears to be stopped but actually takes place in both the directions with the same speed. (b) Chemical equilibrium can be approached from both sides 2HI H2 + I2 or H2 + I2 2HI At equilibrium, each reactant and product have a constant concentration and this is independent of the fact whether the reaction starts from forward direction or backward direction with the reactant or with the product. (c) Equilibrium is not affected by the presence of catalyst. The catalyst only helps in attaining equilibrium rapidly. (d) The measurable properties of the system like temperature, concentration, colour, density etc. don't undergo any change with time at the chemical equilibrium conditions. (e) Homogeneous equilibrium is the equilibrium in which the reactants and products are in the same phase. H CH COOC H CH COOH C H OH 3 2 5( ) 3 ( ) 2 5 ( ) + + (f) Heterogeneous equilibrium is the equilibrium in which the reactants and products are in two or more phases. Zn(s) + CO2(g) ZnO(s) + CO(g) f b r r A B C D + + Rate Equilibrium State (rf=rb) Time
NEET : Chemistry [ 108 ] www.allendigital.in  Digital ➢ ACTIVE MASS: The term active mass means the concentration of the reactants expressed in moles per litre (molar concentration) or the pressure of the reacting gas in atmosphere. In case of gases and solutions, the molar concentration means the number of gram molecules present per litre. Active mass is usually expressed by enclosing the symbol of the reactant in square bracket [ ]. Active mass = Number of gram moles of the substance Volume (in L) = w Weight of substance(in grams) Molecular weight (M ) Volume (in L)  = w w M V(L)  = w w 1000 M V(mL)   ➢ The active mass of solids and pure liquids is a constant quantity (unity) because it is an intensive property i.e. number of molecules present per unit volume do not change because density and molecular weight of solids and pure liquids are constant. But it does not apply for gaseous substances because for them number of molecules present per unit volume change with change in volume of vessel. Molar concentration = w w M V(litre)  = Mw  (where  = density in g L–1) = w 1000 M   (where  = density in g/mL) Active mass = 1 density of the substance (in g L ) molecular mass of the substance − Other names of active mass can also be used as following: (i) mole/litre (ii) gram mole/litre (iii) gram molecules/litre (iv) molarity (v) Concentration (vi) Effective concentration (vii) active quantity (viii) n/v (ix) C (x) M (xi) [ ] Illustration 1: In any chemical reaction, equilibrium is supposed to be established when: (1) Mutually opposite reactions occur. (2) concentration of reactants and resulting products are equal. (3) Velocity of mutual opposite reactions become equal. (4) The temperature of mutual opposite reactions becomes equal. Solution: (3) Illustration 2: If 8.5 g ammonia is present in a vessel of 0.5 L capacity then find out the active mass of ammonia. Solution:   1 3 8.5 NH 1 mol L 17 0.5 − = = 