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www.tiwariacademy.in, UNIT 7 : EQUILIBRIUM, CONCEPT WISE HANDOUTS, KEY CONCEPTS :, 1. Equilibrium in Physical Processes, 2. Equilibrium in Chemical Processes, 3. Law of chemical equilibrium and equilibrium constant, 4. Applications of Equilibrium Constant, 5. Factors affecting Equilibrium, 6. Acids , Bases & Salts, 7. Ionic Equilibrium in Solution, 8. Ionization of Acids & Bases, 9. Hydrolysis of Salts, 10. Buffer Solutions, Concept 1. Equilibrium in Physical Processes, , Chemical equilibrium is the condition which occurs when the concentration of reactants and, products participating in a chemical reaction exhibit no net change over time., , , The state of equilibrium is a state in which the measurable properties of the, system will not undergo any observable change under a given set of conditions., , , , All these observable properties of a system become constant at the state of, equilibrium., , , , 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., , www.tiwariacademy.com, Focus on free education

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www.tiwariacademy.in, SOLID - LIQUID EQUILIBRIUM, When a solid – liquid system at melting point is taken in a well – insulated container, then, this system constitute a system in which solid is in dynamic equilibrium with liquid., For Example: If ice and water at 273K (melting point of ice) taken in a perfectly insulated, thermos flask. It may be noted that temperature as well as masses of ice and water remain, constant. This represents a dynamic equilibrium between ice and water., Ice ⇌ Water, Since there is no change in mass of ice and water, the number of molecules going from ice, into water is equal to number of molecules going from water into ice. Thus, at equilibrium,, , Rate of melting = Rate of freezing, , LIQUID – GAS EQUILIBRIUM, Evaporation of water in a closed evacuated vessel at room temperature continues for some, time leading to the gradual decrease in level of water. After some time the level of water, becomes constant indicating that a state of equilibrium has attained between water and, water vapours., H2O (l), , ⇌, , H2O (g), , Rate of evaporation = Rate of condensation., SOLID – GAS EQUILIBRIUM, Volatile solids sublime and form vapours. When kept in closed container at, constant temperature equilibrium is established., I2 (s) ⇌ I2(g), Characteristics of Physical Equilibria, ,  In the case of Liquid ⇌ vapour equilibrium, the vapour pressure is a constant at a, given temperature.,  For solid ⇌Liquid equilibrium , there is only one temperature(melting point) at 1, atm at which the two phases can coexist without any exchange of heat with the, surroundings, the mass of the two phases remain constant.,  For dissolution of solids in liquids, the solubility is constant at a given temperature.,  For dissolution of gases in liquids the concentration of a gas in liquid is proportional, to the pressure(concentration) of the gas over the liquid., www.tiwariacademy.com, Focus on free education

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www.tiwariacademy.in, Concept 2. Equilibrium in Chemical Processes, A large number of chemical reactions do not go to completion and attain a state of equilibrium after, some time. The state of equilibrium involving a chemical system is referred to as chemical, equilibrium., Reversible reaction : N2 + 3 H2 ⇌ 2 NH3, Irreversible reaction : NaCl(s) + NaNO3(aq) AgCl  AgNO3(aq) + NaCl(aq), A reversible reaction can be made irreversible if one of the product (s) which is gaseous is allowed to, escape out. This is why reversible reactions are done in a closed vessel., Characteristics of Chemical Equilibrium, The important characteristics of chemical equilibrium are :, , (i) At equilibrium the concentration of each component of reactants and products becomes, constant., (ii) The rate of forward reaction will be equal to rate of backward reaction. So, a chemical, equilibrium is said to be dynamic in nature., (iii) A chemical equilibrium is established provided none of the products is allowed to, escape, i.e., only in a closed system a chemical equilibrium can be attained and not in an, open system., (iv) Chemical equilibrium can be attained either from the direction of reactants or from the, direction of products depending upon the conditions like temperature and pressure., (v) A catalyst does not alter the state of equilibrium. It is due to fact that a catalyst, influences both the forward and backward reaction to the same extent. It only helps to, attain the equilibrium rapidly., , www.tiwariacademy.com, Focus on free education

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www.tiwariacademy.in, Concept 3. Law of Chemical Equilibrium & Equilibrium Constant, Two Norwegian chemists Guldberg and Waage (1864) suggested a quantitative relationship known, as the Law of Mass Action between rates of reactions and the concentrations of the reacting species., This law states that:, At constant temperature, the rate of a chemical reaction is directly proportional to the product of the, molar concentrations of reacting species with each concentration term raised to the power equal to, the numerical coefficient of that species in the chemical equation representing the chemical change., ⇌, , aA + bB, if concentrations, [C]c [D]d, ------------ = Kc, [A]a [B]b, , cC + dD, , if gases, [pCc] [pCc], ------------ = Kp, [pAa] [pBb], , Kp = Kc (RT)∆n, The concentrations of pure liquids and pure solids is taken as unity., , Application of Law of Mass Action To Chemical Equilibrium, A+B⇌C+D, , , Rate of forward reaction α [A] [B] = kf [A] [B], where kf is the rate constant for forward reaction., , , , Rate of backward reaction α [C] [D] = kb [C] [D], where kb is the rate constant for the backward reaction, , At equilibrium Rate of forward reaction = Rate of backward reaction, kf[A][B] = kb[C] [D], , Since kf and kb are constants the ratio kf / kb is also constant and it is represented by KC., The constant KC is called Equilibrium constant., , The equilibrium constant K may be defined as the ratio of the product of equilibrium, concentrations of the products to that of the reactants with each concentration term raised to the, power equal to the stoichiometric coefficient of the substance in balanced chemical equation., , www.tiwariacademy.com, Focus on free education

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www.tiwariacademy.in, The Magnitude of Equilibrium Constants, •, , The equilibrium constant, K, is the ratio of products to reactants., , •, , Therefore, the larger K the more products are present at equilibrium., , •, , Conversely, the smaller K the more reactants are present at equilibrium., , •, , If K >> 1, then products dominate at equilibrium and equilibrium lies to the right., , •, , If K << 1, then reactants dominate at equilibrium and the equilibrium lies to the left., , Applications of Equilibrium Constant, Prediction of direction of the reaction, Applications of Equilibrium Constant, •, , Predicting the extent of a reaction:, , o, , If KC > 103, then the products predominate over the reactants., , o, , If KC < 10−3, then the reactants predominate over the products., , o, , If 10−3 < KC< 103, then appreciable concentrations of both reactants and products are, present., , •, , Predicting the direction of a reaction:, Reaction quotient, Q (QC with molar concentration and Qp with partial pressure), For a general reaction,, The concentrations are not necessarily equilibrium values., If QC > KC , then the reaction will proceed in the reverse direction., If QC< KC , then the reaction will proceed in the forward direction., If QC = KC , then the reaction is at equilibrium., , •, , Calculating equilibrium concentrations, , Homogenous & Heterogeneous Equilibrium, Heterogeneous Equilibrium : One or more reactants or products are in a different phase., , , CaCO3(s), , ⇌ CaO(s) + CO2(g), , The amount of CO2 formed will not depend greatly on the amounts of CaO and CaCO3 present., Kc = [pCO2], , , CO2(g) + H2(g) ⇌ CO(g) + H2O(l), , Homogeneous Equilibrium : All reactants and products are in same phase., , , N2O4(g) ⇌2NO2(g), , , , [pNO2]2, Kc =, --------[N2O4], 4NH3(g) + 5O2(g), , 4NO(g) + 6H2O(g), , www.tiwariacademy.com, Focus on free education

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www.tiwariacademy.in, Concept 5. Factors affecting Equilibrium, 1884 Le Chatelier's Principle: A system in equilibrium that is subjected to a stress reacts in a, way that counteracts the stress., When a system in equilibrium is subjected to any change in factors like concentration,, temperature, pressure the system will act in such a way to counteract the effect of the, change., OR, A change in any of the factors that determine the equilibrium conditions of a system will, shift the equilibrium in such a way so as to reduce or to counteract the effect of the change., Changes in temperature:, o, , If the forward reaction is exothermic, an increase in temperature causes the system, to shift in the reverse direction., , o, , If the forward reaction is endothermic, an increase in temperature causes the, system to shift in the forward direction., , o, , The opposite effects would be observed for a decrease in temperature., Ex. N2O4(g) ⇌ 2NO2(g), Forward reaction N2O4(g) 2NO2(g) ∆Ho = + 58.0 kJmol-1, Backward reaction 2NO2(g)  N2O4 (g) ∆Ho = - 58.0 kJmol-1, Increase in temperature favours endothermic reaction., , Changes in concentration:, o, , An increase in the concentration of one or more reactants, or a decrease in the, concentration of one or more products, causes the system to shift in the forward, direction., , o, , A decrease in the concentration of one or more of reactants, or an increase of the, concentration of one or more products, causes the system to shift in the reverse, direction., Ex. H2(g) + I2 (g) ⇌ 2HI(g), If H2(g) is added at equilibrium , then forward reaction will be favoured whereas, addition of HI(g) will favour backward reaction., , www.tiwariacademy.com, Focus on free education

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www.tiwariacademy.in, Changes in pressure : (of homogenous gaseous systems), Pressure does not have any effect on concentrations of species that are present in solid,, liquid or solution form. On the other hand, the change in pressure affects the, concentrations of gases. According to ideal gas law, pressure and volume are inversely, proportional to each other; the greater the pressure, the smaller the volume, and vice, versa., , o, , An increase in pressure causes the system to shift in the direction of fewer gas, molecules., , o, , A decrease in pressure causes the system to shift in the direction of more gas, molecules., , o, , If the number of particles on each side of the reaction is equal, then a change in, pressure has no effect., Ex. N2 (g) + 3H2 (g) ⇌ 2NH3(g), 1vol, , 3vol, , 2vol, , The increase in pressure always affects the side that has more volume. Hence,, increase in pressure shifts the equilibrium from left to right., The pressure has no effect if the total volume of reactants is equal to the total, volume of, the products as in the following example., H2(g) + I2 (g) ⇌ 2HI(g), , The Effect of Catalyst :, A catalyst changes the rate of a reaction by providing an alternative pathway with a, lower activation energy. The lower-energy pathway is available to both the forward, and the reverse reactions of the equilibrium. The addition of a catalyst to a system in, equilibrium does not favor one reaction over the other. Instead, it increases equally, the rates of both the forward and the reverse reactions. The rate at which, equilibrium is reached is increased, but the relative concentrations of reactants and, products at equilibrium, and hence the equilibrium constant, are unchanged., , www.tiwariacademy.com, Focus on free education

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www.tiwariacademy.in, Concept 6. Acids , Bases & Salt, , An electrolyte may be defined as substance which either in molten state or in aqueous, solution can conduct electricity. Acids ,bases and salts are mostly electrolytes. There are two, types of electrolytes:, •, , Strong electrolytes-The electrolytes which are almost completely ionized in aqueous, Solution. Ex. H2SO4 , HCl, , •, , Weak electrolytes-The electrolytes which are ionized to small extent in aqueous, Solution. Ex.CH3COOH,NH4OH, Arrhenius ACIDS & BASES, , An Arrhenius acid is a compound that increases the concentration of H+ ions that are present when, added to water. These H+ ions form the hydronium ion (H3O+) when they combine with water, molecules., ., , HCl (aq)+H3O+(l)→H3O+(aq)+Cl−(aq), Strong Acids, Weak Acids, , HCl , HNO3, H2SO4, HBr , HI, HClO4, All other acids, such as HCN, HF, H2S, HCOOH, , An Arrhenius base is a compound that increases the concentration of OH- ions that are present when, added to water., , NaOH(aq) → Na+(aq)+OH−(aq), , Bronsted – Lowry ACIDS & BASES, Any hydrogen containing species (a molecule, a cation or an anion), which is capable of donating one, or more protons to any other substance, is called an acid., Any species (molecule, cation or anion), which is capable of accepting one or more protons from an, acid, is called a base., , Bronsted Acid  Conjugate Base of the Acid + H+, Bronsted Base + H+  Conjugate acid, www.tiwariacademy.com, Focus on free education

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www.tiwariacademy.in, Lewis ACIDS & BASES, An acid is a substance which can accept a pair of electrons., A base is a substance which can donate a pair of electron., , Concept 7. Ionic Equilibrium in aqueous solution, When weak acid AB( acetic acid )is dissolved in water it dissociates into A+and B- and, following eq. is obtained., AB ⇌ A+ + Bdissociation constant may be given as, Ka =, , [A+][B-], --------[AB], , (Cα * Cα), = -------C(1-α ), , α = √Ka/C, When weak base BOH (NH4OH) is dissolved in water it dissociates into BH+and OH- and, following eq. is obtained., BOH(aq) ⇌ B+(aq) + OH–(aq), Kb =, , [BH +] [OH–], ------------[BOH], , SELF IONIZATION OF WATER: Pure water is poor conductor of electricity. This shows that, water is weak electrolyte and dissociates to a small extent., H2O ⇌ H+ + OHH2O + H2O ⇌ H3O+ + OH-, , www.tiwariacademy.com, Focus on free education

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www.tiwariacademy.in, +, , Keq =, , –, , [H3O ] [OH ], ------------;, [H2O]2, , [H2O]2 = constant = K, , Keq x K = Kw ( Ionic product of water ) =, , +, , [H3O ] [OH–], , It must be noted that ionic product of water is constant only at constant temperature . Its value at, 298 K is found to be 1.008 x10-14, Kw = 1.008 x10-14, , Kw = 1.008 x10-14 =[H3O +] [OH–], [H3O]+ = [OH-], Kw= [H3O+] [OH-] =1.0 x10 -14, [H3O+]2 =1.0 x10 –14, [H3O+] =1.0 x10 –7 mol l-1, Thus in pure water at 298 K, [H3O]+ = [OH-] = 1.0 x10 –7 mol l-1, In general, •, , For acidic solution: [H3O+] > [OH-], , •, , For basic solution: [H3O+] < [OH-], , •, , For neutral solution: [H3O+] = [OH-], , FACTORS INFLUENCING THE DEGREE OF IONIZATION, 1. Dilution, 2. Temperature, 3. Nature of solute, 4. Nature of solvent, 5. Presence of other substances, , www.tiwariacademy.com, Focus on free education

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www.tiwariacademy.in, One measure of the strength of an acid is the acid-dissociation equilibrium constant Ka., When Ka is relatively large, the acid is strong , HCl : Ka = 1 x 103, When it is small, we have a weak acid, CH3COOH: Ka = 1.8 x 10-5, When it is very small, we have a very weak acid,H2O: Ka = 1.8 x 10-16, , Concept of pH, The symbol pH has been taken from danish word ‘potenz the hydrogen ion’which means power of, hydrogen ion.The pH of solution may be defined as negetive logarithm of hydrogen ion, concentration., pH = -log [ H3O+ ], , = log 1 / [ H3O+ ], , log [ H3O+ ] = -pH, , The pH scale range from 0 to 14., , , Numbers lower than 7.0 have higher concentrations of hydrogen ions and therefore are, more acidic., , , , Numbers higher than 7.0 have lower concentrations of hydrogen ions and therefore are, alkaline or basic., , www.tiwariacademy.com, Focus on free education

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www.tiwariacademy.in, Concept 10. Common Ion Effect & Buffer Solutions, The Common Ion Effect, “The extent of ionization of a weak electrolyte is decreased by adding to the solution a, strong electrolyte that has an ion in common with the weak electrolyte.”, Ionization of a weak acid such as HF, HF(aq) ⇌ H+(aq) + F–(aq), The concentration of F– ion can be increased by adding sodium fluoride, a strong electrolyte:, NaF(aq) ⇌ Na+(aq) + F–(aq), According to Le Chatelier's principle, the shift in equilibrium caused by the addition of an ion, common to one of the products of a dissociation reaction is called the common ion effect., , Buffer Solutions: Buffers are solutions of a weak conjugate acid-base pair and are particularly, resistant to pH changes, even when strong acid or base is added., A buffer must contain an acid to react with any OH– ions that may be added, and a base to react, with any added H+ ions. A buffer solution may contain a weak acid and its salt (for example HF and, NaF), or a weak base and its salt (for example NH3 and NH4Cl)., , There are two types of buffers, acid buffer and basic buffer., Acid buffer : A buffer solution containing a large amounts of a weak acid, and its salt with a strong, base, is termed as an acid buffer. Such buffer solutions have pH less than 7 at 298 K., Basic buffer : A buffer solution containing relatively large amounts of a weak base and its salt with a, strong acid, is termed as a basic buffer. Such buffers have pH higher than 7 at 298 K., Mechanism of Buffer Action, Action of an acid buffer : An acid buffer contains relatively large amounts of a weak acid (CH3COOH), and its salt with a strong base (CH3COONa). The buffer solution thus contains large concentration of, CH3COOH and CH3COO– (due to the dissociation of the salt), apart from H3O+ and OH-., An addition of small amount of a strong acid causes the following reaction, H3O+ ( from added acid) + CH3COO– ⇌ CH3COOH + H2O (from buffer), , www.tiwariacademy.com, Focus on free education

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www.tiwariacademy.in, An addition of small amount of a strong base causes the following reaction, –, , –, , CH3COOH (from buffer) + OH (added from base) ⇌ CH3COO + H2O, Since the ratio of weak acid to salt remains virtually unchanged. As a result, no observable change in, the pH value is seen., Action of basic buffer : A basic buffer contains a weak base (NH4OH), and its salt with strong acid, (HCl). The buffer solution thus contains large amounts of the weak base NH4OH, and the cation, NH4+ (coming from the dissociation of the salt NH4Cl), in addition to H3O+ and OH-., The addition of an acid or a base to the basic buffer causes the following reactions:, H3O+ ( from added acid) + NH4OH (in buffer) ⇌ NH4+ + 2H2O, OH– (added from base) + NH4+(in buffer) ⇌ NH4OH, , Buffering Capacity represents the amount of H+ or OH- the buffer can absorb without a significant, change in pH., , A buffer with a large capacity contains large concentrations of buffering components, , and can absorb a relatively large amount of protons or hydroxide ions without much pH change., , Buffers,  resist changes in pH from the addition of acid orbase, , , in the body absorb H3O+ or OH from foods and cellular processes to maintain pH, , , , are important in the proper functioning of cells and blood, , , , in blood maintain a pH close to 7.4; a change in the pH of the blood affects the uptake of, oxygen and cellular processes ., , www.tiwariacademy.com, Focus on free education