EQUILIBRIUM

I MPORTANT T ERMS Equilibrium State- When the number of molecules leaving the liquid to vapour equals the number of molecules returning to the liquid from vapour, it is said to be EQILIBRIUM STATE. Hence at equilibrium state, the rate of evaporation is equal to the rate of condensation. Equilibrium Mixture- The mixture of reactants and products in the equilibrium state is called EQUILIBRIUM MIXTURE.

Dynamic Equilibrium- Any equilibrium in which opposite changes occur simultaneously; an equilibrium in which two reversible reactions occur at the same rate is known as dynamic equilibrium. Chemical Equilibrium- In a chemical process, chemical equilibrium is the state in which the chemical activities or concentrations of the reactants and products have no net change over time. This would be the state that results when the forward chemical process proceeds at the same rate as their reverse reaction.

Classification of Chemical Equilibrium- - The reaction that proceeds nearly to completion and only negligible concentration of the reactants are left. - The reaction in which only small amounts of products are formed and most of the reactants remain unchanged at equilibrium state. - The reaction in which the concentration of reactants and products are comparable, when the system is in equilibrium. Importance of Chemical Equilibrium- Chemical equilibria are important in numerous biological and environmental processes. For e.g.., equilibria involving oxygen molecules and the protein hemoglobin play a crucial role in the transport and delivery of oxygen from our lungs to muscles.

EQUILIBRIUM IN PHYSICAL PROCESSES Solid-Liquid Equilibrium- For any pure substance at atmospheric pressure, the temperature at which the solid and liquid phases are at equilibrium is called the normal melting point or normal freezing point of a substance. Liquid-Vapour Equilibrium- Vapor-liquid equilibrium is a condition where a liquid and its vapor are in equilibrium with each other, a condition or state where the rate of evaporation equals the rate of condensation on a molecular level such that there is no net (overall) vapor- liquid interconversion.

Solid-Vapour Equilibrium- When substances get sublimated, the solid gets converted into vapour without passing through the liquid phase. Sublimation thus involves solid vapour equilibrium. On cooling the vapours, the solid is given back. This equilibrium is obtained in closed systems only.

EQUILIBRIUM INVOLVING DISSOLUTION OF SOLID OR GASES IN LIQUIDS Solids in liquids- Sugar (solution) Sugar (solid) The rate of dissolution of sugar = Rate of crystallization of sugar Gases in liquids- CO 2 (gas) CO 2 (in solution) This equilibrium is governed by HENRY’S LAW which states that “ the mass of a gas dissolved in a given mass of solvent at any temperature is proportional to the pressure of the gas above the solvent.”

S OME F EATURES OF P HYSICAL E QUILIBRIA PROCESS Liquid Vapour H 2 O (l) H 2 O(g) Solid Liquid H 2 O(s) H2O Solute(s) Solute(solution) Gas(g) Gas (aq) CONCLUSION Pressure of water is constant at a given temperature. Melting point is fixed at constant pressure. Concentration of solute in solution is constant at a given temperature. Gas is constant at a given temperature.

E QUILIBRIUM IN CHEMICAL PROCESSES C or D A or B concentratio n time equilibrium Attainment of Chemical Equilibrium

LAW OF CHEMICAL EQUILIBRIUM AND EQUILIBRIUM CONSTANT Law of Chemical Equilibrium - The principle that in a reversible reaction the ratio of the rate of the forward reaction to the rate of the reverse reaction is a constant for that reaction. Equilibrium Constant - According to the law of mass action, for any reversible chemical reaction: aA + bB = cC + dD the equilibrium constant (K)is defined as: K = ([C]c[D]d)/([A]a[B]b)

E QUILIBRIUM S YSTEM IN G ASEOUS S YSTEM To express the gas equilibrium constant expression is with partial pressures of the gases (Kp) in the reaction instead of the concentrations. N 2(g) + 3H 2(g) 2NH 3(g) For this reaction, the Kc and Kp expressions are: K c = [NH 3 ] 2 /[H 2 ] 3 [N 2 ] and K p = [P NH3 ] 2/ [P H2 ] 3 [P N2 ] The relationship between Kc and Kp is described in the following equation: K p = K c * (RT) n

APPLICATIONS OF EQUILIBRIUM CONSTANT Equilibrium constant is applicable only when concentration of the reactants and the products have attained their equilibrium state. The value of the equilibrium constant is independent of initial concentration of the reactants and the products. Equilibrium constant is temperature dependent which have unique value for reactions. The equilibrium constant for the reverse reaction is equal to the inverse of the equilibrium constant for the forward reaction.

P REDICTING D IRECTION OF R EACTION ReactantsProducts Reactants and products are at equilibrium ReactantsProducts

L E C HATELIER ’ S P RINCIPLE ‘ It states that a change in any of the factors that determines the equilibrium conditions of a system will cause the system to change in such a manner so as to reduce or to counteract the effect of the change.’ This is applicable to all physical and chemical equilibria.

B UFFER S OLUTIONS The solutions which resist change in pH on dilution or with the addition of small amounts of acid or alkali are called Buffer solutions. A mixture of acetic acid and sodium acetate acts as buffer solution around pH 4.75 and a mixture of ammonium chloride and ammonium hydroxide acts as a buffer around pH 9.25.