Download Equilibrium Constants and Reaction Quotient and more Study notes Chemistry in PDF only on Docsity! Ch. \3. EquilibriuJP II At e uilib ' th ' q ~um e rate at which products are produced from reactants equals the rate at which reactants are produced from products. It AĀ¢B c .g Variation in Ā«: c:.... concentrations in the approach OJ' to equilibrium for Nz + Hz ~ u C 2NH 3ā¢ (a) The equilibrium iso U approached beginning with Hz .and N1 in the ratio 3:1, (b) The equilibrium is approached beginning with NH 3. A. Equilibrium expression - Rate law cannot be determined by stoichiometry. _ Equilibrium ex~sions are determined by stoichiometry. _ For a general equilibrium a A + b B ~ c C + d D [CnD]d Note: product over reactants. K _.:..........c"--_ C Ā [A]a[B]b TimeĀ (a) TimeĀ (b) ā¢ The value of the equilibrium constant at any given temperature does not depend on the initial concentrations of reactants and products. It also does not matter whether oth~r substances are present, as long as they do not react with a reactant or product. The value of the equilibrium constant does vary with temĀ perature, however.-The common practice is to write equilibrium constants as diĀ mensionless quantities. For ~es, equilibrium constants can be expressed in terms of pressure rather than concentration. For a general gas- phase equilibrium: aA+bB =:;cC+dD r--------~ ,.. o r; .= ~ [B] ~ : I3 / I 0 TimeĀ (a) IEquilibrium achieved : (rates are equal): Achieving chemical equilibrium for the reaction A ~ B. (a) The reaction of pure compound A. with initial concentration [AlQ , After a time the concentrations of A and B do not change. The reason is that (b) the rates of the forward reaction (k~A]) and the reverse reaction (k,fB]) become equal. (b) Be Careful!! Kp is usually not equal to ~. - Direction of chemical equation and ~. 028 consider 2 03 (g) !:; 3 02 (g) Kc =4.38 xl [02r f' h vil f: d.K = [0 y Note: Formation 0 oxygen is ea y avore c 3 What if we wrote equilibrium as 3 02 (g) !:; 2 03 (g)? ,_[03Y _ 1 =228x10-29 Kc - [02Y - 4.38 X 1028 . Note: Kc still indicates that oxygen is heavily favored. - 1 K'=Ā c K c Heterogeneous Equilibria . - Equilibria expressions depend on concentration. - Consider: Ni (s) + 4 CO (g) !::;Ni(CO)4 (g) - What is the concentration ofthe solid? - Since moles of solid Ni per volume is constant, concentration is constant. - The constant concentration is incorporated into the equilibrium constant. lV~ [Ni(CO)4 ] K = (PNi(CO)<) . [J~ .' ,K = c [COr p' ( r I .?Peo rr C. - " Heterogeneous equilibrium is equilibrium between different phases. L-Solid components or liquid solvent components are usually ignored in writing equilibrium expressio 11' Relationship between .& and &. 1"1. () \ " 1" . Recall for molarity [A]=~ V For ideal gases, pV =nRT 2:.=..E.... or [A]= PA :=} PA=[A]RT V RT RT Recall for general gas-phase equilibria: aA+bB !:;cC+dD (Per(PD t ([C]RT)"([D]RTt Kp =(pJa(PBt =([A]RT)"([B]RT)b [C]"(RTt[ D]d(Rt)d [Ct[D]d(RT)"+d K~ =[Ar(RT)a[B]b(RT)b =[A]a[B]b(RT)a+b \S:::" Kc(RTf+j-(a+b) = K c RT.).f where tĀ§l =coefficients of products - coefficien f reactants . ~< /'/ I .. ~r:
