Notes

Outline

Molecular Equilibria: Gas Systems

Dynamic Equilibrium Dynamic Equilibria-Two opposing prosesses that occur simultaneous at equal rates aA  +  bB = cC Rf = k1 [A]a [B]b Rr = k-1[C]c At equilibrium: Rf = Rr k1[A]a [B]b = k-1[C]c K1 /k-1 = [C]c / [A]a [B]b Kc =K-1 /k1 = [C]c / [A]a [B]b

T-148 conc vs time and rate vs time graph of equilibrium

T-149 The Haber Process

T-152 Haber Reactor

Law of Chemical Equilibrium aA(g)  +  bB(g)  =  pP(g)  +  qQ(g) Based on Molar Concentration Kc = [P]p [Q]q  /  [A]a [B]b Based on Partial Pressures Kp = Pp Pq  /  Pa Pb Kc and Kp remains constant at a fixed temperature

T-150 Equilibrium Data for NO2/ N2O4 System

Writing Equilibrium Expressions 3H2(g)  +  N2(g) = 2NH3(g) Kc = [NH3]2 / [N2] [H2]3 Kp = (PNH3)2 / (PH2)3 PN2

Magnitude of Equilibrium Constant K increases then components on the right side predominate in the equilibrium mixture K decreases then the components on the left side predominate in the equilibrium mixture Example

Reversed Equilibria aA(g) = bB(g)  K1 = [B]b / [A]a bB(g) = aA(g)  K2 = [A]a / [B]b Therefore: K1 = 1 / K2

Heterogeneous Equilibria Heterogeneous Equilibrium –equilibria with two or more physical states represented. Liquid and solid states do not affect equilibria because of incompressibility Examples CaCO3(s) = CaO(s)  +  CO2(g) Kc = [CO2] Kp = PCO2 FeO(s)  +  H2(g) = Fe(s)  +  H2O(g) Kc = [H2O] / [H2] Kp = PH2O / PH2

Determining the Equilibrium Constant Given the equilibrium concentrations(or Partial Pressures) of all components Given the initial concentration (or partial pressure)  of components

Relationship Between Kc and Kp Kp = Kc (RT) Dn where Dn = mols of gaseous component on right – mols of gaseous components on left Example:

Predicting The Direction A Reaction Must Proceed To Reach Equilibrium For H2(g)  +  I2(g) = 2HI(g) Q = quotient = [HI]2 / [H2] [I2] If Kc = Q Rx in Equilibrium If Kc < Q then Rx must proceed Left (favor reverse reaction) to reach equilibrium (Kc = Q) If Kc > Q then Rx must proceed to Right (favor forward reaction) to reach equilibrium Example

Determining Equilibrium Concentrations Given the equilibrium concentrations(or partial pressures) of all components but one and the Kc(or Kp) Given the initial concentration (or partial pressures) of the reactants and the Kc(or Kp)

LeChatlier’s Principle When an external stress  is applied to a system in equilibrium, the equilibrium is upset and will respond by shifting in such a way as to undo the stress applied.

External Stresses Change in the concentration of the components (adding or removing) Change in the volume of the container (alters pressures of reacting gases) Change in the temperature

Changing The Concentration of A Component Increasing the concentration of a component (adding component) of a component drives the reaction to the opposite side (uses up the component and relieves the stress) Decreasing the Concentration of a component(removing component) drives the reaction to the same side (produces more component thereby relieving the stress by replacing what was removed) Examples

T-151  Changing Concentrations In An Equilibrium

Changing The Volume of The Container Increasing the volume of a container decreases the pressures of the gaseous components(Boyles Law) which drives the reaction toward the side with the most total mols of gas(brings pressure back up) Decreasing the volume of the container increases the pressure of the gaseous components (Boyles Law) which drives the reaction toward the side with the least total mols of gas (brings pressure back down) Examples

T-14.7 Pressure Effect On Equilibrium

Temperature Effect On Equilibrium Increasing the temperature will always favor the endothermic process (uses up energy added by increased temperature) Decreasing the temperature will always favor the exothermic process(replaces the energy removed by stress) Examples

T-14.8 Effect on Equilibrium on Changing The Temperature

Catalytic Effect On Equilibrium Catalyst alters the Energy of Activation of both processes Therefore equilibrium is not changed