Equilibrium March 28, 2007

Chemical Reactions Reactants  Products A + 3B  2C But the reverse can also happen 2C  3B + A So we can describe the reaction as equilibrium A + 3B 2C Forward Reaction Reverse Reaction

rate of forward reaction = rate of reverse reaction Equilibrium rate of forward reaction = rate of reverse reaction

Equilibrium - Concentration At equilibrium, the concentration of product and reactants stays constant

Equilibrium Visualization http://www.dlt.ncssm.edu/TIGER/chem5.htm

Law of Mass Action Once a reaction has reached equilibrium, the relative concentration of products remains constant We call this constant K, the equilibrium constant Example: N2 + 3H2  2NH3

Equilibrium Expression Things that appear in the equilibrium expression Concentration of solutions Pressure of gases Reaction coefficient Things that do NOT appear Pure liquids Pure solids Units

Greater number of reactants – K < 1 Learning Check Does this graph represent a K > 1, K< 1, or K =1 ? Greater number of reactants – K < 1

Learning Check Determine the Equilibrium Expression (K) for each of the following reactions CaCO3 (s) CaO (s) + CO2(g) 2NO2(g) N2O4(g) H2CO3(aq) CO2(g) + H2O(l)

Calculating the Equilibrium Constant Calculate the equilbrium constant if the equilibrium concentrations of NO2 and N2O4 are 2.0 mol/L. 2NO2(g) N2O4(g)

Equilibrium Position At constant temperature….reaction can only have one equilibrium constant but many equilibrium positions N2 + 3H2 2NH3 K = 640 (25 °C) Equilibrium concentration of each product can be… 675 45 5 800 40 10 900 30 NH3 (mol/L) H2 (mol/L) N2 (mol/L)

Value of Equilibrium Constant A + 2B 2C + D If K >>>>1, forward reaction is favored Large concentration of products If K <<<<1, reverse reaction predominates Large concentration of reactants If K = 1, reverse reaction and forward reaction equal Equal concentration of reactants

Changing the Equilibrium Constant Change the temperature Change the reaction coefficients N2 + 3H2 2NH3 2N2 + 6H2 4NH3 Relationship between K1 and K2 K2 = K12 K2 = (640)2 = 4.096 x 105

Changing the Equilibrium Constant Change the temperature Change the reaction coefficients N2 + 3H2 2NH3 2NH3 N2 + 3H2 Relationship between K1 and K3 K3 = 1/K1 K3 =1/ 640 = 0.00156

Learning Check Determine the value of the equilibrium constant for the following reaction 2NO2 N2O4 ½ N2O4 NO2

Disturbing the Equilibrium

Heat as a Reactant/ Product UO2(s) + 4HF(g)  UF4(g) + 2H2O(g

FeSCN2+ Equilibrium KSCN + Fe(NO3)3  FeSCN2+ + KNO3 SCN- + Fe3+  FeSCN2+ Fe3+ + HPO42-  FeHPO4+ Ag+ + SCN-  AgSCN Ag+ + Cl-  AgCl Spectator ions….ignore