CHEM 104 1st Edition Lecture 19 Magnitude of kc gives information about amount of reactants products at equilibrium o If kcis much larger than 1 the reaction is strongly product favored at equilibrium o If kc is much smaller than 1 the reaction is strongly reactant favored at equilibrium o If kc is approximately 1 there is a significant amount of both reactants and products present at equilibrium Equilibrium constants depend on temperature o Example 2NO2 g N2O4 gas Forward rate k1 NO2 2 and reverse rate k 1 N2O4 Forward rate reverse rate at equilibrium Kc N2O4 NO2 2 k1 k 1 At 25 C kc 3 5 108and at 450 C kc 0 16 there s more NO2 at higher temp and more N2O4 at lower temp At equilibrium forward and reverse reactions occur at the same rate For gases only equilibrium constants can be expressed using partial pressures o Example CO g 3H2 g CH4 g H2O g Kp P CH4 P H2O P CO P H2 3 Kc CH4 H2O CO H2 3 o Relationship between partial pressure and concentration derived from PV nRT P n v RT P x RT o Kp kc RT change in n Change in n moles of gaseous product moles of gaseous reactant o Example CS2 g 4H2 g CH4 g 2H2S g Kc 0 28 at 900 K Kp kc RT n 0 28 0 082 1173 2 3 02 10 5 o Kc kp when there are equal number of moles on each side of equation LeChatelier s Principle If a system is at equilibrium and a change in conditions is made so that the system isn t at equilibrium the system will react to the new equilibrium such as to partially counteract the change o Changes in concentration or partial pressure of reactants or products o Example cisbutane transbutanekc trans cis 1 5 at 600 K If we increase cis the equilibrium will shift right If we increase trans the equilibrium will shift left o The Reaction Quotient Q aA bB cC dD Kc C c D d A a B b Q C c D d A a B b Difference is Q is actual concentration when system isn t at equilibrium whereas kc is equilibrium concentration Q kc at equilibrium Q kc fewer products equilibrium will shift right Q kc fewer reactants equilibrium will shift left o Example Cr2O72 aq 2HO aq 2CrO42 aq H2O l If we add NaOH to double HO Kc CrO42 2 Cr2O72 HO 2 Q CrO42 Cr2O72 2 HO 2 kc 4 Q kc so there are fewer products
View Full Document
Unlocking...