Section 7.3 Qualitative Changes in Equilibrium Systems Le Châtelier’s Principle

What is Le Châtelier’s Principle (LCP)? When a chemical system at equilibrium is disturbed by a change in a property, the system adjusts in a way that opposes the change.

LCP provides a method of predicting the response of a chemical system to a change of conditions i.e. the equilibrium shift. Practical Use: can produce more of the desired product and making processes more efficient/economical

LCP and Concentration Changes If there is an increase in concentration of an entity then the reaction will shift to consume what was added (i.e. shifts to opposite side of the arrow). If there is a decrease in concentration of an entity then the reaction will shift to replace what was taken away (i.e. shifts to the same side of the arrow).

LCP and Temperature Changes Equate temperature (energy) as either a reactant or product (treat like a conc. change) Endothermic (+H): reactants + energy ⇌ products Exothermic (-H): reactants ⇌ products + energy Energy can be added or removed by changing the temperature (heating/cooling).

LCP and Gas Volume Changes Boyles’ Law: the conc of a gas in a container is directly related to the pressure of the gas  pressure =  volume so it will shift to the side with fewer molecules to decrease pressure (b/c there would be fewer molecules)  pressure =  volume, so it will shift to the side that produces more gas molecules (b/c this would increase the number of molecules and thus pressure).

Changes that do not affect the position of equil systems Adding catalysts Adding inert gases (i.e. the Noble Gases)

Drawing Conc. Vs. Time graphs Reactants will go down & Products will go up At a certain time the lines will straighten out =equil. If equil. is shifted then draw a dashed vertical line and make appropriate changes and re-establish equil.

Predict the effect of the following changes on the reaction: 2 SO3(g) + 197.78 kJ 2 SO2 (g) + O2 (g) Increasing the temperature of the reaction. Shifts to the right to use up the extra heat. Increasing the pressure on the reaction. Shifts to the left b/c there are fewer gas molecules Adding more O2 when the reaction is at equilibrium. Shifts to the left to use up O2 Removing O2 from the system when the reaction is at equilibrium. Shifts to the right to make more O2

Seatwork Check out the summary chart on Pg. 456 Pg. 457 #1,2 & Pg. 459 #2-6,8,12