Qualitative Changes in Equilibrium Systems Chapter 7.4

Disturbing Equilibrium A chemical equilibrium can be disturbed by changes in: Concentration Temperature Pressure/Volume

Restoring Balance: Le Châtelier’s Principle An equilibrium shift is a change in concentrations of reactants and products in order to restore an equilibrium state If you somehow change a system at equilibrium, it will shift left or right to restore equilibrium Henry Louis Le Chatelier (1850-1936) French chemist

Le Chatelier’s Principle: Seesaw Analogy Recall that at equilibrium a system is in balance; Forward rate = reverse rate [reactants] and [products] are constant A chemical reaction at equilibrium is like a seesaw; If we added more reactants, the reaction will become unbalanced; To restore equilibrium, the reaction will shift right to make more products and bring the seesaw back to balance!

N2(g) + 3H2(g) ↔ 2NH3(g) ΔH = -92.4kJ/mol Le Chatelier’s Principle: Changes in Concentration For example, the Haber process: N2(g) + 3H2(g) ↔ 2NH3(g) ΔH = -92.4kJ/mol What happens when concentration changes are made to equilibrium? Add [N2] Decrease [H2] Add [NH3] System will want to produce more NH3; Equilibrium shifts right! System will want to produce more H2; Equilibrium shifts left! System will want to produce more reactants; Equilibrium shifts left!

Le Chatelier’s Principle: Changes in Temperature The Haber process again: N2(g) + 3H2(g) ↔ 2NH3(g) ΔH = -92.4kJ/mol What happens when temperature changes are made to equilibrium? Increase temperature Decrease temperature Rewrite to include heat in reaction. Since it is exothermic: N2(g) + 3H2(g) ↔ 2NH3(g) + 92.4kJ/mol System wants to absorb extra heat; Since reverse reaction absorbs heat (endothermic) Equilibrium shifts left! System wants to release more heat; Since forward reaction releases heat (exothermic) Equilibrium shifts right!

Le Chatelier’s Principle: Changes in Pressure Changes in pressure only apply to reactions involving gases The Haber process again: N2(g) + 3H2(g) ↔ 2NH3(g) ΔH = -92.4kJ/mol What happens when pressure changes are made to equilibrium? Increase pressure Decrease pressure What if there are equal number of moles on left and right? Look at number of moles on each side of reaction. 1N2(g) + 3H2(g) ↔ 2NH3(g) ∴ 4 molecules ↔ 2 molecules System wants to lower the pressure; Since fewer molecules reduces pressure Equilibrium shifts right! System wants to increase the pressure; Since more molecules increases pressure Equilibrium shifts left! The system cannot move in any way to reduce the pressure again! Therefore, there is no shift!

Increasing the temperature Decreasing the pressure Adding more O2 Practice: Predict the effect of the following changes on the following reaction Increasing the temperature Decreasing the pressure Adding more O2 Removing SO2 Adding SO3 Decreasing the container volume

Analyzing Concentration vs Time Graphs At equilibrium, more CO2 added. System responds by shifting right; thus the concentration of products increases At equilibrium, CO2 removed. System responds by shifting left; thus the concentration of products decreases

Analyzing Concentration vs Time Graphs At equilibrium, temperature is decreased. For this exothermic reaction, system responds by shifting right to restore the energy; thus the concentration of products increases At equilibrium, temperature is decreased. For this endothermic reaction, system responds by shifting left to restore the energy; thus the concentration of reactants increases

Factors that Do Not Affect Equilibrium Position Catalysts In a reversible reaction, catalysts increase forward and reverse rates by following the lower energy path However, it does not change ∆H so the overall energy change is not affected As a result, the position of equilibrium is not affected

Factors that Do Not Affect Equilibrium Position 2. Inert Gas Inert gases are not reactive and will not enter a chemical reaction Adding an inert gas will result in more collisions but since those collisions do not result in reactions, the actual number and frequency of collisions between reactants and products stays the same Therefore, adding an inert gas does not affect equilibrium position

Factors that Do Not Affect Equilibrium Position 3. State of Reactants Equilibrium is only affected by changes in concentration of entities that are in the same state of matter For example: H2(g) + I2(s) ↔ 2HI(g) Adding more solid iodine DOES NOT change the concentration, therefore it has NO EFFECT on equilibrium!

Homework Watch the following video: Questions p. 446 #1-4 Le Chatelier's Principle Explained Questions p. 446 #1-4 Assignment (Thurs and Friday)