Le Chatelier's Principle in AP Chemistry
In AP Chemistry, Le Chatelier's Principle is an essential concept that explains how a system at equilibrium reacts to external stress. This principle states that if a stress is applied to a system
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About Le Chatelier's Principle in AP Chemistry
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Slide1Le Chatelier’s PrincipleAP Chemistry
Slide2Le Chatelier’s PrincipleIf a stress is applied to a system at equilibrium, the system will change to relieve that stress and re – establish equilibrium It is like the “undo” button on your computer!
Slide3Factors that Affect Equilibrium• Concentration • Temperature • Pressure – For gaseous systems only! • The presence of a catalyst
Slide4Concentration Changes• Add more reactant Shift to products • Remove reactants Shift to reactants • Add more product Shift to reactants • Remove products Shift to products
Slide5Reaction Quotient• The reaction quotient for an equilibrium system is the same as the equilibrium expression, but the concentrations are NOT at equilibrium! N 2 O 4 (g) 2NO 2 (g) Q = [NO 2 ] 2 [N 2 O 4 ]
Slide6Changes in ConcentrationChanges in concentration are best understood in terms of what would happen to “Q” if the concentrations were changed. N 2 O 4 (g) 2NO 2 (g) Q = [NO 2 ] 2 [N 2 O 4 ] • Q = Keq at equilibrium • If Q< K then there are too many reactants, the reaction will shift in the forward direction (the products) • If Q>K then there are too many products, the reaction will shift to the reactants .
Slide7Temperature ChangesExothermic Reactions • Consider heat as a product in exothermic reactions. – Add heat Shift to reactants – Remove heat Shift to products A + B = AB + Heat
Slide8Temperature ChangesEndothermic Reactions • Consider heat as a reactant in endothermic reactions. – Add heat Shift to products – Remove heat Shift to reactants A + B + heat = AB
Slide9Pressure Changes• Only affects equilibrium systems with unequal moles of gaseous reactants and products.
Slide10N2 (g) + 3H 2 (g) = 2NH 3 (g) • Increase Pressure – Stress of pressure is reduced by reducing the number of gas molecules in the container . . . . . .
Slide11N2 (g) + 3H 2 (g) = 2NH 3 (g) • There are 4 molecules of reactants vs. 2 molecules of products. – Thus, the reaction shifts to the product ammonia.
Slide12PCl5 (g) = PCl 3 (g) + Cl 2 (g) • Decrease Pressure – Stress of decreased pressure is reduced by increasing the number of gas molecules in the container .
Slide13PCl5 (g) = PCl 3 (g) + Cl 2 (g) • There are two product gas molecules vs. one reactant gas molecule. • Thus, the reaction shifts to the products.
Slide14Presence of a Catalyst• A Catalyst lowers the activation energy and increases the reaction rate. • It will lower the forward and reverse reaction rates, • Therefore, a catalyst has NO EFFECT on a system at equilibrium! • It just gets you to equilibrium faster!
Slide15Presence of an Inert Substance• An inert substance is a substance that is not- reactive with any species in the equilibrium system. • These will not affect the equilibrium system. • If the substance does react with a species at equilibrium, then there will be a shift!
Slide16•Given: • S 8 (g) + 12O 2 (g) 8 SO 3 (g) + 808 kcals • What will happen when …… • Oxygen gas is added? • The reaction vessel is cooled? • The size of the container is increased? • Sulfur trioxide is removed? • A catalyst is added to make it faster? Shifts to prodcuts Shifts to Products – to replace heat V increases, Pressure decreases, shifts to more particles – to reactants! Shift to products to replace it! No change!
Slide17Given 2NaHCO 3 (s) Na 2 CO 3 (s) + H 2 O (g) + CO 2 (g) • What will happen when . . . . . . . • Carbon dioxide was removed? • Sodium carbonate was added? • Sodium bicarbonate was removed? Shift to products – to replace it No Change – solids to not affect equilibrium No Change
Slide18•Given Ca 5 (PO 4 ) 3 OH(s) 5Ca 2+ (aq) + 3PO 4 3- (aq) + OH- (aq) • What will happen when. . . . . . • Calcium ions are added? • NaOH is added? • 1 M HCl is added? • Na 3 PO 4 (aq) is added? Shift to the reactants Adding OH- , shifts to reactants H+ + OH- H2O (removes OH-, shifts to products) Adds PO 4 3- ions, shifts to reactants