Reaction Equilibrium Unit 17 Daily 2

Key Question: What is Le Chatelier’s Principle?

Today’s Goals: Explore factors affecting equilibrium Predict direction of equilibrium shift Predict resulting changes in concentration Make equilibrium serve your own evil purposes

Factors Affecting Equilibrium Equilibrium is a RATE balance. Anything that can affect the rate of the forward or reverse reaction can throw equilibrium off. What are the factors that affect reaction rate? Concentration Pressure or Volume (gases) Temperature Surface Area Catalysts/Inhibitors

Le Chatelier’s Principle “If a system at equilibrium is subjected to an external stress, the equilibrium will shift to minimize the effects of that stress” The Steps of Le Chatelier’s Principle: Stress: Adding or removing a reactant, adding or removing a product, changing the pressure, volume, or temperature. Shift: The equilibrium will shift away from what is added and towards what is removed. Change in Concentration: The concentration of what is being shifted towards increases, the concentration of what is being shifted away from decreases.

Changes in Concentration Fe3+(aq) + SCN-(aq)  FeSCN2+(aq) (colorless) (red) If more Fe3+ is added to the system, what will happen? equilibrium shifts to the right equilibrium shifts to the product side the forward reaction is favored (forward shift) How are concentrations affected? [Fe3+] increases, [SCN-] decreases, [FeSCN2+] increases How is Keq affected? Keq does not change when changes in concentration cause a shift in equilibrium

Changes in Concentration Fe3+(aq) + SCN-(aq)  FeSCN2+(aq) (colorless) (red) If more FeSCN2+ is added to the system, what will happen? equilibrium shifts to the left equilibrium shifts to the reactant side the reverse reaction is favored (reverse shift) How are concentrations affected? [Fe3+] increases, [SCN-] increases, [FeSCN2+] increases

Changes in Concentration Fe3+(aq) + SCN-(aq)  FeSCN2+(aq) (colorless) (red) If SCN- is removed from the system (by adding AgNO3 so that AgSCN(s) precipitate forms), what will happen? equilibrium shifts to the left equilibrium shifts to the reactant side the reverse reaction is favored (reverse shift) How are concentrations affected? [Fe3+] increases, [SCN-] increases (but also decreases since it was removed), [FeSCN2+] decreases

Changes in Volume & Pressure Only affect an equilibrium system if gases are present What is the relationship between moles of gas and volume/pressure? P = (nRT)/(V) If pressure is increased (or volume decreased): equilibrium will shift towards the side with FEWER moles of gas to bring the pressure back down If pressure is decreased (or volume increased): equilibrium will shift towards the side with MORE moles of gas to bring the pressure back up

Changes in Volume & Pressure N2(g) + 3H2(g)  2NH3(g) (4 moles) (2 moles) If pressure is increased (volume decreased) on the system, what will happen? equilibrium shifts to the right equilibrium shifts to the product side the forward reaction is favored (forward shift) How are concentrations affected? [N2] decreases, [H2] decreases, [NH3] increases

Changes in Volume & Pressure Notes: If the number of moles of gas are equal on both sides, then the equilibrium can’t shift H2(g) + I2(g)  2HI(g) Make sure to only count the moles of GAS when considering pressure effects on equilibrium

Changes in Temperature If temperature is increased in the system, what will happen? equilibrium will shift in the endothermic direction the endothermic direction is away from the side with heat If temperature is decreased in the system, what will happen? equilibrium will shift in the exothermic direction the exothermic direction is towards the side with heat

For Those Who Prefer Visuals…

See Saw H2O H OH

Add More Hydrogen H2O OH H H Which way does the see saw have to shift to reach equilibrium again?

Add More Hydrogen H2O OH H H Which way does the see saw have to shift to reach equilibrium again?

Add More Hydrogen H2O OH SHIFT LEFT H H More H2O is produced due to the shift in equilibrium

Synthesis of Ammonia H2 H2 NH3 NH3 N2 H2 N2 (g) + 3H2 (g)  2NH3 (g)

SHIFT will occur to side with the least number of MOLES Increase Pressure SHIFT RIGHT H2 NH3 NH3 H2 H2 N2 SHIFT will occur to side with the least number of MOLES N2 (g) + 3H2 (g)  2NH3 (g)

Decreasing Volume (Think about how P and V relate with Boyle’s Law) NH3 NH3 N2 H2 N2 (g) + 3H2 (g)  2NH3 (g)

Decrease Volume = Increase Pressure SHIFT RIGHT H2 NH3 NH3 H2 H2 N2 SHIFT will occur to side with the least number of MOLES N2 (g) + 3H2 (g)  2NH3 (g)

Change in Temperature Depends if reaction is Endothermic or Exothermic. Endothermic: Heat on LEFT Exothermic: Heat on RIGHT

Exothermic Increase Temperature… which way will the see saw tip? D A C HEAT D A C B

Which way will the reaction shift? Exothermic A HEAT B C D Which way will the reaction shift?

Reaction will SHIFT LEFT Exothermic A HEAT B C D Reaction will SHIFT LEFT

Exothermic DecreaseTemperature… which way will the see saw tip? D A C HEAT D A C B DecreaseTemperature… which way will the see saw tip?

Which way will the reaction shift? Exothermic HEAT D C B A Which way will the reaction shift?

Reaction will SHIFT Right Exothermic HEAT D C B A Reaction will SHIFT Right

Endothermic Increase Temperature… which way will the see saw tip? D A HEAT D A C B

Which way will the reaction SHIFT? Endothermic Which way will the reaction SHIFT? HEAT D C B A

Reaction will SHIFT Right Endothermic Reaction will SHIFT Right HEAT D C B A

Endothermic Decrease Temperature… which way will the see saw tip? D A HEAT D A C B

Which way will the reaction SHIFT? Endothermic HEAT Which way will the reaction SHIFT? A B C D

Reaction will SHIFT Left Endothermic HEAT Reaction will SHIFT Left A B C D

Demo: Changes in Temperature 2NO2(g)  N2O4(g) + energy (Dark Brown) (Clear)