Le Chatelier’s Principle

 When a system at equilibrium is subjected to a stress, the system “shifts” to relieve effects of the stress and restore equilibrium   “

Shift

” = forward or reverse reaction speeds up “

Stress

” = something that causes a reaction (forward or reverse) to speed up or slow down.

http://www.learnerstv.com/animation/anim ation.php?ani=120&cat=chemistry  Possible Stresses:  Change in Conc.

 Change in Pressure  Change in Temperature https://www.youtube.com/watch?v=dIDgP FEucFM (2 min)

Effects of Stresses on Equilibrium Ex: A + B ↔ C + D

 If forward rate increases  C + D produced at greater rate  amounts increase  A + B used up faster  amounts decrease

Change in Concentration



Will affect only gas and aqueous

INCREASE Concentration

 Reaction shifts in direction that consumes extra amount A + B ↔ C + D (Increase [A], shifts )

DECREASE Concentration

 Reaction shifts in direction that produces more of what was taken away A + B ↔ C + D (Decrease [B], shifts )

Note:  Changing amounts of a

pure liquid or solid

will

not cause a shift

 That will not change their concentrations, just their amounts.

Change in Temperature Increase Temp.

 Favors ENDOTHERMIC 

Direction that consumes excess heat

 A + B ↔ C + D + energy

Decrease Temp.

 Favors EXOTHERMIC 

Direction that produces heat

 A + B ↔ C + D + energy

Pressure Changes



Affect gases only



Count total MOLES of gas on each side of equation

 Increase pressure = Decrease volume  Shifts in direction that forms fewer moles of gas  Decrease pressure = Increase volume  Shifts in direction that forms greater moles of gas NOTE: If moles of gas equal on both sides NO SHIFT

http://www.kentchemistry.com/links/Ki netics/LeChatelier.htm

 Ex: N 2 (g) + 2O 2 (g) ↔ N 2 O 4 (g)

3 moles gas 1 mole gas

 Increase Pressure: Shifts  Decrease Pressure: Shifts

 How might you optimize the production of ammonia in the Haber reaction?

N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g) + 91.8kJ

What could we do to: concentration, temp. and pressure?

To shift right

Increase [N 2 ], [H 2 ], Decrease [NH 3 ] Decrease Temp Increase Pressure Haber reaction http://www.youtube.co

m/watch?v=NWhZ77Q m5y4&safe=active

Common Ion Effect

 Substance is added to an equilibrium system that has a “common ion” with the reaction.

 This will increase the concentration of that aqueous ion and will cause a shift.

 Ex: Saturated solution of AgCl AgCl(s) ↔ Ag +1 (aq) + Cl -1 (aq)      Stress: Add NaCl (s). It breaks apart in water forming Na +1 Common Ion = Cl -1 Shift and Cl -1 This causes a decrease in the solubility of AgCl

Effect of Catalyst on an Equilibrium System

 Increases rate of the forward and reverse reactions

equally

.

 There is

no overall “shift

”  Lowers activation energy the same for forward and reverse reaction

 Le Chat’s Overview Part 1  http://www.youtube.com/watch?v=7zuUV4 55zFs&safe=active  Le Chat’s Overview Part 2 http://www.youtube.com/watch?v=XhQ02e gUs5Y&safe=active

Practice Regents Questions

Given the a system at equilibrium:

N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g) + energy

Which changes occur when the temperature of this system is decreased?

(1) The conc. of H 2 (g) increases and the conc. of N 2 (g) increases.

(2) The conc. of H 2 (g) decreases and the conc. of N 2 (g) increases.

(3) The conc. of H 2 (g) decreases and the conc. of NH 3 (g) decreases.

(4) The conc. of H 2 (g) decreases and the conc. of NH 3 (g) increases.

Given the equation representing a reaction at equilibrium:

N 2 (g) + 3H 2 (g) ↔2NH 3 (g) + energy

Which change causes the equilibrium to shift to the right?

(1) decreasing the concentration of H 2 (g) (2) decreasing the pressure (3) increasing the concentration of N 2 (g) (4) increasing the temperature

Given the system at equilibrium:

2POCl 3 (g) + energy ↔ 2PCl 3 (g) + O 2 (g)

Which changes occur when O 2 (g) is added?

(1) The equilibrium shifts to the right and the concentration of PCl 3 (g)increases.

(2) The equilibrium shifts to the right and the concentration of PCl 3 (g)decreases.

(3) The equilibrium shifts to the left and the concentration of PCl 3 (g) increases.

(4) The equilibrium shifts to the left and the concentration of PCl 3 (g) decreases.

Given the reaction at equilibrium:

N 2 (g) + 3H 2 (g) -->2NH 3 (g) + 91.8 kJ

What occurs when the conc. of H 2 (g) is increased?

(1) The rate of the forward reaction increases and the concentration of N 2 (g) decreases.

(2) The rate of the forward reaction decreases and the concentration of N 2 (g) increases.

(3) The rate of the forward reaction and the concentration of N 2 (g) both increase.

(4) The rate of the forward reaction and the concentration of N 2 (g) both decrease.

Given the system at equilibrium:

N 2 (g) + O 2 (g) + energy ↔ 2 NO(g)

Which changes will result in a

decrease

in the amount of NO(g) formed?

(1) decreasing the pressure.

(2) decreasing the concentration of N 2 (g).

(3) increasing the concentration of O 2 (g) .

(4) increasing the temperature

Given the equilibrium reaction in a closed system:

H 2 (g) + I 2 (g) + heat ↔ 2 HI(g)

What will be the result of an increase in temperature?

(1) The equilibrium will shift to the left and [H 2 ] will increase.

(2) The equilibrium will shift to the left and [H 2 ] will decrease.

(3) The equilibrium will shift to the right and [HI] will increase.

(4) The equilibrium will shift to the right and [HI] will decrease.

Given the reaction at equilibrium: the concentration of A(g) can be increased by A. lowering the temperature B. adding a catalyst C. increasing the concentration of AB(g) D. increasing the concentration of B(g)

Le Chats: (Honors)  http://www.mhhe.com/physsci/chemistry/essentialchemis try/flash/lechv17.swf

Crash Course: Equilibrium http://www.youtube.com/watch?v=g5wNg_dKsYY&safe=active Le Chat’s (University of Surrey) http://www.youtube.com/watch?v=dIDgPFEucFM&safe=active