Le Chatelier's Principle

Additional KEY Terms

• Use Le Chatelier’s principle to predict and explain shifts in equilibrium.

Include: temperature, pressure/volume, reactant/product concentration, catalyst, inert gas  • Interpret concentration versus time graphs. Include: temp, concentration, catalyst changes. • Describe practical applications of Le Chatelier’s

principle.

Le Chatelier's Principle (1884)

When a system at equilibrium is subjected to a stress, the system will adjust to relieve

the stress and return to equilibrium.

Remember: Kc value is constant. BEFORE the stress, and AFTER the reaction adjusts.

Types of Stress

1. Concentration Stress

Stress: a change in concentration of products or reactants by adding or removing.

Adjustment: change in collision rate and redistribution of particles.

• [Add] – system shifts to use it up.• [Remove] – system shifts to make more.

• More C means increased rate of reverse reaction.

Kc =

[C] [A][B]

CBA +

Kc = 1.35

We say “shifts left”

We mean:

• Excess C used up until ratio of product to reactant concentrations is equal to Kc once again.

Increase [C]:

Kc =

[C] [A][B]B CA +

Kc = 1.35

• Forward reaction is favoured

We say “shifts right”

We mean:

• New concentrations re-establish Kc.

Increase [B]:

Kc =

[C] [A][B]B CA +

Kc = 1.35

Removing a particle is like decreasing [ ].

• Decreased rate of forward reaction collisions.

We say “shifts left”

We mean:

• Reverse is favoured, ↑ reactants, Kc the same.

Decrease [A]:

2 NO2 (g) N2O4 (g) car exhaust smog

Huge spike indicates that [ ] was changed by adding more particles.

2 NO2 (g) N2O4 (g) car exhaust smog

A huge spike indicates that [ ] was changed by removing particles.

Temperature

Temperature stress addressed the SAME way as concentration by changing collision rates.

**Re-establishes new eqlbm (with new [ ]s) at new temperature – SO…changes the Kc.

Exothermic: A B (- ∆H )

Endothermic: A B (+ ∆H)HEAT +

+ HEAT

2. Temperature stress

Temperature increase / add heat • Reaction shifts left.• Endothermic collisions (reverse) favored.

Temperature decrease / removing heat• Reaction shifts right.• Exothermic collisions (forward) favored.

+ heat

heat

A B

+A B

= [B] [A]

= [B] [A]

Kc

Kc

∆H = -58 kJ2 NO2 (g) N2O4 (g) car exhaust smog

Initial drop in ALL rates can only occur through temperature decrease.

∆H = -58 kJ2 NO2 (g) N2O4 (g) car exhaust smog

Initial spike in ALL rates can only occur through temperature decrease.

Volume/Pressure

Changing the pressure of a system only affects those equilibria with gaseous reactants and/or products.

3. Volume stress

Rates of collisions change with pressure and effect all concentrations – BUT, Kc will re-establish***.

A + 2 B C

A + 2 B C

Volume increase – (↓P ):

A

B

B

C

Decreased rate of forward reaction. (fewer collisions, in larger space)Reverse rate favoured – shifts left(pressure increases with more particles)

BBA

A + 2 B CA

BBC

C

Volume decrease– (↑P ):

Increased rate of forward reaction. (MORE collisions, in smaller space)Forward rate favoured – shifts right(pressure reduced with fewer particles)

Which way with the system shift IF the size of the container is cut in half?

Reverse reaction favoured (increased likelihood of collisions in a smaller space)

Shifts left

2 NH3(g) N2(g) + 3 H2(g)

Equilibrium position unchanged.

H2(g) + I2(g) 2 HI(g)

Which way with the system shift IF the pressure is decreased?

1 + 1 : 2

Pressure changes have NO effect on this eqlbm – Same # of particles, same collision effects.

Factors (stresses) that do not affect

Equilibrium Systems

Catalysts

Lowers activation energy for both forward and reverse reaction equally.

Equilibrium established more quickly, but position and ratios of concentrations will remain the same.

K value remains the same.

Inert Gases (noble gases)

Unreactive – are not part of a reaction, therefore can not affect equilibrium of a concentration-based equation.

Catalysts, inert gases, pure solids or pure liquids do NOT appear in the Equilibrium Law - so they have no

effect if altered.

Le Chatelier's AND

life

Appliance - NO energy - forward reaction favoredEnergy released to run appliance.

Outlet (recharge) – HIGH energy - reverse favoredReformes reactants, storing energy for use.

Rechargeable Batteries

Lead-acidPbO2 + Pb + 4 H+ + 2 SO4

2- 2 PbSO4 + 2 H2O + energy

Nickel-cadmium Cd + 2 NiO(OH) + 2 H2O 2 Ni(OH) + Cd(OH)2 + energy

Electrical energy (like heat) is written in the reaction.

Haemoglobin protein used to transport O2 from lungs to body tissue.

Lungs - [O2] is high - forward reaction favored Haemoglobin binds O2

Tissue - [CO2] is high and [O2] is low - reverse reaction favored. Hb releases O2

Hb (aq) + O2 (g) HbO2 (aq)

Haemoglobin AND Oxygen

CAN YOU / HAVE YOU?

• Use Le Chatelier’s principle to predict and explain shifts in equilibrium.

Include: temperature, pressure/volume, reactant/product concentration, catalyst, inert gas  • Interpret concentration versus time graphs. Include: temp, concentration, catalyst changes. • Describe practical applications of Le Chatelier’s

principle.