Unit 17

Daily 2

Reaction Equilibrium

What is Le Chatelier’s Principle?

Key Question:

1. Explore factors affecting equilibrium2. Predict direction of equilibrium shift3. Predict resulting changes in concentration4. Make equilibrium serve your own evil

purposes

Today’s Goals:

• 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?1) Concentration2) Pressure or Volume (gases)3) Temperature4) Surface Area5) Catalysts/Inhibitors

Factors Affecting Equilibrium

“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:1) Stress: Adding or removing a reactant, adding or

removing a product, changing the pressure, volume, or temperature.

2) Shift: The equilibrium will shift away from what is added and towards what is removed.

3) Change in Concentration: The concentration of what is being shifted towards increases, the concentration of what is being shifted away from decreases.

Le Chatelier’s Principle

Fe3+(aq) + SCN-

(aq) FeSCN2+(aq)

(colorless) (red)

If more Fe3+ is added to the system, what will happen?equilibrium shifts to the rightequilibrium 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 leftequilibrium shifts to the reactant sidethe 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 leftequilibrium shifts to the reactant sidethe 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 Concentration

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 rightequilibrium shifts to the product sidethe 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 shiftH2(g) + I2(g) 2HI(g)

Make sure to only count the moles of GAS when considering pressure effects on equilibrium

Changes in Volume & Pressure

If temperature is increased in the system, what will happen?equilibrium will shift in the endothermic

directionthe 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

directionthe exothermic direction is towards the side

with heat

Changes in Temperature

For Those Who Prefer Visuals…

See Saw

H2O H OH

Add More Hydrogen

H2O

HOH

H

Which way does the see saw have to shift to reach equilibrium again?

Add More Hydrogen

H2O

HOH

H

Which way does the see saw have to shift to reach equilibrium again?

Add More Hydrogen

H2O H OH

H

More H2O is produced due to the shift in equilibrium

SHIFT LEFT

Synthesis of Ammonia

N2 H2 NH3

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

H2 H2

NH3

Increase Pressure

N2H2

NH3

SHIFT will occur to side with the least number of MOLESN2 (g) + 3H2 (g) 2NH3 (g)

H2H2 NH3

SHIFT RIGHT

Decreasing Volume(Think about how P and V relate with Boyle’s Law)

N2 H2 NH3

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

H2 H2

NH3

Decrease Volume = Increase Pressure

N2H2

NH3

SHIFT will occur to side with the least number of MOLESN2 (g) + 3H2 (g) 2NH3 (g)

H2H2 NH3

SHIFT RIGHT

Change in TemperatureDepends if reaction is Endothermic or

Exothermic.

Endothermic: Heat on LEFTExothermic: Heat on RIGHT

Exothermic

Increase Temperature… which way will the see saw tip?

A BDC

HEAT

Exothermic

Which way will the reaction shift?

AB

DC

HEAT

Exothermic

Reaction will SHIFT LEFT

AB

DC

HEAT

Exothermic

DecreaseTemperature… which way will the see saw tip?

A BDC

HEAT

Exothermic

Which way will the reaction shift?

A

B

DC

HEAT

Exothermic

Reaction will SHIFT Right

A

B

DC

HEAT

Endothermic

Increase Temperature… which way will the see saw tip?

A BDC

HEAT

Endothermic

Which way will the reaction SHIFT?

A B

DC

HEAT

Endothermic

Reaction will SHIFT Right

A B

DC

HEAT

Endothermic

Decrease Temperature… which way will the see saw tip?

A BDC

HEAT

Endothermic

Which way will the reaction SHIFT?

A B

DC

HEAT

Endothermic

Reaction will SHIFT Left

A B

DC

HEAT

2NO2(g) N2O4(g) + energy

(Dark Brown) (Clear)

Demo: Changes in Temperature