• Demo of FeSCN2+ eqm• Add Fe3+• Add SCN2+• Add NaOH

LeChatelier’s Principle (p.50t)

LeChatelier’s Principle (p.50t)• LeChatelier’s Principle says that if a closed system at eqm is

subjected to a change, the system will try and counteract that change (do the opposite).• Cross off the paraphrase.• This principle will allow us to predict the effect of changing

temperature, concentration and pressure on an eqm system.• In each case we will look at the eqm:

2 NO(g) + Cl2(g) 2 NOCl(g) + 76 kJ

Effect of Temp Change (p.50b)

2 NO(g) + Cl2(g) 2 NOCl(g) + 76 kJ• If the temperature is decreased (heat lost):• How do you undo that stress?• Which way does the eqm shift?

• If the temp. is increased, the system will shift left to consume that extra heat.• Think about the demo with the brown and colorless gas

and the color change with temperature.• Hebden doesn’t really discuss why this happens. Here’s

the reason:

Reasoning for Shift w/ Changing Temp• The system is initially at eqm so both forward and reverse rxn

rates are equal.• The rxn is exothermic in the forward direction (draw PE

diagram).• If temp is decreased, both forward and reverse rxns will slow

down.• Which rxn is affected (slowed down) more?• Which rate is now faster?• Which way will the eqm shift?• What happens to the rates as the system shifts?

• The opposite is true if temp is increased

Graphical Analysis of Temp Change (p.51t)• 2 NO(g) + Cl2(g) 2 NOCl(g) + 76 kJ

Effect of Conc Change (p.51m)

2 NO(g) + Cl2(g) 2 NOCl(g) + 76 kJ

• If the concentration of Cl2 is increased:

• How do you undo that stress?• Which way does the eqm shift?

• Again, Hebden doesn’t really discuss why. Here’s the reason:

Reasoning for Shift w/ Changing Conc• 2 NO(g) + Cl2(g) 2 NOCl(g) + 76 kJ

• The system is initially at eqm so both forward and reverse rxn rates are equal.• If [Cl2] is increased,

• What happens to the forward rate?• What happens to the reverse rate?• Which way will the eqm shift?• What happens to the [ ] of each species?• What happens to the rates as the system shifts?

• The opposite is true if [Cl2] is decreased

Graphical Analysis of Conc Change (p.52t)• 2 NO(g) + Cl2(g) 2 NOCl(g) + 76 kJ

Effect of Pressure Change (p.52m)

2 NO(g) + Cl2(g) 2 NOCl(g) + 76 kJ

• If the pressure of the system is increased (or volume is decreased):• How do you undo that stress?• How do you do that?• Which way does the eqm shift?

• Again, Hebden doesn’t really discuss why. Here’s the reason:

Reasoning for Shift w/ Changing Pressure• The system is initially at eqm so both forward and reverse rxn

rates are equal.• If pressure is increased:• What happens to all concentrations?• What happens to both rates?• Which rxn is affected (sped up) more?• Which way will the eqm shift?• What happens to the rates as the system shifts?

• The opposite is true if pressure is decreased.

Graphical Analysis of Pressure Change (p.53t)• 2 NO(g) + Cl2(g) 2 NOCl(g) + 76 kJ

Effect of Adding a Catalyst (p.53m)• A catalyst lowers the overall activation energy for both

forward and reverse reactions.• Therefore, a catalyst speeds up both rxns equally• There is no shift in an equilibrium when a catalyst is

added.• If a system is not at eqm yet, a catalyst can help it reach

eqm faster.

Summary (p.53b)• Changing the temperature: All concentrations slowly change

and there is no sudden change on the graph.• Changing the concentration: One species will suddenly jump

up or down.• Changing the pressure: All concentrations will immediately

jump or drop.• Click for link to animation

Industrial Uses of LeChat’s Principle (p.57t)• Video of the Haber Process.

Homework

• Hebden #17, 19, 21, 23, 24, 26-30.• Quiz coming up on p. 37-56 (in 3 classes).