Equilibrium Equilibrium and and

Le Chatelier’s PrincipleLe Chatelier’s Principle

What is Equilibrium?

Equilibrium Crash Course

Chemical EquilibriumChemical Equilibrium

Reversible ReactionsReversible Reactions: :

A chemical reaction in which the productsA chemical reaction in which the productscan react to re-form the reactantscan react to re-form the reactants

Chemical EquilibriumChemical Equilibrium: :

When the rate of the forward reactionWhen the rate of the forward reactionequals the rate of the reverse reactionequals the rate of the reverse reactionand the concentration of products andand the concentration of products andreactants remains unchangedreactants remains unchanged

2HgO(s) 2Hg(l) + O2(g)

Arrows going both directions ( ) indicates equilibrium in a chemical equation

Approaching Equilibrium Many chemical reactions are reversible if the activation energy is low and the system is closed.

Reactants ⇌ Products 

 

Approaching EquilibriumMany chemical reactions are reversible if the activation energy is low. 

Reactants ⇌ Products 

Forward Reaction is left to right

Reverse Reaction is right to left

Approaching Equilibrium

Reactant concentrations start high and decrease as the reaction proceeds. The forward rate, which depends on collisions of the reactants, also decreases.  Product concentrations start at zero and increase as the reaction proceeds. The reverse rate, which depends on collisions of the products, also increases.  Eventually the forward rate is equal to the reverse rate and the concentrations are constant. This is equilibrium.

 

Graphing the Approach to Equilibrium

 Concentration

Time

Forward rate

Reverse rate

Overall rate

LeChatelier’s PrincipleLeChatelier’s Principle

When a system atWhen a system atequilibrium is placed equilibrium is placed under stress, the under stress, the system will undergo a system will undergo a change in such a way change in such a way as to relieve that as to relieve that stress.stress.

Henry Le Chatelier

When you take something away from a system at When you take something away from a system at equilibrium, the system equilibrium, the system shifts shifts in such a way as to in such a way as to replace what you’ve taken away.replace what you’ve taken away.

LeChatelier Translated:

When you add something to a system at When you add something to a system at equilibrium, the system equilibrium, the system shiftsshifts in such a way as in such a way as toto use up what you’ve added.use up what you’ve added.

Video Clip

http://www.youtube.com/watch?v=7zuUV455zFs

LeChatelier Example #1LeChatelier Example #1

A closed container of ice and water at equilibrium. The temperature is raised.

Ice + Energy Ice + Energy Water Water

The equilibrium of the system shifts to the _______ to use up the added energy.rightright

LeChatelier Example #2LeChatelier Example #2

A closed container of N2O4 and NO2 at equilibrium. NO2 is added to the container.

NN22OO4 4 (g)(g) + Energy + Energy 2 NO 2 NO22 (g)(g)

The equilibrium of the system shifts to the _______ to use up the added NO2.leftleft

LeChatelier Example #3LeChatelier Example #3

A closed container of water and its vapor at equilibrium. Vapor is removed from the system.

water + Energy water + Energy vapor vapor

The equilibrium of the system shifts to the _______ to replace the vapor.rightright

LeChatelier Example #4LeChatelier Example #4

A closed container of N2O4 and NO2 at equilibrium. The pressure is increased.

NN22OO4 4 (g) + Energy (g) + Energy 2 NO 2 NO22 (g) (g)

The equilibrium of the system shifts to the _______ to lower the pressure, because there are fewer moles of gas on that side of the equation.

leftleft

Now you try…..

Predict what will happen in each of the following situations:

The Forward and Reverse Rates and Shifting Explain in terms of the immediate changes to the forward and reverse rates after each stress why each shift occurs

2CO(g) + O2(g) ⇄ 2CO2(g) + energy

 

ShiftForwardReverse

No change

Increases

Right

Some CO is added- more reactant collisions.

The Forward and Reverse Rates and Shifting Explain in terms of the immediate changes to the forward and reverse rates after each stress why each shift occurs

2CO(g) + O2(g) ⇄ 2CO2(g) + energy

 .

CO is removed

ShiftForwardReverse

left

decreases

same

The Forward and Reverse Rates and Shifting Explain in terms of the immediate changes to the forward and reverse rates after each stress why each shift occurs

2CO(g) + O2(g) ⇄ 2CO2(g) + energy

Temperature is increased-

Shift

ForwardReverse

increases increases by more

left

both forward and reverse rates increase.

We know it shifts left