Lecture 14: LeChatelier's Principle - Pressure Effect on Equilibrium - Temperature Effect on Equilibrium

Reversible Reactions

a chemical reactions in which the products can react to re-form the reactants

Chemical equilibrium

When the rate of the forward reaction equals the rate of the reverse reaction and the concentration of products and reactants remains unchanged

What happens to reactant concentration as you approach equilibrium?

reactant concentrations start high and decrease as the reaction proceeds.

What happens to the forward rate as a reaction approaches equilibrium?

The forward rate depends on collisions of the reactants, it also decreases

What happens to product concentrations when approaching equilibrium?

start at zero and increase as the reaction proceeds

What happens to reverse rate when approaching equilibrium?

reverse rate depends on collisions of the products, it also increases

LeChatelier’s Principle

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

What happens when you take something away from a system at equilibrium?

system shifts in such a way to replace what you’ve taken away

What happens when you add something to a system at equilibrium?

The system shifts in such a way as to use up what you’ve added

What factors affect equilibrium?

• concentration

• temperature

• pressure (for gaseous systems only)

• presence of a catalyst

Concentration: add more reactant

shift to products

concentration: remove reactants

shift to reactants

concentration: add more product

shift to reactants

concentration: remove products

shift to products

An inert substance

a substance that has no effect on the equilibrium

How do catalysts effect equilibrium?

since it will lower forward and reverse reaction rates, it has no effect on a system at equilibrium, just gets you to equilibrium faster

exothermic reactions favor…

low temperature reaction conditions