Equilibrium
Reversible Reactions
• Some reactions go in one
direction to COMPLETION, and
are not reversible
• Some reactions are reversible,
and both reactants and products
are formed simultaneously
Dynamic Equilibrium
• Must have a closed system so
reactants and products can’t
escape or be added
• RATE (speed) of
forward reaction =
RATE of reverse
reaction
Phase Change Equilibrium
• https://phet.colorado.edu/sims/
html/states-ofmatter/latest/states-ofmatter_en.html
Dynamic Equilibrium
• QUANTITIES of reactants and
products do not need to be
EQUAL
• RATES of reactions (forward and
reverse) MUST be EQUAL
H2O (l)
H20 (g)
Phase Change Equilibrium
• At 0oC in a closed container:
• H20 (s)
H20 (l)
• Rate of melting = rate of freezing
• Amount of water
Amount of ice!
Solution Equilibrium
NaCl (s)
Na+ (aq) + Cl- (aq)
Solution Equilibrium
• Must be at the Saturation point
• NaCl (s)
Na+ (aq) + Cl- (aq)
• Rate of dissolving = rate of precipitation
• Amount of solid
Amount of dissolved
Chemical Equilibrium
• Mix reactants A and B
• With time A + B
C+D
• C and D will start to form
• As more C and D form, reverse
reaction will start up
• More A and B will form
• Eventually, an equilibrium will be
reached (if all other conditions
remain the same ) when
rate of forward = rate of reverse
A+B
C+D
Le Chatelier’s Principle
When any system at equilibrium is subjected to
change in concentration, temperature, volume,
or pressure, then the system readjusts itself to
(partially) counteract the effect of the applied
change and a new equilibrium is established.
A day at the beach
Morning: cold water
Afternoon: hot sun
Shark sighting!
Reaction goes to completion: everyone
out!
Change in Concentration
CH4 + H2O
• Add more CH4
• Rate of FORWARD will increase
• More products will form
• As more products form, rate of
REVERSE reaction will increase,
more reactants will form
• New EQUILIBRIUM will be
reached
H2 + CO2
“Shift to the RIGHT”
More PRODUCTS present
Change in Concentration
CH4 + H2O
• decrease CH4
• Rate of FORWARD will decrease
• More reactants will form
• As more reactants form, rate of
FORWARD reaction will increase,
more productas will form
• New EQUILIBRIUM will be
reached
H2 + CO2
“Shift to the left”
More REACTANTS present
Increase the Temperature
• A+B
C + D + HEAT
• Rates of both forward and reverse
reactions will increase
• BUT not equally!
• Higher temp favors the
ENDOTHERMIC direction more
• Shifts to the left
Decrease the Temperature
• A+B
C + D + HEAT
• Rates of both forward and reverse
reactions will decrease
• BUT not equally!
• Lower temp favors the
EXOTHERMIC direction more
• Shifts to the right
Increase in Pressure (GASES ONLY!)
CO2 (g)
CO2 (aq)
• Pressure increases the
concentration of the gas
phase only.
• Shifts to the right (more
aqueous)
Decrease in Pressure (GASES ONLY!)
CO2 (g)
CO2 (aq)
• Low Pressure decreases
the concentration of the
gas phase only.
• Shifts to the left (more
gas)
Increase in Pressure ( ALL GASES)
2A (g) + 4B (g)
6 molecules
• Increase in concentration
on both sides
3 molecules • BUT not equally
• High pressure favors the
side with the least
molecules (less crowded)
• Shift to the right
3C (g)
Decrease in Pressure ( ALL GASES)
2A (g) + 4B (g)
6 molecules
• decrease in
concentration on both
3 molecules sides
• BUT not equally
• Low pressure favors the
side with more molecules
(more crowded)
• Shift to the left
3C (g)
Change in Pressure: same # of molecules
A (g) + B (g)
2C (g)
2 molecules
2 molecules
NO EFFECT!!!!
Adding a Catalyst
•NO Effect
•But reaches
equilibrium
faster
Haber Process
N2 (g) + 3H2 (g)
2NH3 (g) + 22 kcal