1. Le Chatelier’s Principle

2. Reversible Reactions
Thus far, we have considered only one-way reactions:
A + B → C + D
Some reactions are reversible: a chemical reaction in which the products can react to re-from the reactants
They go forward (“to the right”) : A + B → C + D
and backwards (“to the left”) : A + B ← C + D
Written with a two-way arrow:
A + B ↔ C + D

3. Examples of reversible reactions:
Melting & Freezing
Recharging a rechargeable battery
Examples of irreversible reactions:
Burning paper
Dropping an egg

4. Chemical Equilibrium A + B ↔ C + D + + + + + +
For a reversible reaction, when the forward rate equals the backward rate, a chemical equilibrium has been established.
A + B ↔ C + D A B + A B + A B + C D + C D + C D +

5. Le Chatelier’s Principle
When a stress is applied to a system (i.e. reactants and products) at equilibrium, the system responds to relieve the stress.
The system shifts in the direction of the reaction that is favored by the stress.
A stress is a change in:
Concentration
Temperature (ENERGY)
Volume
Pressure

6. Equilibrium LeChatelier’s Principle LEFT CO2 + CaO CaCO3 I WISH I HAD
SWEAT GLANDS.
LeChatelier’s Principle
CO CaO CaCO3
“chicken
breath”
“food”
“egg shell”
Le Chatelier’s Principle
“Henri Louis Le Chatelier ( ) was a French professor of chemistry. He studied the gases that caused explosions in mines and the reactions that occur in a blast furnace. These studies led him to put forward the principle that bears his name. The principle explains that if conditions such as temperature or pressure are changed in a system containing reactants and products, then any chemical reaction occuring will tend to counteract the change and return the conditions to equilibrium.”
Eyewitness Science “Chemistry” , Dr. Ann Newmark, DK Publishing, Inc., 1993, pg 39
As temperature increases, chickens “pant” more.
This stresses the system and shifts the equilibrium to the
LEFT
This makes the egg shells THIN and fragile.

7. In a chicken… CaO + CO2 CaCO3 (eggshells)
I wish I had
sweat glands.
In a chicken… CaO CO CaCO3
(eggshells)
In summer, [ CO2 ] in a chicken’s blood due to panting.
-- shift ; eggshells are thinner
How could we increase eggshell thickness in summer?
[ CO2 ] , shift
-- give chickens carbonated water
[ CaO ] , shift
-- put CaO additives in chicken feed
-- air condition the chicken house
TOO much $$$
-- pump CO2 gas into the chicken house
would kill all the chickens!

8. Le Chatelier’s principle
When a system at equilibrium is disturbed, it shifts to a
new equilibrium that counteracts the disturbance.
N2(g) H2(g) NH3(g)
Disturbance Equilibrium Shift
Add more N2…………………..
“ “ H2…………………..
“ “ NH3…………………
Remove NH3…………………..
Add a catalyst…………………
no shift
Increase pressure…………….

9. LeChatelier’s Principle
N H NH3 + heat
Raising the temperature…
…favors the endothermic reaction (the reverse
reaction) in which the rise in temperature is
counteracted by the absorption of heat.
Increasing the pressure…
…favors the forward reaction in which 4 mol
of gas molecules is converted to 2 mol.
Decreasing the concentration
of NH3…
…favors the forward reaction in order to
replace the NH3 that has been removed.
Dorin, Demmin, Gabel, Chemistry The Study of Matter 3rd Edition, page 532

10. Equilibrium Expression
Haber Process
N H NH3 + heat
Dorin, Demmin, Gabel, Chemistry The Study of Matter 3rd Edition, page 532

11. Light-Darkening Eyeglasses
AgCl + energy Ago + Clo
(clear) (dark)
Go outside…
Sunlight more intense than inside light;
“energy”
shift to a new equilibrium:
GLASSES DARKEN
Sensitive Sunglasses
Oxidation-reduction reactions are the basis for many interesting and useful applications of technology. One such application
is photochromic glass, which is used for the lenses in light sensitive glasses. Lenses manufactured by the Corning Glass Company
can change from transmitting 85% of light to only transmitting 22% of light when exposed to bright sunlight.
Photochromic glass is composed of linked tetrahedrons of silicon and oxygen atoms jumbled together in a disorderly array,
with crystals of silver chloride caught in between the silica tetrahedrons. When the glass is clear, the visible light passes right
through the molecules. The glass absorbs ultraviolet light, however, and this energy triggers an oxidation-reduction reaction
between Ag+ and Cl-:
Ag Cl- --> Ag0 + Cl0
To prevent the reaction from reversing itself immediately, a few ions of Cu+ are incorporated into the silver chloride crystal.
These Cu+ ions react with the newly formed chlorine atoms:
Cu Cl0 --> Cu Cl-
The silver atoms move to the surface of the crystal and form small colloidal clusters of silver metal. This metallic silver
absorbs visible light, making the lens appear dark (colored).
As the glass s removed from the light, the Cu2+ ions slowly move to the surface of the crystal where they interact with
the silver metal:
Cu Ag0 --> Cu Ag+
The glass clears as the silver ions rejoin chloride ions in the crystals.
Then go inside…
“energy”
shift to a new equilibrium:
GLASSES LIGHTEN

12. Maintaining Blood pH HCO31- + H+ H2CO3 H2O + CO2
Acid entering the blood stream
Carbon dioxide is exhaled
HCO H H2CO H2O CO2
Bicarbonate ion circulates in the blood stream where it is in equilibrium with H+ and OH-.
In the lungs, bicarbonate ions combine with a hydrogen ion and lose a water molecule
to form carbon dioxide, which is exhaled.
Kelter, Carr, Scott, Chemistry A World of Choices 1999, page 291

13. Carbon dioxide is removed from the blood too quickly.
Alkalosis
If our breathing becomes too fast (hyperventilation)…
Carbon dioxide is removed from the blood too quickly.
This accelerates the rate of degradation of carbonic acid into carbon dioxide and water.
The lower level of carbonic acid encourages the combination of hydrogen ions and
bicarbonate ions to make more carbonic acid. The final result is a fall in blood H1+
levels that raises blood pH which can result in over-excitability or death.
Kelter, Carr, Scott, Chemistry A World of Choices 1999, page 291

14. Acidosis The normal pH of blood is between 7.2 – 7.4.
If breathing becomes too slow (hypoventilation)…
TAKING A REST
“With strenuous exercise, lack of sufficient oxygen leads to lactic acid being produced in the muscles. As lactic acid builds up in the blood, it produces fatigue. Rest and oxygen allow it to be carried away and oxidized in the liver.”
Eyewitness Science “Chemistry” , Dr. Ann Newmark, DK Publishing, Inc., 1993, pg 41
…free up acid, pH of blood drops, with associated health risks such as depression
of the central nervous system or death.
The normal pH of blood is between 7.2 – 7.4.
This pH is maintained by the bicarbonate ion and other buffers.
Kelter, Carr, Scott, Chemistry A World of Choices 1999, page 291

15. Shifts
If you INCREASE volume, concentration or temperature from the REACTANT side, there will be a shift RIGHT towards the products.
If you DECREASE volume, concentration or temperature from the REACTANT side, there will be a shift LEFT towards the reactants.
*** Opposite is true for compounds on the products side***
If you INCREASE pressure, it shifts to the side with the least amount of moles.
If you DECREASE pressure, it shifts to the side with the most amount of moles.
If a catalyst is present NO shift happens

16. Example CO (g) + 2 H2 (g)  CH3OH (g) + energy
Methanol, CH3OH, can be manufactured using the following equilibrium reaction:
CO (g) + 2 H2 (g)  CH3OH (g) + energy
Predict the direction that the reaction will shift for each of the following stresses.
Decrease in temperature
Increase in pressure
Add more concentration of H2
Add a catalyst
Decrease volume CO