Le Chatelier’s Principle

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

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

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 +

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

Equilibrium LeChatelier’s Principle LEFT CO2 + CaO CaCO3 I WISH I HAD SWEAT GLANDS. LeChatelier’s Principle CO2 + CaO CaCO3 “chicken breath” “food” “egg shell” Le Chatelier’s Principle “Henri Louis Le Chatelier (1850-1936) 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.

In a chicken… CaO + CO2 CaCO3 (eggshells) I wish I had sweat glands. In a chicken… CaO + CO2 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!

Le Chatelier’s principle When a system at equilibrium is disturbed, it shifts to a new equilibrium that counteracts the disturbance. N2(g) + 3 H2(g) 2 NH3(g) Disturbance Equilibrium Shift Add more N2………………….. “ “ H2………………….. “ “ NH3………………… Remove NH3………………….. Add a catalyst………………… no shift Increase pressure…………….

LeChatelier’s Principle N2 + 3 H2 2 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

Equilibrium Expression Haber Process N2 + 3 H2 2 NH3 + heat Dorin, Demmin, Gabel, Chemistry The Study of Matter 3rd Edition, page 532

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 --> Cu2+ + 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: Cu2+ + 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

Maintaining Blood pH HCO31- + H+ H2CO3 H2O + CO2 Acid entering the blood stream Carbon dioxide is exhaled HCO31- + H+ H2CO3 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

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

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

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

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