Valdosta State University Experiment 3 Le Chatelier’s Principle Valdosta State University

Valdosta State University Purpose Demonstrate Le Chatelier’s Principle in equilibrium reactions. Valdosta State University

Background Consider the reaction: N 2 (g) + 3 H 2 (g)  2 NH 3 (g) Once some amount of ammonia is produced a competing reaction starts: 2 NH 3 (g)  N 2 (g) + 3H 2 (g) Valdosta State University

Background Both these reactions occur at the same time in the same vessel: N 2 (g) + 3 H 2 (g)  2 NH 3 (g) Valdosta State University

Background Both these reactions occur at the same time in the same vessel: N 2 (g) + 3 H 2 (g)  2 NH 3 (g) Valdosta State University

Background Dynamic Equilibrium - A process in which no net change occurs in the concentration of the substances in two competing processes. Valdosta State University

Background An equilibrium can be upset when the reaction conditions are changed. How it responds to this change in described by Le Chatelier’s Principle. Valdosta State University

Background Le Chatelier’s Principle – If a stress is applied to a chemical system at equilibrium, the equilibrium will shift to minimize the effect of the stress. Valdosta State University

Background – Le Chatelier’s Principle Types of Stress Change of concentration of reactants. Change of concentration of products. Change of temperature of reaction. Valdosta State University

Background – Le Chatelier’s Principle Change of concentration of reactants. N 2 (g) + 3 H 2 (g)  2 NH 3 (g) If H 2 is added, the reaction shifts to the right, consuming the excess hydrogen and making more ammonia. In other words, the equilibrium shifts away from the added species. Valdosta State University

Background – Le Chatelier’s Principle Valdosta State University

Background – Le Chatelier’s Principle Change of concentration of reactants. N 2 (g) + 3 H 2 (g)  2 NH 3 (g) If additional nitrogen gas is added, the equilibrium will shift to the right. Again, the equilibrium shifts away from the added species. If ammonia is removed from the reaction the equilibrium will shift to the right. In this case, the equilibrium shifts toward the species that was removed. Valdosta State University

Background – Le Chatelier’s Principle Change of concentration of reactants. N 2 (g) + 3 H 2 (g)  2 NH 3 (g) If one of the reactants was removed or the product concentration was increased the reaction would shift to the left. Again, if a reactant is removed, the equilibrium would shift toward the reactant that was removed (left), or if the concentration of the product was increased, the equilibrium will shift away from the added product. Valdosta State University

Prelab 3. Ca 2+ (aq) + CO 3 2- (aq)  CaCO 3 (s) if Ca(NO 3 ) 2 (aq) is added if CaCO 3 (s) is added

Valdosta State University Laboratory Reactions Copper(II) – Ammonia equilibrium CuSO 4 (aq) + NH 3 (aq)  Cu(NH 3 ) 4 2+ (aq) + SO 4 2- (aq) MIX WELL! Cu 2+ (aq) + 4 NH 3 (aq)  Cu(NH 3 ) 4 2+ (aq) The equilibrium can be shifted by adding an acid to remove the ammonia. In this case, think of this as removing the ammonia. NH 3 (aq) + HCl(aq)  NH 4 Cl(aq) Valdosta State University

Laboratory Reactions Silver Equilibrium 2 AgNO 3 (aq) + Na 2 CO 3 (aq)  Ag 2 CO 3 (s) + 2 NaNO 3 (aq) 2 Ag + (aq) + CO 3 2- (aq)  Ag 2 CO 3 (s) The equilibrium can be shifted by adding an acid to remove the carbonate. 2 HNO 3 (aq) + CO 3 2- (aq)  H 2 O(l) + CO 2 (g) + 2 NO 3 - (aq) ADD to the SAME test tube Valdosta State University

Laboratory Reactions Silver Equilibrium Ag + (aq) + Cl - (aq)  AgCl(s) The equilibrium can be shifted by adding ammonia to remove the silver ion. Ag + (aq) + 2 NH 3 (aq)  Ag(NH 3 ) 2 + ADD to the SAME test tube Valdosta State University

Laboratory Reactions Silver Equilibrium Ag + (aq) + l - (aq)  Agl(s) The equilibrium can be shifted by adding a sulfide to remove the silver ion. 2 Ag + (aq) + S 2- (aq)  Ag 2 S ADD to the SAME test tube Valdosta State University

Laboratory Reactions Buffer Solution A solution containing a weak acid (or weak base) and its conjugate (a salt). The important feature of this solution is that it resists large changes in pH. The solution contains an acid and a base to neutralize incoming acid and base. Valdosta State University

Laboratory Reactions Buffer Solution HC 2 H 3 O 2 (aq) + H 2 O(aq)  H 3 O + (aq) + C 2 H 3 O 2 - (aq) pH NaC 2 H 3 O 2 (aq)  Na + (aq) + C 2 H 3 O 2 - (aq) Addition of NaOH neutralizes the acetic acid, shifting the equilibrium. NaOH(aq) + HC 2 H 3 O 2 (aq)  H 2 O(l) + C 2 H 3 O 2 - (aq) + Na + (aq) Valdosta State University

Laboratory Reactions Buffer Solution HC 2 H 3 O 2 (aq) + H 2 O(aq)  H 3 O + (aq) + C 2 H 3 O 2 - (aq) Addition of HCl neutralizes the conjugate base (acetate ion), shifting the equilibrium. C 2 H 3 O 2 - (aq) + HCl(aq)  HC 2 H 3 O 2 (aq) + H 2 O(aq) + Cl - (aq) The changes in equilibrium are followed by monitoring the pH (H 3 O + concentration). Valdosta State University

Laboratory Reactions Prove that acetic acid and acetate make a buffer solution. –Test changes of pH using pure water

Valdosta State University Laboratory Reactions Co(H 2 O) 6 2+ / CoCl 4 2- Co(H 2 O) Cl - (aq)  CoCl 4 - (aq) + 6 H 2 O(l) The equilibrium can be shifted by adding water to the reaction. The forward reaction is endothermic, therefore the addition of heat shifts the reaction to the right. Think of heat as being a reactant / product. If adding heat shifted the equilibrium to the right, then the heat is on the left side of the equilibrium, since the equilibrium shifts away from the added species Valdosta State University

Laboratory Reactions CrO 4 2- / Cr 2 O H + (aq) + 2 CrO 4 2- (aq)  Cr 2 O 7 2- (aq) + H 2 O(l) The equilibrium shifts on the addition of ammonia. Remember that ammonia reacts with the H +. NH 3 (aq) + H + (aq)  NH 4 + (aq) Valdosta State University

Safety The 6M and concentrated acids are corrosive. If you spill any on your skin, wash the affected area for five minutes. If the acids get in your eyes, wash the eyes for ten minutes. Seek immediate medical attention! Concentrated ammonia (NH 3 ) is corrosive. If you spill any on your skin, wash the affected area for five minutes. If the ammonia get in your eyes, wash the eyes for ten minutes. Seek immediate medical attention! The fumes of concentrated ammonia are very irritating. Use adequate ventilation (i.e. mini hoods). Potassium chromate is a strong oxidizing agent. If you spill some on your skin, wash the affected area for five minutes. Silver salts stain the skin brown upon exposure to light, especially UV light (i.e. like tanning beds). Valdosta State University

Waste Disposal All waste materials should be placed in waste containers marked “Recovered Metals and Metal Ions”. Do not dispose of any reagents in the sink or the trash can. Valdosta State University