PHA612 Lab - Expt 2 - Chemical Equilibrium.pdf

EXPERIMENT 2: EQUILIBRIUM PHA612 – Pharmaceutical Inorganic Chemistry (with Qualitative Chemistry) Objectives: 1. Observe color changes as indicators of shifts in equilibrium. 2. Predict shifts in equilibrium by applying Le Chatelier’s principle. Chemical Equilib...

EXPERIMENT 2: EQUILIBRIUM PHA612 – Pharmaceutical Inorganic Chemistry (with Qualitative Chemistry) Objectives: 1. Observe color changes as indicators of shifts in equilibrium. 2. Predict shifts in equilibrium by applying Le Chatelier’s principle. Chemical Equilibrium It is the state in which the rates of the forward and reverse reactions are equal, and the concentration of the reactants remains unchanged with time. Le Chatelier’s Principle States that when an external stress is applied to a system at equilibrium, the system adjusts to partially offset the stress as the system reaches a new equilibrium. Stressors: Changes in Concentration Temperature Pressure and Volume (for gases only) 4 A. Changes in Concentration Concentration Equilibrium Shift Increase reactant Right Increase product Left Ex. N2 (g) + 3H2 (g) ↔ 2NH3 (g); +NH3 = would shift the equilibrium to the left (to relive the stress) 5 B. Changes in Volume and Pressure Boyle’s Law States that P and V are inversely proportional Formula: P1V1 = P2V2 (Constant: Temp) Ideal Gas Law Pressure is directly proportional to gas moles Formula: PV = nRT R (universal gas constant) = 0.0821 Concentration: Equilibrium Shift Incr P, Decr V Shift to lesser moles of gas Decr P, Incr V Shift to greater moles of gas 6 B. Changes in Volume and Pressure Ex. Consider the following equilibrium systems and then predict the direction of the net reaction in each case as a result of increasing the pressure (decreasing the volume) on the system at constant temperature. Equilibrium shift Equilibrium systems: (due to incr P & decr V of system): 2PbS (s) + 3O2 (g) ↔ 2PbO (s) + 2SO2 (g) 3 moles reactant > 2 moles product: Shift to the right PCl5 (g) ↔ PCl3 (g) + Cl2 (g) 1 mole reactant < 2 moles product: Shift to the left H2 (g) + CO2 (g) ↔ H2O (g) + CO (g) 2 moles reactant = 2 moles product: No shift 7 C. Changes in Temperature Enthalpy (∆H) Represents the heat/ energy of reaction Endothermic (+∆H) ∆H > 0 Positive change in enthalpy Absorb heat → Cold Exothermic (-∆H) ∆H < 0 Negative change in enthalpy Release heat → Hot 8 C. Changes in Temperature Thermal reactions Equilibrium shift (due to incr Temp) Effect on Kc Endothermic (+∆H) Right Increases (Heat is considered as reactant) Reactant + Heat ↔ Product Exothermic (-∆H) Left Decreases (Heat is considered as product) R ↔ Product + Heat *Vice versa (when T is decreased) 9 C. Changes in Temperature Endothermic Exothermic ↑ temp. favors forward reaction ↑ temp. favors backward reaction ↓ temp. favors backward reaction ↓ temp. favors forward reaction A (g) + B(g) ↔ C (g) ∆H = + 54.5 cal. A (g) + B (g) ↔ C (g) ∆H = - 54.5 cal. A (g) + B (g) + 54.5 cal. ↔ C(g) A (g) + B (g) - 54.5 cal. ↔ C (g) A (g) + B(g) ↔ C (g) - 54.5 cal. A (g) + B (g) ↔ C (g) + 54.5 10 C. Changes in Temperature Ex. What effect would an increase in temperature have on the position of the equilibrium in these reactions? Equilibrium shift Equilibrium systems: (due to incr T): 4HCl (g) + O2 (g) - 95.4 kJ ↔ 2H2O (g) + 2Cl2 (g) Left H2(g) + Cl2 (g) + 185 kJ ↔ 2HCl (g) Right CH4 (g) + 2O2 (g) ↔ CO2 (g) + 2H2O (g) + 890 kJ Left N2O4 (g) ↔ 2NO2 (g) - 58.6 kJ Right 11 C. Changes in Temperature Ex. What effect would a decrease in temperature have on the position of the equilibrium in these reactions? Equilibrium shift Equilibrium systems: (due to decr T): 2 CO2 (g) ↔ 2 CO (g) + O2 (g) - 566 kJ Left H2(g) + I2 (g) -51.9 kJ ↔ 2HI (g) Right 2SO2 (g) + O2 (g) ↔ 2 SO3 (g) + 198 kJ Right H2 (g) + CO2 (g) + 41 kJ ↔ H2O + CO (g) Left 12 D. Adding a Catalyst Accelerates forward and backward rates equally (by lowering Ea) No effect/ shift on equilibrium (Kc) 13 Summary of Le Chatelier’s Principle Change Equilibrium External Stressors: Equilibrium shift constant (Kc) Concentration Yes No Temperature Yes Yes Pressure & Volume Yes No (Gases) Catalyst No No 14 Le Chatelier’s Principle Ex. Consider the following equilibrium process between dinitrogen tetrafluoride (N2F4) and nitrogen difluoride (NF2): N2F4 (g) ↔ 2NF2 (g), ΔH° = 38.5 kJ/mol. Predict the changes in the equilibrium if: 1. The reacting mixture is heated at constant volume; Right; Increase Kc 2. Some N2F4 gas is removed from the reacting mixture at Left; Constant Kc constant temperature and volume; 3. The pressure on the reacting mixture is decreased at constant temperature; and Right; Constant Kc 4. A catalyst is added to the reacting mixture. No shift; Constant Kc 15 Applications of Le Chatelier’s Principle 1. Life at High Altitudes and Hemoglobin Production Hb (aq) + O2 (g) ↔ HbO2 (aq) [𝑯𝒃𝑶𝟐 ] 𝑲𝒄 = 𝑯𝒃 [𝑶𝟐 ] 16 Applications of Le Chatelier’s Principle 2. Haber Process N2 (g) + 3H2 (g) ↔ 2NH3 (g), ∆H0 = -92.6 kJ/mol 17 Practice: N2 (g) + 3H2 (g) ↔ 2NH3 (g), - 94.5 kJ H2 gas is added to the system O2 gas is added to absorb N2 Temp. is decreased Pressure is decreased HCl is added to neutralize NH3 Volume is decreased A positive catalyst is added Neon, an inert gas is added NH3 is added to the system Temp. is increased 18  COMMON ION EFFECT TEST TUBE COLOR CONCLUSION pH SHIFT IN EQUILIBRIUM TT1. 10 gtts 0.1M + 1 gtt Methyl (acidic or (Left/Right) CH3COOH red (MR) basic) TT2. 10 gtts 0.1M + 1 gtt Methyl + pinch of (acidic or (Left/Right) CH3COOH red (MR) NaCH3COO crystals basic) CH3COOH ↔ CH3COO- + H+ Colorless at high conc., turns red with MR NaCH3COO à CH3COO- + Na + Color changes of Methyl Red under different pH media  LE CHATELIER’S PRINCIPLE TEST TUBE SHIFT IN EQUILIBRIUM TT1 5 gtts 0.1 M CoCl2 Standard (pink) Control TT2 5 gtts 0.1 M CoCl2 + 12 M HCl until color Standard for color (Left/Right) change is observed comparison (blue) TT3 5 gtts 0.1 M CoCl2 + 12 M HCl until color + H2O until color (Left/Right) change is observed change is observed TT4 5 gtts 0.1 M CoCl2 + 12 M HCl until color + 0.1 M AgNO3 until no (Left/Right) change is observed further color change Co(H2O)6+2 + 4Cl- ↔ CoCl42- + 6H2O Pink blue  SOLUBILITY EQUILIBRIA TEST TUBE SHIFT IN EQUIL. TT1 1 gtt 0.1 M AgNO3 + 5 gtts 2M HCl Standard Control TT2 1 gtt 0.1 M AgNO3 + 5 gtts 2M HCl +1 gtt 12 M HCl (Left/Right) TT3 1 gtt 0.1 M AgNO3 + 5 gtts 2M HCl + 6 M NH4OH TT3: (Left/Right) until ppt Reference disappears. Divide TT4: (Left/Right) into 2 portions + 5 gtts of 12 M HCl AgNO3 + HCl → AgCl + HNO3 AgCl (s) ↔ Ag+ + Cl- AgCl + NH4OH ↔ Ag(NH3)2+ + Cl- white↓ colorless  COMPLEX ION FORMATION TEST TUBE SHIFT IN EQUIL. TT1 5 gtts 0.025M FeCl3 + 5 gtts Standard Control 0.025 M KSCN + 10 gtts H2O TT2 5 gtts 0.025M FeCl3 + 5 gtts + Pinch of Fe(NO3)3 crystal (Left/Right) 0.025 M KSCN + 10 gtts H2O TT3 5 gtts 0.025M FeCl3 + 5 gtts + Pinch of KCl crystal (Left/Right) 0.025 M KSCN + 10 gtts H2O TT4 5 gtts 0.025M FeCl3 + 5 gtts + Pinch of Na2HPO4 crystal (Left/Right) 0.025 M KSCN + 10 gtts H2O FeCl3 + 3KSCN ↔ Fe(SCN)3 + 3KCl Yellow Blood red