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Chemistry 1011

Chemistry 1011. TOPIC Gaseous Chemical Equilibrium TEXT REFERENCE Masterton and Hurley Chapter 12. 12.5 Effect of Changes in Conditions Upon an Equilibrium System. YOU ARE EXPECTED TO BE ABLE TO: Define Le Chatelier’s Principle.

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Chemistry 1011

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  1. Chemistry 1011 TOPIC Gaseous Chemical Equilibrium TEXT REFERENCE Masterton and Hurley Chapter 12 Chemistry 1011 Slot 5

  2. 12.5 Effect of Changes in Conditions Upon an Equilibrium System YOU ARE EXPECTED TO BE ABLE TO: • Define Le Chatelier’s Principle. • Use Le Chatelier’s Principle to predict qualitatively the effect on an equilibrium system of changes in: • concentration (partial pressure) of individual components • total pressure of the system at constant volume • volume of the system • total thermal energy of the system • Predict the effect on an equilibrium of adding a catalyst • Describe industrial processes for the manufacture of ammonia and sulfur trioxide Chemistry 1011 Slot 5

  3. Le Chatelier’s Principle • A chemical equilibrium can be disturbed by changing the external conditions • Changing the pressure or volume • Adding or removing a component • Changing the temperature • When an external change is made to an equilibrium system, the system will alter so as to oppose the change Chemistry 1011 Slot 5

  4. Changing the Pressure or Volume • Changing the pressure or volume of a system will result in compression or expansion • If possible, the system will change and the equilibrium will shift so as to oppose the compression or expansion • This can only occur if the total number of moles or product is different from the total number of moles of reactant Chemistry 1011 Slot 5

  5. Compressing the N2O4 – NO2 Equilibrium System N2O4(g) 2NO2(g) • Compressing the equilibrium system by reducing the volume will increase the pressure • The system will shift so as to reduce the pressure • The reverse reaction will take place since this results in a decrease in the total number of molecules 2NO2 (g) N2O4(g) Chemistry 1011 Slot 5

  6. Effect of Pressure on Equilibrium Position Compression Expansion N2O4(g) 2NO2(g)  N2(g) + 3H2(g) 2NH3(g) H2(g) + I2(g)  2HI(g) no effect no effect N2(g) + O2(g) 2NO(g) no effect no effect Chemistry 1011 Slot 5

  7. Adding or removing a Gaseous Component • Adding a gaseous reactant or product to an equilibrium system will disturb the equilibrium • The system will shift so as to remove the added species • Removing a gaseous reactant or product from an equilibrium system will disturb the equilibrium • The system will shift so as to replace the removed species Chemistry 1011 Slot 5

  8. Modifying the N2O4 – NO2 Equilibrium System by Adding/Removing Components N2O4(g) 2NO2(g) • Adding more N2O4 - reaction occurs in forward direction • Adding more NO2 - reaction occurs in reverse direction • Removing N2O4 - reaction occurs in reverse direction • Removing NO2 - reaction occurs in forward direction Chemistry 1011 Slot 5

  9. Confirming Le Chatelier’s Principle • A determination of the reaction quotient immediately after adding (or removing) a gaseous component will confirm Le Chatelier’s Principle • For N2O4(g) 2NO2(g) Kp = (PNO2)2/PN2O4 Adding NO2 will raise PNO2 and lower PN2O4 Q will be >Kp Reverse reaction will occur Chemistry 1011 Slot 5

  10. Changing the Temperature • Changing the temperature of a system will disturb the equilibrium • The system will change and the equilibrium will shift so as to oppose the change in temperature • If the temperature is raised, the reaction will proceed in the endothermic direction until a new equilibrium is reached at a higher temperature • If the temperature is lowered, the reaction will proceed in the exothermic direction until a new equilibrium is reached at a lower temperature Chemistry 1011 Slot 5

  11. Modifying the N2O4 – NO2 Equilibrium System by Changing the Temperature • The reaction N2O4(g) 2NO2(g)DHo = +57.2kJ (colourless) (brown) is endothermic in the forward direction • An increase in temperature will cause the forward reaction to take place in order to absorb the added heat (Le Chatelier) • A new equilibrium will be established at the higher temperature • PNO2 will be greater; PN2O4 will be less • The gas mixture will become more brown Chemistry 1011 Slot 5

  12. Confirming Le Chatelier’s Principle • The van’t Hoff equation relates the values of the equilibrium constant for a reaction at different temperatures to the value of DHo ln K2=DHo 1 - 1 K1 R T1 T2 • If DH is +ve, then K2 is smaller than K1 if T2 > T1 Chemistry 1011 Slot 5

  13. Effect of Changes in Conditions Upon an Equilibrium System • If the number of reactant molecules is different from the number of product molecules, changing the total pressure at equilibrium will change the equilibrium composition. Kp WILL NOT change • Adding or removing a gaseous reactant or product species will change the equilibrium composition. Kp WILL NOT change • Changing the temperature will change the equilibrium composition. Kp WILL change Chemistry 1011 Slot 5

  14. Effect of Catalysts on Equilibrium • Adding a catalyst will not alter the equilibrium concentrations of reactants or products. Kp WILL NOT change • Adding a catalyst WILL result in a reaction reaching equilibrium more quickly Chemistry 1011 Slot 5

  15. Applying Le Chatelier’s Principle – The Haber Process N2(g) + 3H2(g) 2NH3(g) DH = -92kJ Kp=(PNH3)2 =6.0 x 105 at 25oC PN2 x (PH2)3 • The number of product molecules is 2, the number of reactant molecules is 4 • The forward reaction is exothermic • The value of Kp decreases as temperature rises • At 227oC Kp = 0.10 • The activation energy for the forward reaction is >150kJ Chemistry 1011 Slot 5

  16. Choosing the Best Conditions • At 25oC the equilibrium favours NH3, but at 25oC the reaction rate is almost zero • High temperatures are required in order to have a reasonable number of reactant molecules with energy > activation energy • While the rate will increase at higher temperatures, the equilibrium yield of ammonia will be lower • Raising the pressure both favours a higher equilibrium yield of ammonia and increases the rate • Adding a catalyst will result in a lower activation energy Chemistry 1011 Slot 5

  17. The Haber Process Compromise • Moderate temperature – 450oC • High pressure – 200 to 600 atm • Carefully selected catalyst • Extra nitrogen • Reactants recycled as ammonia removed from system Chemistry 1011 Slot 5

  18. Applying Le Chatelier’s Principle – The Contact Process Sulfur is burned in air S(s) + O2(g) SO2(g) Sulfur dioxide is reacted with more oxygen using a catalyst SO2(g) + 1/2O2(g) SO3(g) DH = -98.9kJ Sulfur trioxide is reacted with water SO3(g) + H2O(l) H2SO4(l) Chemistry 1011 Slot 5

  19. The SO2 -SO3 Equilibrium • The forward reaction is exothermic – higher temperatures favour reactants, low temperatures preferred • (at 200oC Kp = 1.0 x 106; at 600oC Kp = 10) • Low temperatures result in very low rates - high temperatures are required if reactant molecules are to overcome the actvation energy barrier • High pressures favour products and result in faster rates Chemistry 1011 Slot 5

  20. The Contact Process Compromise • Temperature not so high as to favour reactants, but high enough to result in rapid rate • Use of a carefully selected catalyst • Pass reactant mixture over catalyst beds at moderate temperatures – 450oC to 600oC • First pass at high temperature (600oC) results in rapid attainment of equilibrium with 80% conversion of to • Second pass at results in 99% conversion • (Note: SO3 will not react with water! It must be dissolved in concentrated H2SO4. The resulting mixture is then diluted) Chemistry 1011 Slot 5

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