Chemistry 1011 TOPIC Gaseous Chemical Equilibrium TEXT REFERENCE
Chemistry 1011 TOPIC Gaseous Chemical Equilibrium TEXT REFERENCE Masterton and Hurley Chapter 12 Chemistry 1011 Slot 5 1
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 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 2
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 3
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 4
Compressing the N 2 O 4 – NO 2 Equilibrium System N 2 O 4(g) 2 NO 2(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 2 NO 2 (g) N 2 O 4(g) Chemistry 1011 Slot 5 5
Effect of Pressure on Equilibrium Position Compression Expansion N 2 O 4(g) 2 NO 2(g) N 2(g) + 3 H 2(g) 2 NH 3(g) H 2(g) + I 2(g) 2 HI(g) effect N 2(g) + O 2(g) 2 NO(g) no effect Chemistry 1011 Slot 5 no no effect 6
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 7
Modifying the N 2 O 4 – NO 2 Equilibrium System by Adding/Removing Components N 2 O 4(g) 2 NO 2(g) • Adding more N 2 O 4 - reaction occurs in forward direction • Adding more NO 2 - reaction occurs in reverse direction • Removing N 2 O 4 - reaction occurs in reverse direction • Removing NO 2 - reaction occurs in forward direction Chemistry 1011 Slot 5 8
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 N 2 O 4(g) 2 NO 2(g) Kp = (PNO 2)2/PN 2 O 4 Adding NO 2 will raise PNO 2 and lower PN 2 O 4 Q will be >Kp Reverse reaction will occur Chemistry 1011 Slot 5 9
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 10
Modifying the N 2 O 4 – NO 2 Equilibrium System by Changing the Temperature • The reaction N 2 O 4(g) (colourless) • • 2 NO 2(g) DHo = +57. 2 k. J (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 PNO 2 will be greater; PN 2 O 4 will be less The gas mixture will become more brown Chemistry 1011 Slot 5 11
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 K 2 = DHo 1 - 1 ln K 1 R T 1 T 2 • If DH is +ve, then K 2 is smaller than K 1 if T 2 > T 1 Chemistry 1011 Slot 5 12
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 13
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 14
Applying Le Chatelier’s Principle – The Haber Process N 2(g) + 3 H 2(g) 2 NH 3(g) DH = -92 k. J 2 (P ) NH 3 = 6. 0 x 105 at 25 o. C Kp = PN 2 x (PH 2)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 227 o. C Kp = 0. 10 • The activation energy for the forward reaction is >150 k. J Chemistry 1011 Slot 5 15
Choosing the Best Conditions • At 25 o. C the equilibrium favours NH 3, but at 25 o. C 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 16
The Haber Process Compromise • • • Moderate temperature – 450 o. C High pressure – 200 to 600 atm Carefully selected catalyst Extra nitrogen Reactants recycled as ammonia removed from system Chemistry 1011 Slot 5 17
Applying Le Chatelier’s Principle – The Contact Process Sulfur is burned in air S(s) + O 2(g) Sulfur dioxide is reacted with more oxygen using a catalyst SO 2(g) + 1/2 O 2(g) SO 3(g) DH = -98. 9 k. J Sulfur trioxide is reacted with water SO 3(g) + H 2 O(l) H 2 SO 4(l) Chemistry 1011 Slot 5 18
The SO 2 - SO 3 Equilibrium • The forward reaction is exothermic – higher temperatures favour reactants, low temperatures preferred – (at 200 o. C Kp = 1. 0 x 106; at 600 o. C 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 19
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 – 450 o. C to 600 o. C – First pass at high temperature (600 o. C) results in rapid attainment of equilibrium with 80% conversion of to – Second pass at results in 99% conversion • (Note: SO 3 will not react with water! It must be dissolved in concentrated H 2 SO 4. The resulting mixture is then diluted) Chemistry 1011 Slot 5 20
- Slides: 20