Chemistry 1011 Slot 51 Chemistry 1011 TOPIC Gaseous Chemical Equilibrium TEXT REFERENCE Masterton and Hurley Chapter 12

Chemistry 1011 Slot 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 53 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 54 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 55 Compressing the N 2 O 4 – NO 2 Equilibrium System N 2 O 4(g) 2NO 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 2NO 2 (g) N 2 O 4(g)

Chemistry 1011 Slot 56 Effect of Pressure on Equilibrium Position Compression Expansion N 2 O 4(g)   2NO 2(g)  N 2(g) + 3H 2(g) 2NH 3(g) H 2(g) + I 2(g)  2HI (g) no effect no effect N 2(g) + O 2(g) 2NO (g) no effect no effect

Chemistry 1011 Slot 57 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 58 Modifying the N 2 O 4 – NO 2 Equilibrium System by Adding/Removing Components N 2 O 4(g) 2NO 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 59 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) 2NO 2(g) Kp = (P NO 2 ) 2 / P N 2 O 4 Adding NO 2 will raise P NO 2 and lower P N 2 O 4 Q will be >Kp Reverse reaction will occur

Chemistry 1011 Slot 510 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 511 Modifying the N 2 O 4 – NO 2 Equilibrium System by Changing the Temperature The reaction N 2 O 4(g) 2NO 2(g)  H o = +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 P NO 2 will be greater; P N 2 O 4 will be less The gas mixture will become more brown

Chemistry 1011 Slot 512 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  H o ln K 2 =  H o 1  1 K 1 R T 1 T 2 If  H is +ve, then K 2 is smaller than K 1 if T 2 > T 1

Chemistry 1011 Slot 513 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 514 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 515 Applying Le Chatelier’s Principle – The Haber Process N 2(g) + 3H 2(g) 2NH 3(g)  H = -92kJ Kp = (P NH 3 ) 2 = 6.0 x 10 5 at 25 o C P N 2 x (P H 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 >150kJ

Chemistry 1011 Slot 516 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 517 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 518 Applying Le Chatelier’s Principle – The Contact Process Sulfur is burned in air S (s) + O 2(g) SO 2(g) Sulfur dioxide is reacted with more oxygen using a catalyst SO 2(g) + 1 / 2 O 2(g) SO 3(g)  H = -98.9kJ Sulfur trioxide is reacted with water SO 3(g) + H 2 O (l) H 2 SO 4(l)

Chemistry 1011 Slot 519 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 10 6 ; 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 520 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)