Factors Which Affect Le Chatelier s Principle: If a closed system at equilibrium is subjected to a change, processes will occur that tend to counteract that change. OR Whatever we do nature tries to undo 1. Effect of Temperature Example 1: N 2 O 4(g) + heat 2 NO 2(g) colourless brown PE (kj) 2NO 2 N O 2 4 0 Progress of Reaction Disturbance = increasing the temperature = adding heat Counteraction = decrease the temperature = removing heat. Question: How can heat be removed? Answer: Increase the endothermic reaction rate this is the forward / reverse reaction rate Therefore: NO 2 is produced faster than it is consumed so [NO 2 ] will increase. 1
N 2 O 4(g) + heat 2 NO 2(g) N 2 O 4 is consumed faster than it is produced so [N 2 O 4 ] will decrease. Now: more NO2 molecules are available for collision, less N2O4 molecules are available for colliison Therefore the reverse reaction rate will also increase. Since [N2O4] is decreasing, the forward reaction rate will slow down. Eventually: reverse reaction rate = forward reaction rate.. a new equilibrium is acheived! Summariy of Example 1: A new equilibrium is established with more NO 2 and less N 2 O 4. the new equilibrium has more products and less reactants The equilibrium has shifted to the right To the product side SUMMARY: Effect of Temperature on an equilibrium: When tempearture is increased, the endothermic reaction rate will increase and the equilibrium will shift toward the side without / with the heat term. When temperature is decreased, the exothermic reaction rate will increase and the equilibrium will shift toward the side without / with the heat term. 2
2. Effect of Concentration and Partial Pressure Partial Pressure (PP): In a gas mixture, the pressure exerted by one gas in the mixture is called the Partial Pressure of that gas. The more of that gas you have, the greater the pp of that gas. pp of a gas = concentration of a gas eg. Consider the following reaction at equilibrium in a closed container: CO 2(g) + NO (g) CO (g) + NO 2(g) Distrubance: add some CO 2 This will increase the partial pressure of CO 2. which is equivalent to increasing the [CO 2 ] Counteraction: decrease the Partial Pressure of CO 2 How can the PP of CO 2 be decreased? Increase the forward reaction rate Then: the [CO 2 ] will decrease and the [NO] will decrease the [CO] will increase and [NO 2 ] will increase Eventually the reverse reaction will begin, and when the reverse rate becomes equal to the forward rate again, we have reached a new equilibrium Summariy of Example 2: In the new equilibrium, [CO] and [NO 2 ] will be higher than originally and [CO 2 ] and [NO] will be lower. The equilibrium has shifted to the right To the product side 3
3. Effects of Changing the Volume of the Container or Total Pressure A) What is pressure? With a fixed number of moles of gas: as Volume increases, Pressure will decrease as Volume decreases, Pressure will increase Example: Two molecules of NO 2 are bouncing around in a set volume: Diagram 1: NO 2 NO 2 Then, the two molecules of NO 2(g) react to form one N 2 O 4(g) Diagram 2: N O 2 4 Now have only one molecule In diagram 2, half as many molecules are colliding with the sides of container Therefore the pressure will be only half of what is was in diagram 1 Conclusion: In a set volume: The greater the number of moles of gas, the greater the pressure. 4
B) Effect of Pressure Change on Equilbrium Imagine have mixture of NO 2 and N 2 O 4 at equilibrium in a syringe. 2NO 2(g) brown N 2 O 4(g) colourless Disturbance: quickly increase the pressure by pushing in the plunger. Counteraction: decrease the pressure To decrease the pressure: The number of gas molecules must be lowered This can be done by increasing in the forward reaction rate the counteratction is a shift to the side with less moles of gas Summary: When total pressure is increased (by V) in an equilibrium system with gases, the equilib will shift toward the side with less moles of gas. When total pressure is decreased (by V) in an equilibrium system with gases, the equilib will shift toward side with more moles of gas. 4. Effect of Catalysts Adding a catalyst will lower the E a by providing another mechanism by which the reaction can occur. Catalysts lower the E a for both the forward reaction and reverse reaction equally. forward rate and reverse rate will increase by the same amount! the equilibrium will / will not shift! 5
SUMMARY OF FACTORS AFFECTING EQUILIUBRIUM: 1. Temperature If T is increased, the equilibrium will shift toward side the heat term. If T is decreased, the equilibrium will shift toward side the heat term. 2. Concentration If [reactant] is increased, the equilibrium equilib will shift toward the side. If [product] is increased, the equilibrium equilib will shift toward the side. 3. Partial Pressure of Gases - the same effects as concentration. 4. Total Volume and Total Pressure If pressure is increased (by decreasing volume), the equilibrium will shift toward the side with moles of gas. If pressure is decreased (by increasing volume), the equilibrium will shift toward the side with moles of gas. 5. Catalysts - Have no effect on equilibrium. 6