Factors Which Affect Equilibrium

Transcription

1 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 Progress of Reaction Disturbance = increasing the temperature = adding heat Counteraction = the temperature = heat. Question: How can heat be removed? Answer: Increase the thermic reaction rate this is the forward / reverse reaction rate Therefore: NO 2 is faster than it is, so [NO 2 ] will crease. 1

2 N 2 O 4 is faster than it is, so [N 2 O 4 ] will crease. Now: more NO2 molecules are available for collision, less N2O4 molecules are available for colliison Therefore the reaction rate will also increase. Since [N2O4] is decreasing, the reaction rate will be. 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 side SUMMARY: Effect of Temperature on an equilibrium: When tempearture is increased, the thermic reaction rate will and the equilibrium will shift toward the side without / without the heat term. When temperature is decreased, the thermic reaction rate will and the equilibrium will shift toward the side without / without the heat term. 2

3 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 the pp of that gas. eg. Consider the equilibrium in a pp of a gas = concentration of a gas following reaction at closed container: CO 2(g) + NO (g) CO (g) + NO 2(g) Distrubance: add some CO 2 This will increase the of CO 2. which is equivalent to ing the [CO 2 ] Counteraction: decrease the Partial Pressure of CO 2 How can the PP of CO 2 be decreased? crease the forward reaction rate Then: the [CO 2 ] will and the [NO] will the [CO] will and [NO 2 ] will Eventually the reaction will begin, and when the reverse rate becomes equal to the forward reate again, we have reached a new! Summariy of Example 2: In the new equilibrium, [CO] and [NO 2 ] will be than originally and [CO 2 ] and [NO] will be. The equilibrium has shifted to the right To the side 3

4 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. as Volume decreases, Pressure will. 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 molecule In diagram 2, half as many molecules are colliding with the sides of container Therefore the pressure will be only of what is was in diagram 1 Conclusion: In a set volume: The the number of moles of gas, the the pressure. 4

5 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: the pressure To decrease the pressure: The number of gas molecules must be. This can be done by increase in the reaction rate the counteratction is a to the side with 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 the E a by providing another by which the reaction can occur. Catalysts the E a for both the reaction and reaction equally. 5

6 forward rate and reverse rate will by the same amount! the equilibrium will / will not shift! 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