1 Today s Objectives Qualitatively predict the following using Le Chatelier s principle: shifts in equilibrium caused by changes in temperature, pressure, volume, concentration, or the addition of a catalyst describe how these changes affect the equilibrium constant, K c Section 15.2 (pp. 690-699)
2 Le Châtelier worked to maximize yield in the chemical industry by manipulating rxns He discovered a pattern and stated it as a generalization that is supported by extensive experimental evidence and is now accepted in the scientific community Provides a method of predicting the response of a chemical system to an imposed change Principle applied in industry to allow engineers to efficiently produce of the desired product
3 when a chemical system at equilibrium is disturbed by a change in property of the system, the system always appears to react in the direction that opposes the change, until a new equilibrium is reached (Figure 2 p. 690) Changes to chemical systems include: Δ [reagent] Δ T Δ V (g) catalyst added
4 the application of this principle involves three stages: initial equilibrium state, shifting non-equilibrium state (change imposed), and a new equilibrium state with no macroscopic (observable) change equilibrium shift occurs to counteract the change imposed the system changes in such a way as to oppose the change introduced rxn shifts to oppose change to the system i.e. proceeds predominantly in a different direction
Concentration Changes If a system at equilibrium is disturbed by the addition of a reactant (or the removal of a product), then Le Chatelier s principle predicts that the equilibrium will shift right in order to restore balance in the rxn 2N 2 O (g) + 3O 2(g) 4 NO 2(g) If the disturbance is the removal of a reactant (or the addition of a product), then Le Chatelier s principle predicts that the equilibrium will shift left. 2N 2 O (g) + 3O 2(g) 4 NO 2(g) Since concentrations of solids are constants and do not appear in expressions for K c removing or adding some solid does not cause shifts.
6 Example: see Figure 3 p. 691 Concentration Changes Addition of the HF (g) reactant will shift equilibrium to the right More products will be produced and a new equilibrium is established i.e. add more HF (g) reactant to increase freon production If you see spike in either a reactant or product it means that a substance has been added or removed, which causes a gradual change in the other entities.
7 Example: see Figure 4 p. 692 Concentration Changes Removal of the HCl (g) product will shift the equilibrium to the right More products will be produced and a new equilibrium is established i.e. product removal shifts rxn fwd until reactants are exhausted and equilibrium is restored
8 Example: see Figure 5 p. 692 Concentration Changes oxygenated hemoglobin biochemical application In the lungs, a high [O 2(g) ] shifts equilibrium fwd and the blood becomes oxygenated. Cell reactions consume O 2(g) and shifts equilibrium left, releasing more O 2(g) equilibrium shift accommodates the addition (or removal) of a reagent
Concentration Changes In industry, engineers design processes that continually add reactants and remove products 9 Creates an open system (no longer closed) so the rxn never establishes equilibrium and the rxn always shifts forward, thus making product (and $$$)
Collision-Reaction Theory and Concentration Changes Concentration Changes More entities results in more frequent collisions, thus increasing rxn rate rxn continues until balance reestablished between fwd and rev rxns at the new equilibrium faster rates at the new equilibrium than at the original equilibrium because there are more particles in dynamic equilibrium i.e. reagent, collisions, rxn rate 10
11 Temperature Changes Consider energy as a reagent Endothermic (absorb): Reactants + Energy Products positive std heat of reaction (Enthalpy change, r H) Calculate in Energetics Unit Exothermic (release): Reactants Products + Energy negative std heat of reaction (measure of energy, kj)
With endothermic reactions, heat acts like a reactant. Increasing the temperature shifts the reaction right. Decreasing the temperature, shifts the reaction left Temperature Changes Heat + 2N 2 O (g) + 3O 2(g) 4 NO 2(g) In exothermic reactions, heat acts like a product. Increasing the temperature shifts the reaction left. Decreasing the temperature, shifts the reaction right. 4 NO 2(g) 2N 2 O (g) + 3O 2(g) + Heat
Temperature Changes add or remove thermal energy, equilibrium shifts to minimize change in the system system heated, shifts to absorb energy ( T ) if endothermic, produce more yield to consume E added Example: see p. 693 (salt-sulfuric acid process) 13 system cooled, shifts to produce energy ( T ) if exothermic, produce more yield while replacing E removed Example: see Figure 6 p. 693 (cool exothermic rxn to make more product)
Example: see Figure 6 p. 693 temperature decreases at the time indicated by the dotted line results in equilibrium shifting to the right creates more products until a new equilibrium is established Temperature Changes 14 + energy A gradual change in the reactants with an opposite change in the products indicates that there is a temperature change occurring.
Equilibrium constant (K c ) is temperature dependent see Learning Tip p. 694 Temperature Changes change in temperature results in a change in the K c value No other change imposed on a system at equilibrium changes the numerical value of K c 15 T of exothermic rxn shifts equilibrium left (towards reactants) and K c T of endothermic rxn shifts towards products and K c expression numerator increases
The equilibrium constant (K c ) is temperature dependent Temperature Changes 16 Reaction Type Role of Heat Effect of T Effect of T Endothermic reactants + heat products K K Exothermic reactants products + heat K K
Collision-Reaction Theory and Energy Changes Shift occurs as a result of imbalance of rxn rates Temperature Changes Example: lower temperatures of exothermic rxn results in less collisions, but decreases reverse rxn rate more than fwd, so shifts to produce more energy until rates are equal again at a lower temperature 17
Temperature Changes Industrial exothermic reactions often occur at high temperatures, even though adding heat shifts equilibrium toward reactants (lower %Yield) Example: Haber Process p. 325 (Chemistry 20 Case Study) add energy b/c fwd and rev rxn rates too slow at low temp to reach equilibrium in a reasonable time making large quantity in a short time is more valuable to manufacturer 18
Gas & Volume Changes 19 consider Boyle s Law Chem 20 pressure inversely proportional to volume amount concentration of a gas is directly proportional to its pressure use to predict effect of container volume on equilibrium position
Gas & Volume Changes If volume is decreased, then pressure increases (Boyle s Law) rxn will shift in the direction which contains the fewest moles of gas 20 Pressure 2N 2 O (g) + 3O 2(g) 4 NO 2(g) 4 moles (right) are fewer than 5 moles (left) If volume is increased, then pressure decreases (Boyle s Law) rxn will shift in the direction which contains the most moles of gas Pressure 2N 2 O (g) + 3O 2(g) 4 NO 2(g) 5 moles (left) are more than 4 moles (right) If both sides of the equation have the same number of moles of gas, then the change in volume of the container has no effect on the equilibrium.
Gas & Volume Changes consider the total chemical amount (moles) of gas reactants and products i.e. add rxn coefficients Example: see Figure 7 p. 694 (decrease volume of sulfur dioxide and oxygen rxn) 21
22 Example: see Figure 7 p. 694 Gas & Volume Changes V, overall P and [amt] so rxn shifts fwd and decreases the total number of reactant gas molecules, resulting in a reduced pressure (restore equilibrium) fewer gas molecules lowers pressure If you see a spike in all of the entities = P, V If you see a drop in all of the entities = P, V
Gas & Volume Changes 23 if equal number of gas molecules on each side of equation, then V does NOT affect system similarly, systems involving only (s) or (l) reagents are NOT affected by P add or remove a gaseous substance not involved in rxn (like an inert gas) P in container, but will NOT shift equilibrium b/c does not change the reagent gases concentrations
Collision-Reaction Theory and Gas Volume Changes Explain again by an imbalance of rxn rates Gas & Volume Changes consider previous example: V caused both fwd and rev rxn rates to increase b/c increase in concentrations of both reactants and products, but fwd rate increase more than rev rate because more particles are involved in the fwd rxn (greater total number of collisions with more particles). Observe production of more product from shift until rates equal again 24
25 Catalyst & Equilibrium Catalysts decrease the time required to reach an equilibrium position, but do NOT affect the concentration of entities at the final position of equilibrium lowers activation energy equally for both fwd and rev rxns Discuss more in Chapter 12 useful in industry b/c allows more rapid overall production of desired product
27 HOMEWORK Practice Qs p. 695 #1-3 Section 15.2 Review p. 699 #1-7 Section 15.2 Extra Exercises handout Chapter 15 Review p. 705 #1-41 Case Study p. 696 #1-2 (Urea Production in Alberta) DUE: Tuesday, September 29 Lab Exercise 15.C p. 698 (Equilibrium of Nitrogen Compounds) DUE: Tuesday, September 29