Presentation is loading. Please wait.

Presentation is loading. Please wait.

o If you recall, we mentioned that there are rxns that bounce back and forth from forming products to reforming reactants A.K.A. reversible rxns A.K.A.

Similar presentations


Presentation on theme: "o If you recall, we mentioned that there are rxns that bounce back and forth from forming products to reforming reactants A.K.A. reversible rxns A.K.A."— Presentation transcript:

1

2 o If you recall, we mentioned that there are rxns that bounce back and forth from forming products to reforming reactants A.K.A. reversible rxns A.K.A. reversible rxns o Reversible reactions occur simultan- eously in both directions An example of a reversible rxn involves reactants A and B producing C. An example of a reversible rxn involves reactants A and B producing C. A + B C A + B C Reversible Reactions

3

4 o At chemical equilibrium there is no net change in the actual amounts of the components of the system. And although the rates of the forward & reverse rxns are equal at chemical equilibrium, the concentrations of the components on both sides of the chem- ical eqn are not necessarily the same. And although the rates of the forward & reverse rxns are equal at chemical equilibrium, the concentrations of the components on both sides of the chem- ical eqn are not necessarily the same. ̵ In fact they can be dramatically different. Chemical Equilibrium

5 o Consider a set of escalators as being like the double arrows in a dynamic equilibrium. o The # of people using the up escalator must be the same as the # of people using the down escalator for equilibrium to have been established However, the # of people upstairs do not have to equal the # of people downstairs Just the transfer between floors must be consistent

6 o The equilibrium position of a rxn is given by the concentrations of the system’s components at equilibrium o The equilibrium position indicates whether the components on the left or right side of a reversible rxn are at a higher concentration. If A reacts to give B and the mixture at equilibrium contains more of B – say 1% of A vs. 99% of B –the formation of B is said to be favored. If A reacts to give B and the mixture at equilibrium contains more of B – say 1% of A vs. 99% of B –the formation of B is said to be favored. Which Direction is Favored?

7 o On the other hand, if the mixture contains 99% of A and 1% of B at equilibrium then the formation of A is favored. Reverse direction is favored! Forward direction is favored! Which Direction is Favored?

8 o In principle, almost all rxns are reversible to some extent under the right conditions o In practice, one set of components is often so favored at equilibrium that the other set cannot be detected. If one set of components has established equilibrium by converting mostly into products, the rxn has gone to completion If one set of components has established equilibrium by converting mostly into products, the rxn has gone to completion When no products can be detected, you can say there is no rxn When no products can be detected, you can say there is no rxn Reversibility vs. Reality

9 o Reversible rxns occupy a middle ground between the theoretical extremes of irreversibility and no rxn. o The addition of a catalyst will speed up forward and reverse rxns equally By reducing the energy needed to activate the rxn in both forward and reverse directions. By reducing the energy needed to activate the rxn in both forward and reverse directions. Does not effect the amount of reactants and products present at equilibrium; simply decreases the time it takes to establish equilibrium Does not effect the amount of reactants and products present at equilibrium; simply decreases the time it takes to establish equilibrium o Reversible rxns occupy a middle ground between the theoretical extremes of irreversibility and no rxn. o The addition of a catalyst will speed up forward and reverse rxns equally By reducing the energy needed to activate the rxn in both forward and reverse directions. By reducing the energy needed to activate the rxn in both forward and reverse directions. Does not effect the amount of reactants and products present at equilibrium; simply decreases the time it takes to establish equilibrium Does not effect the amount of reactants and products present at equilibrium; simply decreases the time it takes to establish equilibrium Reversibility vs. Reality

10 Equilibrium Expression o Chemist’s can express the equilibrium position in terms of a numerical constant The equilibrium constant shows the relationship between the amount of product and reactant at equilibrium The equilibrium constant shows the relationship between the amount of product and reactant at equilibrium o Consider this hypothetical rxn… o Chemist’s can express the equilibrium position in terms of a numerical constant The equilibrium constant shows the relationship between the amount of product and reactant at equilibrium The equilibrium constant shows the relationship between the amount of product and reactant at equilibrium o Consider this hypothetical rxn… aA + bBcC + dD o We can write an expression to show the ratio of product concentrations to reactant concs called a mass action expression [C] c [C] c [D] d [A] a [A] a [B] b

11 Equilibrium Expression o The conc of each substance is raised to a power equal to the # of mols of that substance in the balanced rxn eqn. o The square brackets indicate concentration in Molarity (mol/L)

12 o Molarity is a measure of how much “stuff” is dissolved in water. ̵ The more stuff dissolved, the more concentrated the solution ̵ The higher the molarity o The quotient ratio of the equilibrium is called the equilibrium constant or K When the reactants and products amnts are in molarity the constant is called a K c When the reactants and products amnts are in molarity the constant is called a K c When the reactants and products amounts are in pressure units is called a K p When the reactants and products amounts are in pressure units is called a K p K= [C] c [C] c [D] d [A] a [A] a [B] b

13 o The constant is dependent on the temp If the temp changes so does the constant If the temp changes so does the constant o The constant is dependent on the temp If the temp changes so does the constant If the temp changes so does the constant Equilibrium Expression NOTE: water and solid materials are not included in mass action expressions. Write the mass action expression for each of the following reactions: A.2SO 2 (g) + O 2 (g) 2SO 3 (g) B.Bi 2 S 3 (s) 2Bi +3 (aq) + 3S -2 (aq)

14 Balance and write the mass action expression for each of the following reactions: 1. C 4 H 10 (g) + O 2 (g) CO 2 (g) + H 2 O(g) 2. Al(s) + O 2 (g) Al 2 O 3 (s) 3. Mn +2 (aq)+ BiO 3 -1 (aq) 3. Mn +2 (aq)+ BiO 3 -1 (aq) MnO 4 -1 (aq) + Bi +3 (aq) Classwork:

15 o Equilibrium constants provide valuable chemical information o They show whether the products or the reactants are favored in a rxn (spontaneus or nonspontaneous) always written as a ratio of products over reactants always written as a ratio of products over reactants a value of K > 1 means that products are favored a value of K > 1 means that products are favored K < 1 than reactants are favored K < 1 than reactants are favored o Equilibrium constants provide valuable chemical information o They show whether the products or the reactants are favored in a rxn (spontaneus or nonspontaneous) always written as a ratio of products over reactants always written as a ratio of products over reactants a value of K > 1 means that products are favored a value of K > 1 means that products are favored K < 1 than reactants are favored K < 1 than reactants are favored Equilibrium Constant

16

17 Sample Problem 1 Dinitrogen tetroxide (N 2 O 4 ), a colorless gas, and nitrogen dioxide (NO 2 ), a brown gas, exist in equilibrium with each other according to the following eqn: N 2 O 4 (g) 2NO 2 (g) A 1.0 liter of gas mixture at 10  C at equilibrium contains.0045 mol N 2 O 4 &.030 mol NO 2. Write the mass action expression and calculate K for the rxn.

18 o Known: [N 2 O 4 ] =.0045 mol/1.0 L [N 2 O 4 ] =.0045 mol/1.0 L [NO 2 ] =.030 mol/1.0 L [NO 2 ] =.030 mol/1.0 L o Unknown: Mass action expression = ? Mass action expression = ? K = ? K = ? o Known: [N 2 O 4 ] =.0045 mol/1.0 L [N 2 O 4 ] =.0045 mol/1.0 L [NO 2 ] =.030 mol/1.0 L [NO 2 ] =.030 mol/1.0 L o Unknown: Mass action expression = ? Mass action expression = ? K = ? K = ? Analyze: list what we know o At equil, there is no net change in the amount of N 2 O 4 or NO 2 at any given instant

19 The only product of the rxn is NO 2, which has a coefficient of 2 in the balanced eqn The only product of the rxn is NO 2, which has a coefficient of 2 in the balanced eqn The only reactant N 2 O 4 has a coefficient of 1 in the balanced eqn The only reactant N 2 O 4 has a coefficient of 1 in the balanced eqn o The mass action expression is: The only product of the rxn is NO 2, which has a coefficient of 2 in the balanced eqn The only product of the rxn is NO 2, which has a coefficient of 2 in the balanced eqn The only reactant N 2 O 4 has a coefficient of 1 in the balanced eqn The only reactant N 2 O 4 has a coefficient of 1 in the balanced eqn o The mass action expression is: Calculate: solve for unknowns [.030M] 2 [.0045M] 1 K =K =K =K = [NO 2 ] 2 [N 2 O 4 ] 1 K = o K is equal to: K = 0.20 o K < 1, therefore rxn doesn’t favor products

20 1. Find the equilibrium constant if [SO 2 ] = 1.0 M; [O 2 ]=1.0 M; [SO 3 ]=2.0 M; using the mass action expression written in the examples 2. Find the equilibrium constant if [Bi +3 ] = M; [S -2 ] = M; using the mass action expression written in the examples 1. Find the equilibrium constant if [SO 2 ] = 1.0 M; [O 2 ]=1.0 M; [SO 3 ]=2.0 M; using the mass action expression written in the examples 2. Find the equilibrium constant if [Bi +3 ] = M; [S -2 ] = M; using the mass action expression written in the examples Classwork:

21 o Another type of equilibrium is the equilibrium of dissolving o This is the equilibrium of a solid and its aqueous form o Even the most insoluble salts will dissolve to some extent in water For example, when AgCl is mixed with water a tiny amount of Ag +1 and Cl -1 exist For example, when AgCl is mixed with water a tiny amount of Ag +1 and Cl -1 exist o Another type of equilibrium is the equilibrium of dissolving o This is the equilibrium of a solid and its aqueous form o Even the most insoluble salts will dissolve to some extent in water For example, when AgCl is mixed with water a tiny amount of Ag +1 and Cl -1 exist For example, when AgCl is mixed with water a tiny amount of Ag +1 and Cl -1 exist Solubility Product AgCl(s) Ag + (aq) + Cl - (aq)

22 [Ag + ][Cl - ] [AgCl] K sp = ο If we exclude the solid “reactant” because of its constant conc. We get a special kind of equilibrium constant called the solubility product constant (K sp ) We get a special kind of equilibrium constant called the solubility product constant (K sp ) The lower the solubility of a substance the smaller the K sp The lower the solubility of a substance the smaller the K sp Solubility Product

23 ο If there are coefficients in the dissociation equation (from balancing) they become exponents that the conc are raised to. K sp = [Ag + ] 1 [Cl - ] 1 ο The solubility product (K sp ) for AgCl at 25  C is 1.8x M indicates a very small conc of silver and chloride ions indicates a very small conc of silver and chloride ions Solubility Product

24 The equilibrium concentration of hydroxide ions in a saturated solution of iron(II) hydroxide is 1.2 x M at a certain temperature. Calculate the K sp of Fe(OH) 2 at this temperature. Solubility Product: example 1

25

26 What is the concentration of silver ions in a saturated solution of silver carbonate? The K sp of Ag 2 CO 3 is 8.1 x Solubility Product: example 2

27 o We can also determine if a reaction has reached equilibrium by calculating a reaction quotient (Q). It’s like taking a snapshot of a reaction at a given time and interpreting how far along the reaction is. It’s like taking a snapshot of a reaction at a given time and interpreting how far along the reaction is. o Once the reaction quotient is solved, it is compared to the equilibrium constant The following picture helps us decide how to interpret the direction the reaction will continue. The following picture helps us decide how to interpret the direction the reaction will continue. o We can also determine if a reaction has reached equilibrium by calculating a reaction quotient (Q). It’s like taking a snapshot of a reaction at a given time and interpreting how far along the reaction is. It’s like taking a snapshot of a reaction at a given time and interpreting how far along the reaction is. o Once the reaction quotient is solved, it is compared to the equilibrium constant The following picture helps us decide how to interpret the direction the reaction will continue. The following picture helps us decide how to interpret the direction the reaction will continue. Reaction Quotient

28

29 Sample Problem 3 Will a precipitate of PbSO 4 form when ml of M MgSO 4 is mixed with ml of M Pb(NO 3 ) 2 ? The K sp of PbSO 4 = 6.3x10 -7.

30 Sample Problem 3

31 moles of SO 2, moles of SO 2 Cl 2, and moles of Cl 2 are combined in an evacuated 5.00 L flask and heated to 100°C. What is Q before the reaction begins? Which direction will the reaction proceed in order to establish equilibrium? SO 2 Cl 2 (g) SO 2 (g) + Cl 2 (g) K c = at 100°C moles of SO 2, moles of SO 2 Cl 2, and moles of Cl 2 are combined in an evacuated 5.00 L flask and heated to 100°C. What is Q before the reaction begins? Which direction will the reaction proceed in order to establish equilibrium? SO 2 Cl 2 (g) SO 2 (g) + Cl 2 (g) K c = at 100°C Classwork:

32 Manipulating the Equilibrium… o There is a principle that can be studied to govern changes in equilibrium Le Chatelier’s Principle. o Le Chatelier’s Principle states: “If a stress is applied to a system in dynamic equilibrium, the system changes to relieve the stress.” “If a stress is applied to a system in dynamic equilibrium, the system changes to relieve the stress.” Stresses are changes in temperature, pressure, concentration of reactants, or concentration of products Stresses are changes in temperature, pressure, concentration of reactants, or concentration of products o There is a principle that can be studied to govern changes in equilibrium Le Chatelier’s Principle. o Le Chatelier’s Principle states: “If a stress is applied to a system in dynamic equilibrium, the system changes to relieve the stress.” “If a stress is applied to a system in dynamic equilibrium, the system changes to relieve the stress.” Stresses are changes in temperature, pressure, concentration of reactants, or concentration of products Stresses are changes in temperature, pressure, concentration of reactants, or concentration of products

33 o Adjusting the concentrations of either reactants or products can have dramatic impact on the equilibrium If we add more of reactant A to a system at equilibrium the system will strive to reestablish equilibrium at a new equilibrium position. If we add more of reactant A to a system at equilibrium the system will strive to reestablish equilibrium at a new equilibrium position. The reaction will push to use up the extra A and generate more C The reaction will push to use up the extra A and generate more C o Adjusting the concentrations of either reactants or products can have dramatic impact on the equilibrium If we add more of reactant A to a system at equilibrium the system will strive to reestablish equilibrium at a new equilibrium position. If we add more of reactant A to a system at equilibrium the system will strive to reestablish equilibrium at a new equilibrium position. The reaction will push to use up the extra A and generate more C The reaction will push to use up the extra A and generate more C Concentration & Equilibrium [A]↑, rxn will shift toward products

34 o Adjusting the concentrations of either reactants or products can have dramatic impact on the equilibrium If we add more of product C to a system at equilibrium the system will strive to reestablish equilibrium at a new equilibrium position. If we add more of product C to a system at equilibrium the system will strive to reestablish equilibrium at a new equilibrium position. The reaction will push to use up the extra C and generate more A and B The reaction will push to use up the extra C and generate more A and B If we add more of product C to a system at equilibrium the system will strive to reestablish equilibrium at a new equilibrium position. If we add more of product C to a system at equilibrium the system will strive to reestablish equilibrium at a new equilibrium position. The reaction will push to use up the extra C and generate more A and B The reaction will push to use up the extra C and generate more A and B Concentration & Equilibrium [C]↑, rxn will shift toward reactants [C]↑, rxn will shift toward reactants

35

36 o The impact of temperature changes on an equilibrium is dependent on if the process is endothermic or exothermic Endothermic processes use energy as a reactant, while exothermic processes produce energy Endothermic processes use energy as a reactant, while exothermic processes produce energy K eq is temperature dependent K eq is temperature dependent Temp effects on Equilibrium 250 kJ is a product If T↑, the equilibrium shifts left

37 o The impact of temperature changes on an equilibrium is dependent on if the process is endothermic or exothermic Endothermic processes use energy as a reactant, while exothermic processes produce energy Endothermic processes use energy as a reactant, while exothermic processes produce energy Temp effects on Equilibrium energy is a reactant energy is a reactant If T↑, the equilibrium shifts right If T↑, the equilibrium shifts right

38

39 o If A, B, and C are all gases, then the equil they establish is pressure dependent o When the pressure is increased, the system relieves the pressure by favoring the direction that produces fewer gas molecules. Pressure is # of particles dependent, the more particles the higher the pressure Pressure is # of particles dependent, the more particles the higher the pressure Fewer gas molecules will exert less pressure. Fewer gas molecules will exert less pressure. o So, more product is formed, which overall reduces the pressure, this is a shift right o If A, B, and C are all gases, then the equil they establish is pressure dependent o When the pressure is increased, the system relieves the pressure by favoring the direction that produces fewer gas molecules. Pressure is # of particles dependent, the more particles the higher the pressure Pressure is # of particles dependent, the more particles the higher the pressure Fewer gas molecules will exert less pressure. Fewer gas molecules will exert less pressure. o So, more product is formed, which overall reduces the pressure, this is a shift right Pressure & Equilibrium

40 o Conversely, a decrease in pressure will favor the rxn that produces the most molecules So we have a shift to the left So we have a shift to the left o Conversely, a decrease in pressure will favor the rxn that produces the most molecules So we have a shift to the left So we have a shift to the left Pressure & Equilibrium P↑, this equilibrium shifts right If P↓, this equilibrium shifts left

41

42

43 Predict the effect of the following changes on the reaction in which SO 3 decomposes to form SO 2 and O 2. 2SO 3 (g) 2SO 2 (g) + O 2 (g) H o = kJ (a) Increasing the temperature of the reaction. (b) Increasing the pressure on the reaction. (c) Adding more O 2 when the reaction is at equilibrium. (d) Removing O 2 from the system when the reaction is at equilibrium. Predict the effect of the following changes on the reaction in which SO 3 decomposes to form SO 2 and O 2. 2SO 3 (g) 2SO 2 (g) + O 2 (g) H o = kJ (a) Increasing the temperature of the reaction. (b) Increasing the pressure on the reaction. (c) Adding more O 2 when the reaction is at equilibrium. (d) Removing O 2 from the system when the reaction is at equilibrium. ClassworkClasswork


Download ppt "o If you recall, we mentioned that there are rxns that bounce back and forth from forming products to reforming reactants A.K.A. reversible rxns A.K.A."

Similar presentations


Ads by Google