2A + B C Reversible Reactions If you recall, we mentioned that there are rxns that bounce back and forth from forming products to reforming reactantsA.K.A. reversible rxnsReversible reactions occur simultan-eously in both directionsAn example of a reversible rxn involves reactants A and B producing C.A + B C
4Chemical EquilibriumAt 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.In fact they can be dramatically different.
5Consider a set of escalators as being like the double arrows in a dynamic equilibrium. The # of people using the up escalator must be the same as the # of people using the down escalator for equilibrium to have been establishedHowever, the # of people upstairs do not have to equal the # of people downstairsJust the transfer between floors must be consistent
6Which Direction is Favored? The equilibrium position of a rxn is given by the concentrations of the system’s components at equilibriumThe 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.
7Which Direction is Favored? 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!AB99%1%
8Reversibility vs. Reality In principle, almost all rxns are reversible to some extent under the right conditionsIn 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 completionWhen no products can be detected, you can say there is no rxn
9Reversibility vs. Reality Reversible rxns occupy a middle ground between the theoretical extremes of irreversibility and no rxn.The addition of a catalyst will speed up forward and reverse rxns equallyBy 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
10Equilibrium Expression Chemist’s can express the equilibrium position in terms of a numerical constantThe equilibrium constant shows the relationship between the amount of product and reactant at equilibriumConsider this hypothetical rxn…aA + bBcC + dDWe can write an expression to show the ratio of product concentrations to reactant concs called a mass action expression[C]c[D]d[A]a[B]b
11Equilibrium Expression The conc of each substance is raised to a power equal to the # of mols of that substance in the balanced rxn eqn.The square brackets indicate concentration in Molarity (mol/L)
12Molarity is a measure of how much “stuff” is dissolved in water. The more stuff dissolved, the more concentrated the solutionThe higher the molarityThe quotient ratio of the equilibrium is called the equilibrium constant or KWhen the reactants and products amnts are in molarity the constant is called a KcWhen the reactants and products amounts are in pressure units is called a KpK=[C]c[D]d[A]a[B]b
13Equilibrium Expression The constant is dependent on the tempIf the temp changes so does the constantNOTE: water and solid materials are not included in mass action expressions.Write the mass action expression for each of the following reactions:2SO2(g) + O2(g) <==> 2SO3(g)Bi2S3(s) <==> 2Bi+3(aq) + 3S-2(aq)
14Classwork:Balance and write the mass action expression for each of the following reactions:C4H10(g) + O2(g) <==> CO2(g) + H2O(g)Al(s) + O2(g) <==> Al2O3(s)Mn+2(aq)+ BiO3-1(aq) <==>MnO4-1(aq) + Bi+3(aq)
15Equilibrium ConstantEquilibrium constants provide valuable chemical informationThey show whether the products or the reactants are favored in a rxn (spontaneus or nonspontaneous)always written as a ratio of products over reactantsa value of K > 1 means that products are favoredK < 1 than reactants are favored
16products favored at equil reactants favored at equil K > 1products favored at equilK < 1reactants favored at equil
17Sample Problem 1 N2O4(g) 2NO2(g) Dinitrogen tetroxide (N2O4), a colorless gas, and nitrogen dioxide (NO2), a brown gas, exist in equilibrium with each other according to the following eqn:N2O4(g) NO2(g)A 1.0 liter of gas mixture at 10C at equilibrium contains mol N2O & .030 mol NO2. Write the mass action expression and calculate K for the rxn.
18Analyze: list what we know Known:[N2O4] = mol/1.0 L[NO2] = .030 mol/1.0 LUnknown:Mass action expression = ?K = ?At equil, there is no net change in the amount of N2O4 or NO2 at any given instant
19Calculate: solve for unknowns The only product of the rxn is NO2, which has a coefficient of 2 in the balanced eqnThe only reactant N2O4 has a coefficient of 1 in the balanced eqnThe mass action expression is:[NO2]2[.030M]2[.0045M]1K =K =[N2O4]1K is equal to: K = 0.20K < 1, therefore rxn doesn’t favor products
20Classwork:Find the equilibrium constant if [SO2] = 1.0 M; [O2]=1.0 M; [SO3]=2.0 M; using the mass action expression written in the examplesFind the equilibrium constant if [Bi+3] = M; [S-2] = M; using the mass action expression written in the examples
21AgCl(s) Ag+(aq) + Cl-(aq) Solubility ProductAnother type of equilibrium is the equilibrium of dissolvingThis is the equilibrium of a solid and its aqueous formEven the most insoluble salts will dissolve to some extent in waterFor example, when AgCl is mixed with water a tiny amount of Ag+1 and Cl-1 existAgCl(s) Ag+(aq) + Cl-(aq)
22Solubility Product [Ag+][Cl-] Ksp= [AgCl] If we exclude the solid “reactant” because of its constant conc.We get a special kind of equilibrium constant called the solubility product constant (Ksp)The lower the solubility of a substance the smaller the Ksp
23Solubility Product Ksp = [Ag+]1[Cl-]1 If there are coefficients in the dissociation equation (from balancing) they become exponents that the conc are raised to.Ksp = [Ag+]1[Cl-]1The solubility product (Ksp) for AgCl at 25C is 1.8x10-10 M210-10 indicates a very small conc of silver and chloride ions
24Solubility Product: example 1 The equilibrium concentration of hydroxide ions in a saturated solution of iron(II) hydroxide is 1.2 x 10-5 M at a certain temperature. Calculate the Ksp of Fe(OH)2 at this temperature.
26Solubility Product: example 2 What is the concentration of silver ions in a saturated solution of silver carbonate? The Ksp of Ag2CO3 is 8.1 x 10-12
27Reaction QuotientWe 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.Once the reaction quotient is solved, it is compared to the equilibrium constantThe following picture helps us decide how to interpret the direction the reaction will continue.
31SO2Cl2(g) <==> SO2(g) + Cl2(g) Classwork:0.035 moles of SO2, moles of SO2Cl2, and moles of Cl2 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?SO2Cl2(g) <==> SO2(g) + Cl2(g)Kc = at 100°C
32Manipulating the Equilibrium… There is a principle that can be studied to govern changes in equilibrium Le Chatelier’s Principle.Le Chatelier’s Principle states:“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
33Concentration & Equilibrium Adjusting the concentrations of either reactants or products can have dramatic impact on the equilibriumIf 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[A]↑, rxn will shift toward products
34Concentration & Equilibrium Adjusting the concentrations of either reactants or products can have dramatic impact on the equilibriumIf 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[C]↑, rxn will shift toward reactants
36Temp effects on Equilibrium The impact of temperature changes on an equilibrium is dependent on if the process is endothermic or exothermicEndothermic processes use energy as a reactant, while exothermic processes produce energyKeq is temperature dependent250 kJ is a productIf T↑, the equilibrium shifts left
37Temp effects on Equilibrium The impact of temperature changes on an equilibrium is dependent on if the process is endothermic or exothermicEndothermic processes use energy as a reactant, while exothermic processes produce energyenergy is a reactantIf T↑, the equilibrium shifts right
38Endothermic or Exothermic? Keq is TEMP dependentEndothermic or Exothermic?
39Pressure & Equilibrium If A, B, and C are all gases, then the equil they establish is pressure dependentWhen 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 pressureFewer gas molecules will exert less pressure.So, more product is formed, which overall reduces the pressure, this is a shift right
40Pressure & Equilibrium Conversely, a decrease in pressure will favor the rxn that produces the most moleculesSo we have a shift to the leftP↑, this equilibrium shifts rightIf P↓, this equilibrium shifts left
432SO3(g) <=> 2SO2 (g) + O2 (g) Ho = 197.78 kJ ClassworkPredict the effect of the following changes on the reaction in which SO3 decomposes to form SO2 and O2.2SO3(g) <=> 2SO2 (g) + O2 (g) Ho = kJ(a) Increasing the temperature of the reaction.(b) Increasing the pressure on the reaction.(c) Adding more O2 when the reaction is at equilibrium.(d) Removing O2 from the system when the reaction is at equilibrium.