Chemical Equilibrium. aa bb cc dd [C] [D] [A] [B] A generalized balanced chemical reaction: Is this chemical system at equilibrium?

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1 A geralized balanced chemical reaction: aa bb cc dd Chemical Equilibrium Is this chemical system at equilibrium? A chemical system is at equilibrium a. when no of it s thermodynamic properties change with time (macroscopic) b. when the rate of the forward reaction exactly equals the rate of the reverse reaction. Why is chemical equilibrium important? All chemical transformations occur because they tend to reach a state of chemical equilibrium. aa bb cc dd At equilibrium microscopically, transformation occur in both directions at equal rates. (dynamic equilibrium). No changes of macroscopic concentration values occur, if the reaction system is unperturbed from equilibrium state. At equilibrium all species shown in the equation co-exist, however small/large the concentrations are. Every system (reaction or otherwise) strives to reach stability, which is the state of equilibrium. aa bb cc dd At equilibrium the concentration of species are related by way of the equilibrium constant, defid as; [C] [D] [ ] equilibrium concentration value [A] [B] = equilibrium constant (is a function of temperature) measures the equilibrium position of the reaction Large more products than reactants AT EQUILIBRIUM, and vice versa. (Expression for is associated with alanced equation as written). Reactions with v.large are arly complete (in the forward direction); forward reaction much favored. Amount of product is determid by quantity of limiting reactant only. Such reactions are ideal for quantitative (gravimetric and volumetric) analysis. Reactions with very small have much less tendency to go forward and the reverse reaction is favored!! A chemical system at equilibrium can be perturbed to a noquilibrium state. Then the system would readjust to reach a w equilibrium state, in order to offset the disturbance (Le Chaterlier s Principle). Systems not at equilibrium - Reaction Quotient Q: A chemical reaction system that is not at equilibrium would have the concentrations not conforming to the equilibrium constant expression. [C] [D] [A] [B] c c d b [C],[D],... non- equilibrium concentration values

2 Any reaction strives to reach the equilibrium state. aa bb cc dd Systems not at equilibrium - Reaction Quotient Q: [C] [D] Q [X] non- equilibrium ( ) concentration values [A] [B] At equilibrium the concentrations of the species would satisfy the equilibrium expression. aa bb cc dd [C] [D] [ ] equilibrium concentration value [A] [B] Reaction species mixed with their concentrations not satisfying the equilibrium concentration expression (Q ) would change their concentrations so as to reach the equilibrium expression values (Q ). Q = conc. ratio of non. eq. system Q< ; Q> ; non equilibrium: so that Q= For and Q expressions use the numerical value of the concentrations expressed in units of ; solutions value of molarity (mol/l) M {std.state - M}; gases value of partial pressure (atm) {std.state - atm}; for solvents, solids and pure liquids set [ ] =. Examples of expression: and Q are dimensionless quantities. Q captures the state of the reaction system on it s way to equilibrium. Q is a measure of the how far the reaction state is away from the equilibrium state. H C (aq) = HC - (aq) H (aq) [ HC ][ H ] [ H C ] Bi S (s) 6H (aq) = Bi (aq) H S(aq) [ Bi ] [ H S] 6 [ H ] Manipulating equilibrium constants: aa bb cc dd [ C] [ D] [ A] [ B] cc dd aa bb; for the reverse reaction ' [ A] [ B] [ C] [ D] ' If a reaction is a summation of two or more reactions ; then their equilibrium constants are related; ABCD [ C][ D] [ A][ B] () CDE F G [ F][ G] [ C][ D][ E] () A BE F G [ F][ G] [ A][ B][ E] () Because () () () [ C][ D] [ F][ G] [ F][ G] [ A][ B] [ C][ D][ E] [ A][ B][ E]

3 [ C][ D] ABCD [ A][ B] [ C] [ D] ABC D = [ A] [ B] [ C][ D] na nb nc nd = [ A][ B] n Aqueous - Solution Chemistry Most reactions in quantitative analysis are performed in aqueous solutions. Reactants that are gerally water soluble are; electrolytes, polar molecules or small molecules. dissociation: ~00% strong electrolyte % fairly weak electrolyte The ions of dissolved weak electrolytes exist in equilibrium with it s un-dissociated molecules. (Expression for is associated with alanced equation as written). AB(aq) A (aq) B - (aq) Examples of equilibria: Acid dissociation a : HA H A - H (note: species solvent) acid conjugate base [A ][H ] a,ha [HA] note: [H ] solvent; set to. stronger HA (larger a ) ~ weaker base, A - weaker HA (smaller a ) ~ stronger base, A - Structure of hydronium ion H - log a = p a Bronsted Acid is a substance which increase [H ] in water. Strong acids dissociate arly completely in solution. Strong acids consumes/reacts with basic groups when encountered, almost completely. Strengths of acids/bases expressed by their values. (only a few strong acids are encountered in aqueous phase) now all strong acids and bases. a >>0. All other acids and bases are weak; a small.

4 Bronsted Base is a substance which decreases [H ] in water or increases the [H - ] in water. Strong bases consume acidic hydrogens arly completely, vice versa. A salt is any ionic compound which is ither a Bronsted acid nor a Bronsted base. Strengths of acids/bases expressed by their values. (only a few strong acids are encountered in aqueous phase) Base constant b for ase A: - ; Bases other than simple hydroxides render aqueous solutions basiue to hydrolysis, (breakup of water molecules) A: - H HA H - hydrolysis - [H ][HA] [A ] b,a- - - log b = p b acid-base reaction eq : A - H HA H Base Acid weaker acid [HA] [A ][H ] eq - a,ha note: inverse relationship All solvents with active hydrogens (i.e. protic solvents) undergo auto-protolysis (self ionization). NH NH NH CHCH CHCH CHC H H H 4 Any acid in water (for example) gerates hydronium ions. Acidic species actually available in aqueous solutions is H. H ions in solution is hydrated. Hydration shell. Ideal hydrated hydronium ion. 4

5 a,b,b,auto-protolysis of solvents (water, w ): self dissociation of water: H H - H w = [H-][H ] The strength of an acid or base character of a solute is highly dependent on the solvent in which it is dissolved. It is the solvent that determis the acid or base strength of a solute. Leveling effect: All acids stronger than H is leveled down to the strength of H in water. All bases stronger than H - is leveled down to H - in water. Weak acids and bases: acid dissociation a of a weak acid HA: HA H A - H WA Conjugate base Weaker acid stronger conj. base Stronger acid weaker conj. base a,ha [H ][A ] [HA] - log a = p a ; smaller a larger p a Conjugate pairs shown in color. For aqueous solutions; H H - H w = [H - ][H ] =[H - ][H 5 o C aha, ba, w w p p p w =whaa=ahaa,always exist in water Pure water: [H ] = [H - ] = 0-7 stronger conj. Base weaker acid weaker conj. Base stronger acid Weaker acid stronger conj. base Stronger acid weaker conj. basewsolubility of Ionic Compounds Solubility is an equilibrium process between the ionic compound (solid) and it s dissolved ions. Solubility is expressed as concentrations of solutes in solution at saturation point of dissolution. 5

6 Solubility product sp: BaS4(s) Ba(aq) S4-- (aq) spbas4 =. 0-0 sp = [Ba ][S4 - ] saturated solution concentrations!!! no [BaS4] in sp expression solid, set to Compounds with very small sp are ideal for gravimetric analysis. Eqlm. position can be shifted by disturbing the equilibrium system Le Chaterlier's Principle. note: solubility product gives the concentrations of associated ions existing in equilibrium/their co-existing concentrations. Reaction quotient, Q = conc. product ratio of non. eq. system. For solubility problems note; Q< dissolution; equilibrium; Q> pptn. Example of a solubility equilibrium. Note: Reverse of solubilization is precipitation Ba (aq) S4 - (aq) BaS4(s) =/ sp BaS 4 (s) dissolution Ba (aq) S 4 - (aq) = sp In equilibrium calculations identify the possible equilibria, and pick the concentrations of species relevant to solve the problem. Calculate [H - ] in a. 0.00M NaH? b. 0.00M HCl? Solubility of Hg Cl in a. water? b. 0.00M Cl -? sp,hgcl =. 0-8 HgCl() s Hg ( aq) Cl ( aq) Salts are less soluble in solvents carrying common ions - common ion effect! How much Cl - can be added to a Ag (0.0 M) solution without creating a ppt.? sp,agcl = i.e. to reach saturation solubility of AgCl in water. AgCl(s) = Ag (aq) Cl - (aq) Q Ag Cl 0 sp, AgCl.80 [ ][ ] non-eqlm. geral case What would be the [Cl - ] so the above system is at equilibrium? That is when would Q=. Q< ; Q> ; non equilibrium: so that Q= 0 Q[ Ag ][ Cl ] [ Cl ].80 Add Cl - until [Cl - ] = M 9 [ Cl ].80 [Cl - ] > M pptn; Q> Alternate approach xt page. Ag (aq) Cl - (aq) = AgCl(s) How much Cl - can be added to a Ag solution (0.0 M) solution without creating a precipitate? sp,agcl = Note: Precipitation starts when the concentration just exceeds the saturation point (solubility wise). Ag (aq) Cl - (aq) = AgCl(s) [Ag ]=0.0M Q =? = / sp,agcl Q< ; Q> ; non equilibrium: so that Q= [ Ag ][ Cl ] 9 Q sp, AgCl Ag (aq) Cl - (aq) = AgCl(s) Before adding Cl ; Q [0.] 0 After adding a trace Cl ; Q 0 [0.][ 0 ] 0 adding more Cl ; Q 0 9 [0.][0 ] adding more Cl 9 ; Q [0.][.8 0 ] For [Cl - ] >.80-9, Q < and precipitation occurs. [Ag ]=0.0M 6

7 Can you separate Pb(II) completely from Hg(I) from a mixture using iodide. (PbI (s), Hg I (s))? Initial conc. of both ions Pb(II) and Hg(I) is 0.M. sp,lead iodide = sp,mercury(i) iodide =.0-8 Less soluble compound precipitates first. By inspection, Hg I is more insoluble. In geral o must solve for solubility to make that decision. Criterion: "complete pptn." = 99.99% At the start of 'complete' precipitation [Hg ] in solution [Hg ] The ions in solution (saturation) satisfies; sphgi [Hg ] For the system; PbI (s) = Pb (aq) I (aq), at the point of 'complete' precipitation of mercurous ion; 8 sphgi.0 Q PbI [Pb ] 0. 5 [Hg ] QPbI.0 PbI (PbI 7.90 ) no precipitation because no trend to left in the above system. [Hg ][I ] sphgi [I ] Would Pb(II) remain in solution when 99.99% of Hg(I) is removed from the solution by precipitation? If so; at the point of Hg(I) removal, the Q for the equilibrium; PbI (s) = Pb (aq) l - (aq) sp,pbl must be less than or equal to it s value - ( sp,pbl ) so the equilibrium do not tend to left (i.e. precipitation) To find out we ed to calculate Q PbI at the point of complete precipitation of mercurous ion. Complex Ions: Formation constant of complex ions n; In geral more than o ligand combis to form metal complexes; occurs when [L]>>0. M x nl ML x n [ML n] overall formation constant [M ][L] n x n Q< ; Q> ; non equilibrium: so that Q= Stepwise Formation constants,,,.. n. Complexes are formed in a stepwise manr. Stepwise equilibria exist. Pb I PbI [PbI ] [I ] PbI I PbI molecular species in dissolved state (ion pairs) [] [] [PbI ] PbI I PbI [] [I ] PbI I PbI All aquated species. [I ] [4] 7

8 - cumulative equilibrium constants [] [][] [][][] Sum all Pb I PbI Pb I PbI Pb I PbI Pb 4I PbI verall formation constant, n =.. n, is the overall eq. const. of a series of stepwise formation constants ( = ). Equilibrium constants relate the concentrations of species that can coexist at equilibrium (i.e. if the system is not disturbed). Polyprotic Acids: - H P 4 H P 4 H a H P H H H P H H a - - H P HP 4 4 H a - - HP P 4 4 H a H H P H P H P P H H a a H P HP H H P P H 4 4 a a 4 4 a a a Polybasic species: P H P H H- b P H HP H HP 4 H H H - P 4 b b H P H H P H H- b - H P 4 H H H - P 4 b H P H H H P H H H- b 8

9 For triprotic acids: Learn to recognize the acidic and basic entities in large molecules. a b = w a b = w a b = w H H H NH Note the geral fact, that the product of s of conjugated acid-base pairs in aqueous solution (solvent) is equal to the ion product of water (solvent). H H NH Equivalence point ph - acid/base rxn or ph of salt solutions: Strong (acids bases) salt (e.g. Na Cl - ) water Weak acid strong base Na CHCH H CHC Na H acid base conj. conj. base acid Weaker compont (apart from water) a salt of the weak compont. verall; the product side is weaker than reactant side, if the reaction takes place as written. At least o compont of such salts reacts with water, hydrolysis; CH C H CH CH H good base weak acid At equivalence point; the resultant solution of a weak acid/base titration is not utral, ph 7. i.e. salt of a weak acid or weak base. Conjugate base of weaker acid is a stronger base. Conjugate base of stronger acid is a weaker base. In the above solution; CH C, H, CH CH, H and H from H H H Identify all possible equilibria among species 9