Proerties of an Equilibrium System CHAPTER 6 Chemial Equilibrium Equilibrium systems are DYNAMIC (in onstant motion) REVERSIBLE an be aroahed from either diretion Pink to blue Co(H O) 6 Cl ---> > Co(H O) Cl + H O Blue to ink Co(H O) Cl + H O ---> > Co(H O) 6 Cl Basi Conets Reversible reations do not o to omletion. They an our in either diretion Symbolially, this is reresented as: a A + b B C + d D ( ) ( ) ( ) ( ) Chemial equilibria are dynami equilibria. Moleules are ontinually reatin, even thouh the overall omosition of the reation mixture does not hane. Basi Conets Grahially, this is a reresentation of the rates for the forward and reverse reations for this eneral reation. a A( ) ( ) + b B C( ) + d D( ) Reation Quotient & Equilibrium ahieved Produt on. inreases and then beomes onstant at equilibrium Reatant on. delines and then beomes onstant at equilibrium The For a simle one-ste mehanism reversible reation suh as: The rates of the forward and reverse reations an be reresented as: Rate f k f[ A][ B] whih reresents the forward rate. Rate k [ C][ D] whih reresents the reverse rate. r A r C + D + B DERIVE 6
The Similarly, for the eneral reation: a A we an define a onstant + b B This exression [ C][ D] a [ A] [ B] C + d D d b roduts reatants is valid for all reations. Writin and Maniulatin Exressions Solids and liquids NEVER aear in equilibrium exressions. S(s) + O SO [SO ] [O ] 7 8 Writin and Maniulatin Exressions Solids and liquids NEVER aear in equilibrium exressions. NH (aq) + H O(liq) (aq) [NH + ][OH - ] [NH ] NH + (aq) + OH - Writin and Maniulatin Exressions Addin equations for reations [ SO ] S(s) + O SO [ O ] SO + / O SO Net equation S(s) + / O SO net [SO ] [O ] / net [SO ] [SO ][O ] / 9 0 Writin and Maniulatin Exressions Chanin diretion S(s) + O SO SO S(s) ) + O new [O ] [SO ] [SO ] [O ] new [O ] [SO ] old The Examle: One liter of equilibrium mixture from the followin system at a hih temerature was found to ontain 0.7 mole of hoshorus trihloride,, 0.086 mole of hlorine, and 0.08 mole of hoshorus entahloride. Calulate for the reation. PCl PCl + Cl Equil [ ] s] s 0.08 M 0.7 M 0.086 M
The Examle: Write equilibrium onstant exressions for the followin reations at 00 o C. All reatants and roduts are ases at 00 o C. PCl PCl + Cl [ PCl][ Cl] [ PCl ] Variation of with the Form of the Balaned Equation The value of deends uon how the balaned equation is written PCl PCl + Cl This reation has a [PCl ][Cl ]/[PCl ]0. What is? PCl Cl PCl + The Examle: The deomosition of PCl was studied at another temerature. One mole of PCl was introdued into an evauated.00 liter ontainer. The system was allowed to reah equilibrium at the new temerature. At equilibrium 0.60 mole of PCl was resent in the ontainer. Calulate the equilibrium onstant at this temerature. The Examle: At a iven temerature 0.80 mole of N and 0.90 mole of H were laed in an evauated.00-liter ontainer. At equilibrium 0.0 mole of NH was resent. Calulate for the reation. 6 Tyial Calulations PROBLEM: Plae.00 mol eah of H and I in a.00 L flask. Cal. equilibrium onentrations. H + I + I HI [HI] [H ][I ]. 7 Nitroen Dioxide Equilibrium N O NO [NO ] [N O ] 0.009 at 98 If initial onentration of N O is 0.0 M, what are the equilibrium onentrations? Ste. Set u an ICE table [N O ] [NO ] Initial 0.0 0 Chane Equilib 8
Nitroen Dioxide Equilibrium N O NO [NO ] [N O ] 0.009 at 98 If initial onentration of N O is 0.0 M, what are the equilibrium onentrations? Partial Pressures and the For as hase reations the equilibrium onstants an be exressed in artial ressures rather than onentrations. For ases, the ressure is roortional to the onentration. We an see this by lookin at the ideal as law. PV nrt P nrt/v n/v M P MRT and M P/RT 9 0 Partial Pressures and the Consider this system at equilibrium at 00 0 C. Cl( ) + HO( ) HCl( ) + O( ) [ HCl] [ O] [ Cl ] [ H O] and ( PHCl) ( PO ) ( P ) ( P ) Cl HO Partial Pressures and the P P HCl O ( ) ( ) ( P ) ( P ) RT RT HCl O ( ) RT P P Cl HO ( ) ( ) ( PCl ) ( PH O) ( RT) RT RT ( ) so for this reation (RT) RT - or (RT) L atm Must use R 0.08 mol n ( RT) or ( RT) n (# of moles of aseous roduts) - (# of moles of aseous n reatants) Relationshi Between and Examle: Nitrosyl bromide, NOBr,, is % dissoiated by the followin reation at o C, in a vessel in whih the total ressure is 0. atmoshere. What is the value of? NOBr NO + Br ( ) ( ) ( ) Relationshi Between and Examle: is 9 for the followin reation at 0 o C. If.0 mole of H and.0 mole of I are allowed to reah equilibrium in a.0-liter vessel, H I HI + ( ) ( ) ( ) (a) How many moles of I remain unreated at equilibrium? (b) What are the equilibrium artial ressures of H, I and HI? () What is the total ressure in the reation vessel?
The Reation Quotient The mass ation exression or reation quotient has the symbol Q. Q has the same form as The major differene between Q and is that the onentrations used in Q are not neessarily equilibrium values. For this eneral reation : aa + bb C + dd Q d [ C][ D] [ A] a [ B] b Reation Quotient & At any oint in the reation H + I HI Q reation quotient [HI] [H ][I ] 6 Reation Quotient & The Reation Quotient Equilibrium ahieved In the equilibrium reion [HI] [H ][I ]. equilibrium onstant Q> Q< Q When : The reation ours to the left to a reater extent. The reation ours to the riht to a reater extent. To hel understand this think of The system is at equilibrium. Q and as frations. 7 8 The Reation Quotient Examle: The equilibrium onstant for the followin reation is 9 at 0 o C. If 0. mole of I, 0. mole of H, and 0.66 mole of HI were ut into an evauated.00-liter ontainer, would the system be at equilibrium? If not, what must our to establish equilibrium? Uses of the Equilibrium Constant, Examle: The equilibrium onstant,, is.00 for the followin reation at a iven temerature. If.00 mole of SO and.00 mole of NO are ut into an evauated.00 L ontainer and allowed to reah equilibrium, what will be the onentration of eah omound at equilibrium? SO NO SO + NO + 9 0
Uses of the Equilibrium Constant, Examle: The equilibrium onstant is 9 for the followin reation at 0 o C. If.00 mole of HI is ut into an evauated.00- liter ontainer and allowed to reah equilibrium, what will be the equilibrium onentration of eah substane? H + I HI The Meanin of. Can tell if a reation is rodut-favored or reatant-favored. For N + H NH [NH ]. x 08 [N ][H ] Con. of roduts is muh reater than that of reatants at equilibrium. The reation is stronly rodut-favored favored. The Meanin of For ACl(s) A + (aq) ) + Cl - (aq) [A + ] [Cl[ - ].8 x 0 - Con. of roduts is muh less than that of reatants at equilibrium. The reation with small is stronly reatant-favored favored. A + (aq) ) + Cl Cl - (aq) - ACl(s) is is rodut-favored. EQUILIBRIUM AND EXTERNAL EFFECTS Temerature, atalysts, and hanes in onentration affet equilibria. The outome is overned by LE CHATELIER S PRINCIPLE...if a system at equilibrium is disturbed, the system tends to shift its equilibrium osition to ounter the effet of the disturbane. Equlibrium: : Preditions LeChatelier s Prinile - If a hane of onditions (stress) is alied to a system in equilibrium, the system resonds in the way that best tends to redue the stress in reahin a new state of equilibrium. Some ossible stresses to a system at equilibrium are:. Chanes in onentration of reatants or roduts.. Chanes in ressure or volume (( ) ) (for aseous reations) Chanes in temerature. 6. Chanes in temerature. Equilibrium: Preditions For onveniene we may exress the amount of a as in terms of its artial ressure rather than its onentration. To derive this relationshi, we must solve the ideal as equation. PV nrt n P RT V n Beause has the units mol/l, V P []RT Thus at onstant T, the artial ressure of a as is diretly roortional to its onentration. 6
Equlibrium: : Preditions Chanes in Conentration of Reatants and/or Produts Also true for hanes in ressure for reations involvin ases. Look at the followin system at equilibrium at 0 o C. H + I ( ) ( ) ( ) [ HI] [ H ][ I ] HI 9 Whih way will the system move ifffffff???? 7 Equlibrium: : Preditions Chanes in Volume (and ressure for reations involvin ases) Predit what will haen if the volume of this system at equilibrium is haned by hanin the ressure at onstant temerature: NO N ( ) ( ) O [ NO] [ NO ] 8 Equlibrium: : Preditions Chanin the Reation Temerature Consider the followin reation at equilibrium: SO ( ) + O( ) SO( ) +98 kj Heat is a rodut of this reation, exothermi. Dereasin the reation temerature stresses the reatants. This favors the reatants or forward reation. Equlibrium: : Preditions Introdution of a Catalyst Catalysts derease the ativation enery of both the forward and reverse reation equally. Catalysts do not affet the osition of equilibrium. Equilibrium will be established faster with a atalyst. 9 0 Equilibrium: Preditions Examle: Given the reation below at equilibrium in a losed ontainer at 00 o C. How would the equilibrium be influened by the followin? o N + H NH Hrxn 9 kj/mol Fator a. Inreasin the reation temerature b. Dereasin the reation temerature. Inreasin the ressure by dereasin the volume d. Inrease the onentration of e. Derease the onentration of H NH Effet on reation roedure Equilibrium: Calulations To hel with the alulations, we must determine the diretion that the equilibrium will shift by omarin Q with. Examle: An equilibrium mixture from the followin reation was found to ontain 0.0 mol/l of A, 0.0 mol/l of B, and 0.0 mol/l of C. What is the value of for this reation? A B + ( ) ( ) ( ) If the volume of the reation vessel were suddenly doubled while the temerature remained onstant, what would be the new equilibrium onentrations? C 7
Equilibrium: Calulations Examle: A.00 liter vessel in whih the followin system is in equilibrium ontains.0 moles of COCl, 0.60 moles of CO and 0.0 mole of Cl. Calulate the equilibrium onstant. CO( ) ( ) + Cl COCl ( ) An additional 0.80 mole of Cl is added to the vessel at the same temerature. Calulate the molar onentrations of CO, Cl, and COCl when the new equilibrium is established. Le Chatelier s Prinile Chane T hane in therefore hane in P or onentrations at equilibrium Use a atalyst: reation omes more quikly to equilibrium. not haned. Add or take away reatant or rodut: does not hane Reation adjusts to new equilibrium osition Conentration hanes no hane in only the equilibrium omosition hanes. 8