2Subtopics1.Chemical Potential in an Ideal Gas Mixture. 2.Ideal-Gas Reaction Equilibrium 3.Temperature Dependence of the Equilibrium Constant 4.Ideal-Gas Equilibrium Calculations
31.1 Chemical Potential of a Pure Ideal Gas Expression for μ of a pure gasdG=-S dT + V dPDivision by the no of moles gives:dGm = dμ = -Sm dT + Vm dPAt constant T,dμ = Vm dP = (RT/P) dPIf the gas undergoes an isothermal change from P1 to P2:.μ (T, P2) - μ (T, P1) = RT ln (P2/P1)Let P1 be the standard pressure P˚μ (T, P2) – μ˚(T) = RT ln (P2/ P˚)μ = μ˚(T) + RT ln (P/ P˚) pure ideal gas
41.2 Chemical Potential in an Ideal Gas Mixture An ideal gas mixture is a gas mixture having thefollowing properties:The equation of state PV=ntotRT obeyed for all T, P & compositions (ntot = total no. moles of gas).If the mixture is separated from pure gas i by a thermally conducting rigid membrane permeable to gas i only, at equilibrium the partial pressure of gas i in the mixture is equal to the pure-gas-i system.At equilibrium, P*i = P iMole fraction of i(ni/ntot)
51.2 Chemical Potential in an Ideal Gas Mixture Let μi – the chemical potential of gas i in the mixtureLet μ*i – the chemical potential of the pure gas in equilibrium with the mixture through the membrane.The condition for phase equilibrium:The mixture is at T & P, has mole fractions x1, x2,….xiThe pure gas i is at temp, T & pressure, P*i.P*i at equilibrium equals to the partial pressure of i, Pi in the mixture:Phase equilibrium condition becomes:gas in the mixture pure gas(ideal gas mixture)At equilibrium, P*i = P i
61.2 Chemical Potential in an Ideal Gas Mixture The chemical potential of a pure gas, i:(for standard state, )The chemical potential of ideal gas mixture:
72. Ideal-Gas Reaction Equilibrium All the reactants and products are ideal gasesFor the ideal gas reaction:the equilibrium condition:Substituting into μA , μB , μC and μD :
82. Ideal-Gas Reaction Equilibrium The equilibrium condition becomes:where eq – emphasize that these are partial pressure atequilibrium.
92. Ideal-Gas Reaction Equilibrium Defining the standard equilibrium constant ( ) forthe ideal gas reaction: aA + bB cC + dDThus,
102. Ideal-Gas Reaction Equilibrium For the general ideal-gas reaction:Repeat the derivation above,Then,Define:Standard equilibrium constant:(Standard pressure equilibrium constant)
11Example 1A mixture of mmol of H2S & 5.48mmol of CH4 was placed in an empty container along with a Pt catalyst & the equilibriumwas established at 7000C & 762 torr.The reaction mixture was removed from the catalyst & rapidly cooled to room temperature, where the rates of the forward & reverse reactions are negligible.Analysis of the equilibrium mixture found mmol of CS2.Find & for the reaction at 7000C.1bar =750torr
12Answer (Example 1) Mole fraction: P = 762 torr, Partial pressure: Standard pressure, P0 = 1bar =750torr.
143. Temperature Dependence of the Equilibrium Constant The ideal-gas equilibrium constant (Kp0) is a function of temperature only.Differentiation with respect to T:From
153. Temperature Dependence of the Equilibrium Constant Since ,This is the Van’t Hoff equation.The greater the | ΔH0 |, the faster changes with temperature.Integration:Neglect the temperature dependence of ΔH0,
16Example 2Find at 600K for the reaction by using the approximation that ΔH0 is independent of T;Note:SubstancekJ/molNO2 (g)33.1851.31N2O4 (g)9.1697.89
17Answer (Example 2)If ΔH0 is independent of T, then the van’t Hoff equation givesFrom
183. Temperature Dependence of the Equilibrium Constant Since , the van’t Hoff equation can be written as:The slope of a graph of ln Kp0 vs 1/T at a particular temperature equals –ΔH0/R at that temperature.If ΔH0 is essentially constant over the temperature range, the graph of lnKp0 vs 1/T is a straight line.The graph is useful to find ΔH0 if ΔfH0 of all the species are not known.
19Example 3 Use the plot ln Kp0 vs 1/T for for temperature in the range of 300 to 500KEstimate the ΔH0.Plot of lnKp0 vs 1/T
214. Ideal-Gas Equilibrium Calculations Thermodynamics enables us to find the Kp0 for a reaction without making any measurements on an equilibrium mixture.Kp0 - obvious value in finding the maximum yield of product in a chemical reaction.If ΔGT0 is highly positive for a reaction, this reaction will not be useful for producing the desired product.If ΔGT0 is negative or only slightly positive, the reaction may be useful.A reaction with a negative ΔGT0 is found to proceed extremely slow - + catalyst
224. Ideal-Gas Equilibrium Calculations The equilibrium composition of an ideal gas reaction mixture is a function of :T and P (or T and V).the initial composition (mole numbers) n1,0,n2,0….. Of the mixture.The equilibrium composition is related to the initial composition by the equilibrium extent of reaction (ξeq).Our aim is to find ξeq.
234. Ideal-Gas Equilibrium Calculations Specific steps to find the equilibrium composition of anideal-gas reaction mixture:Calculate ΔGT0 of the reaction using and a table of ΔfGT0 values.Calculate Kp0 using [If ΔfGT0 data at T of the reaction are unavailable,Kp0 at T can be estimated usingwhich assume ΔH0 is constant]
244. Ideal-Gas Equilibrium Calculations Use the stoichiometry of the reaction to express the equilibrium mole numbers (ni) in terms of the initial mole number (ni,0) & the equilibrium extent of reaction (ξeq), according to ni=n0+νi ξeq.(a) If the reaction is run at fixed T & P, use(if P is known)& the expression for ni from ni=n0+νi ξeq to expresseach equilibrium partial pressure Pi in term of ξeq.(b) If the reaction is run at fixed T & V, usePi=niRT/V (if V is known)to express each Pi in terms of ξeq
25Ideal-Gas Equilibrium Calculations Substitute the Pi’s (as function of ξeq) into the equilibrium constant expression & solve ξeq.Calculate the equilibrium mole numbers from ξeq and the expressions for ni in step 3.
26Example 4Suppose that a system initially contains mol of N2O4 (g) and mol of NO2 (g) & the equilibrium is attained at 250C and 2.00atm (1520 torr).Find the equilibrium composition.Note:1.2.3. ni=n0+νi ξeq.4.5.6. Get 𝜉 and find nSubstancekJ/molNO2 (g)51.31N2O4 (g)97.89
29Answer (Example 4)The reaction occurs at: P=2.00atm=1520 torr & P0=1bar=750torr.Clearing the fractions:Use quadratic formula:So, x =Number of moles of each substance present at equilibrium must be positive.Thus,So,As a result,
30Example 5 Kp0 =6.51 at 800K for the ideal gas reaction: If mol of A, mol of B and mol of C are placed in an 8000 cm3 vessel at 800K.Find the equilibrium amounts of allspecies.1.2.3. ni=n0+νi ξeq.4. Pi=niRT/V5.6. Get 𝜉 and find n1 bar= torr,1 atm = 760 torrR=82.06 cm3 atm mol-1 K-1
31Answer (Example 5)Let x moles of B react to reach equilibrium, at the equilibrium:The reaction is run at constant T and V.Using Pi=niRT/V & substituting intoWe get:Substitute P0=1bar= torr, R=82.06 cm3 atm mol-1 K-1,
32Answer (Example 5) We get, By using trial and error approach, solve the cubic equation.The requirements: nB>0 & nD>0, Hence, 0 < x <1.Guess if x=0, the left hand side =Guess if x =1, the left hand side = 0.024Guess if x=0.9, the left hand side =Therefore, 0.9 < x < 1.0.For x=0.94, the left hand side = 0.003For x=0.93, the left hand side=-0.001As a result,nA=1.14 mol, nB=0.07mol, nC=4.93mol, nD=0.93mol.