物理化学052.ppt

1,例题,例3.在一抽成真空的容器中单独放有NH4Cl(s)，在340oC时分解反应NH4Cl(s)=NH3(g)+HCl(g)的平衡压力P1=104.6kPa；同样条件下，若单独放有NH4I(s),分解反应NH4I(s)=NH3(g)+HI(g)的平衡压力P2=18.85kPa。求NH4Cl(s)与NH4I(s)两种纯固态物质同时存在，加热到340oC，当上述两个反应都达到平衡时，系统的总压为多大?设HI(g)不分解，两个盐类不形成固态溶液。,2,Answer 1,NH4Cl(s)alone:NH4Cl(s)=NH3(g)+HCl(g)P(NH3)=P(HCl)=P1/2NH4I(s)alone:NH4I(s)=NH3(g)+HI(g)P(NH3)=P(HI)=P2/2,3,When NH4Cl(ammonium chloride)and NH4I(ammonium iodide)coexist in the system,the partial pressure(分压)of ammonia(NH3)gas must accord with both two equilibrium mentioned above.If the partial pressure is P(NH3)for NH3(g),P(HCl)for HCl(g)and P(HI)for HI,So(1)(2)and P(NH3)=P(HCl)+P(HI)(3)The answer is P(total)=P(NH3)+P(HCl)+P(HI)=106.3kPa,4,Answer 2,Let x and y denote the amounts of HCl and HI when the two equilibrium coexisted in the reaction system.So the amount of NH3 in the system must be x+y and the total amounts of three gas is 2(x+y).Thus,the partial pressure of NH3 is and,5,(we can see the partial pressure of P(NH3)=P(HCl)+P(HI)X=30.83y P(total)=106.3kPa,6,Key problem in this example,1.The partial pressure is a intensive variable(强度变量）,and the amount of species is extensive variable(广度变量）2.Equilibrium constant for heterogeneous reactions(复相反应的平衡常数)3.The calculate rule for the equilibria coexisted in a system(同时平衡的计算原则）,7,Review,1 The criterion of a spontaneous chemical reaction2 The description of an equilibrium of reaction3 The isothermal(constant temperature)equation,8,Review,1 Several symbols1The Gibbs energy of a reaction system,G 2The reaction Gibbs energy:rGm=(G/)P,T3 The standard reaction Gibbs Energy4 is the Gibbs energy of mixing A and B5 Standard Gibbs energy of formation,9,2 Several relations:1 relation of reaction Gibbs energy(rGm)to the chemical potentials of the species,rGm=products-reactants2 The standard equilibrium constant(K)and the standard reaction Gibbs energy=-RT In Ko3 general expression for rGm at an arbitrary stage of the reaction,rGm=+RT In Q,10,Review,5thermodynamic equilibrium constant,(热力学平衡常数)can expressed in terms of activities(or fugacities):K for Perfect gas reaction,solution reaction and heterogeneous reaction has different form.6 in terms of standard Gibbs energies of formation,11,The content for this class,1The standard Gibbs energy of reaction in solution 2 The response of equilibria to the conditions1 How equilibria respond to pressure?2The effect of inert gas on equilibria3 Le Chateliers principle4The response of equilibria to temperature(van t Hoff equation)3 The coexist equilibrium,12,1 The standard Gibbs energy of reaction in solution,(1)Because the value of standard Gibbs energy formation(标准生成吉布斯自由能)is for a pure species,so the value of the standard reaction Gibbs energy(反应的标准吉布斯自由能变)indicated that the standard state of each reactant and product is in the pure state.,13,The standard state for the species in solution reaction maybe not the pure state.(1)The solution need not distinguish solvent(溶剂)and solute(溶质)Equation(1)can be used to calculate the(2)The solvent and solute need distinguishand the solution is dilute(稀溶液).The standard state of each reactant and product is ci=1mol.dm-3 and the solution submit Henrys law hypothetical.,14,Example,Calculate the ionization constant（电离常数）for benzoic acid C6H5COOH in solution at 298 K.C6H5COOH(aq.)=C6H5COO-(aq.)+H+(aq.)Given that for C6H5COOH in the solid state,and for C6H5COO-(C=1.0 mol.dm-3).The solubility of C6H5COOH in water is 0.02787 mol.dm-3 at 298K.,15,2 The response of equilibria to the conditions,1How equilibria respond to pressure(P234):The standard equilibrium constant is independent of the pressure,but the equilibrium composition depends on the pressure.(压力对化学平衡的影响，实际上并非是对平衡常数的影响，而是指对平衡移动的影响）,16,Le Chateliers principle:A system at equilibrium,when subjected to a disturbance,responds in a way that tends to minimize the effect of the disturbance.The principle implies that if a system at equilibrium is compressed,then the reaction will adjust so as to minimize the increase in pressure.,17,2 effect of inert gas(惰性气体)on equilibrium,1.If an inert gas is added to an equilibrium mixture of gases at constant temperature and volume,there is no effect on the equilibrium.Because the addition of an inert gas leaves the molar concentrations of the original gases unchanged,as they continue to occupy the same volume.,18,2.If adding an inert gas at constant temperature and pressure which has the same effect as lowering the original gas partial pressure.If,the addition of an inert gas at constant pressure reduces the sum of the partial pressure of the reactants and products,and the reaction shifts to the left to compensate for this.,19,Example,At 1273 K and a total pressure of 30.4 atm the equilibrium in the reaction CO2(g)+C(s)=2CO(g)is such that 17 mol%of the gas is CO2.(a)What would be CO2 if the total pressure were 20.3 atm?(b)What would be the effect on the equilibrium of adding N2 to the reaction mixture in a closed vessel until the partial pressure of N2 is 10 atm?If the reactant gases contain a mole of N2 in addition to 1 mole of CO2,what is the equilibrium extent of reaction at 30.4 atm?(d)At what pressure of the reactants will 25%of the gas be CO2?,20,4 The response of equilibria to temperature,According to Le Chateliers principle,a system at equilibrium will tend to shift in the endothermic direction if the temperature is raised,while the equilibrium tend to shift in the exothermic direction if the temperature is lowered.,21,The van t Hoff equation,The van t Hoff equation is an expression for the effect of temperature on the standard equilibrium constant Ko.Here is the standard reaction enthapy at the temperature T.,22,the value of K at different temperature,General,the integral(积分)of Vant Hoff Equation is used for gas reaction which the pressure is low.(1)If varies only slightly with temperature over the temperature range of interest,the integral of equation from T1 to T2 yiels,23,(2)If depends on temperature,Specially,when the heat capacities(热容）of reactants and products is the mean value,the value of is constant,then,24,3 The effect of initial composition on equilibrium,For example,consider the equilibrium for the production of methanol from CO and H2:CO(g)+2 H2=CH3OH(g)The value of K at 500 K is 0.00623.(a)A gas stream containing equimolar amounts of CO and H2 is paased over a catalyst at 101.3 kPa.What is the extent of reaction at equilibrium?(b)If the initial composition is 2 mole H2 and 1 mole CO,what is the equilibrium extent?,