化学课件】chapter 5 chemical equilibrium

1、O返回 Chemical EquilibriumO返回5.1 The conditions of chemical equilibrium and affinity of chemical reaction 5.2 The equilibrium constant of a reaction and isothermal equation 5.3 Heterogeneous chemical equilibrium 5.4 Determination of equilibrium constants 5.5 The standard Gibbs function of formationO返回

2、5.8 Coupling reaction5.7 Chemical equilibrium of simultaneous reaction5.6 The response of reactions to the conditionsExpress mathematically how the equilibrium constant depends on the temperature, and derive and use the vant Hoff equation.O返回 We have seen that the natural direction of all change at

3、constant temperature and pressure is towards minimum Gibbs function. The idea is entirely general, and in this chapter we use it as a basis for the discussion of chemical transformations.O返回Is the natural tendency for the mixture A+B to go to the mixture C+D, or the opposite? This can be answered by

4、 calculating the Gibbs function of each mixture.If the sum of the Gibbs functions of A+B is higher than that of C+D, then the reaction has a tendency to roll from left to right.O返回Suppose a reaction：DEFGdefg BBddnBBddnBased on extent of reaction：BBB0Or:O返回BBBddddnpVTSG,BBBBBBdddT pGn （）BB(dd )nAt co

5、nst. T and P，,BBB() (a) TpG ：1 molrm,BBB (b) TpG（）O返回 ,Brm,B() , ()T pBT pGG 或 rm,()0T pGThe reaction spontaneously proceeds from left to right.rm,()0T pGThe reaction spontaneously proceeds from right to left.rm,()0T pGThe reaction is at equilibrium.Use to determine:O返回0)(,pTGFrom left to right0)(,p

6、TGFrom right to left0)(,pTGAt equilibriumPTG,is the slope of G with recpect to the extent of reaction .O返回O返回O返回 1922年，比利时热力学专家德唐德（De donder）首先引进了化学反应亲和势的概念。他定义化学亲和势A为： def ,BBB ()T pGA mr-GA或 A是状态函数，体系的强度性质。用A判断化学反应的方向具有“势”的性质，即：A0 反应正向进行 A0 反应逆向进行A=0 反应达平衡O返回The simplest assumption is to suppose t

7、hat the species are perfect gases, in which case their chemical potentials are determined by their partial pressures:ABmrabGaAAbBBPPRTaPPRTblnlnO返回mrGaAbBABPPPPRTab/lnaAbBmrPPPPRTG/ln0 - Reaction proceeds - Reaction is at equilibriumO返回BBB,mr)(pTGBBBBBB( )lnfTRTp $BBB( , )( )lnfT pTRTp$rmBBB( )( )GT

8、T $Brm,rmBB()( )lnT pfGGTRTp $ is the standard molar Gibbs function of the reactionrm( )GT$fB is the fugacity of BSubstituting this expression, givesO返回ghGHrmdeDrEm(/) (/)( )ln(/) (/)fpfpGTRTfpfpG $rm( )lnfGTRTQ $DEGHdegh Reaction:Where Q is the reaction quotient.O返回When the reaction is at equilibri

9、um， 0mr GghGHrmdeDE(/) (/)ln(/)(/(fpfpRTfpfTpG $lnfRTK $ is the thermodynamic equilibrium constant.fK$ It should be noticed that vides a way of measuring the standard Gibbs functions of reactions.O返回Use the vant Hoff equation：rmlnlnffGRTKRTQ $rmlnlnppGRTKRTQ $For pg reaction:rm0ppKQG$ from lef

10、t to rightrm0ppKQG$from right to leftrm0ppKQG$at equilibrium.O返回Subscript m denoting the extent of reaction is unity i.e. 1 mol .rm( )G T$fK$rm( )lnfGTRTK $rm,2rm,12GG$2,2,1()ffKK$Example：HI(g)2g)(Ig)(H22HI(g)g)(Ig)(H221221（1）（2）is referred toO返回Distinguish the thermodynamic equilibrium constant fro

11、m pressure and composition equilibrium constants. Example：DEGHdeghBHGBdBDEghpeppKppp pK1. Pressure equilibrium constantwhen0BpK(Dimension is unity)O返回BHGBBDEghxdexxKxxxBBpKKpxxK2.mole fraction equilibrium constantA pg obeys Dolton partial pressure law，BBpxp O返回BHGBBDEghcdeccKcccBB)(RTKKpccK3Concentr

12、ation equilibrium constant pg，cRTpO返回BBBaKa ，BBBcac$BB()acrKKKc$aK 4Activity equilibrium constantO返回aAbBmrPPPPRTKG/ln)298(KRT lnaAbBPPPPK/)(abaAbBPPP)()(abPPK)()()()(abaAbBPRTCRTC)()(abaAbBRTPCC)()(abCRTPK)(/abaAbBRTCPCCCCO返回32CaCO (s)CaO(s)CO (g)2(CO )/pKpp$is dissociation pressure )CO(2p) s (CaCO3

13、Consider a reactionEquilibrium between ideal gas and pure condensed phases: If the substances taking part in the chemical equilibrium is of more than one phase, O返回S(g)H)g(NHHS(s)NH234example：S)H()NH(23ppp32(NH )(H S)ppppKp$214(/)p p$It is said earlier that chemical potentials of condensed phases ar

14、e insensitive to the pressure change, unless the pressure is very large.O返回G concentrates our attention on the system, and lets us avoid having to think explicitly about the entropy of the surroundings. In an isothermal change G can be related to the enthalpy and entropy changes of the system by BGG

15、mfBmrO返回rmlnaGRTK $rmexp(/)aKGRT$ Using rGm , calculate the thermodynamic equilibrium constant:O返回22rm(1) C(s)O (g)CO (g)(1)G$122rm2(2) CO(g)O (g)CO (g) (2)G$12rm2(3) C(s)O (g)CO(g)(3)G$rmrmrm(3)(1)(2)GGG $(1)(3)(2)pppKKK$(1) - (2) 得（3） 2. 计算实验不易测定的平衡常数例如，求 的平衡常数122C(s)O (g)CO(g)O返回3近似估计反应的可能性rmrmln

16、pGGRTQ $只能用 判断反应的方向。但是，当 的绝对值很大时，基本上决定了 的值，所以可以用来近似地估计反应的可能性。0,mrf)(wpTGrmG$mrGO返回fmG$（B,T）Define the Gibbs function of reaction and calculate it from tables of Gibbs function of formation.Calculate the equilibrium constants for reactions on the basis of the Gibbs function information given in Table

17、 X.X.O返回fmG$rmBfmB(B)GG$rmG$ At T(1)mrmrSTHdTCKHmpTKrmr,298)298()198(298,TKmprmrTdTCKSTO返回If the equilibrium constant is known at a temperature T*, its value at another temperature T can be found as follows. In each caseRTTGTKmr)()(lnRTTGTKmr)()(lnO返回Subtracting these relates the two equilibrium con

18、stants:RTTGRTTGTKTKmrmr)()()()(lnNow write G = H - T S, and make the approximation that neither H nor S changes significantly over the temperature range of interest:O返回RTSRTTHRTSRTTHTKTKmrmrmrmr)()()()()()(lnTTRTHmr11)()()(lnTKTKThe last equation is very remarkable because it predicts the shift in e

19、quilibrium when the temperature is changed; all we need to known is the standard molar enthalpy of reaction.O返回2)(THTTGmrpmrfmrKRTGlnfmrKRTGlnpfTKR)ln(2lnRTHdTKdmrfO返回The infinitesimal expression of vant Hoff relationrm2dlndpKHTRT$对吸热反应，升高温度，增加，对正反应有利。rm0H$pK$对放热反应，升高温度，降低，对正反应不利。pK$rm0H$O返回If we as

20、sure that does not depend on the temperature this equation integrates to rmH$2rm121()11ln()( )ppKTHRTTKT$CRTHKmrflnO返回当理想气体用浓度表示时，因为 ，可以得到pcRTrm2dlndcKUTRT$2rm121()11ln()( )ccKTURTTKT$这个公式在气体反应动力学中有用处。O返回Provides a way of measuring the enthalpy of a reaction without using a calorimeter, but in pract

21、ice it is inaccurate.Equilibrium compositions are measured over a range of temperatures (at constant pressure), and lnK is plotted against 1/T. It follows from eq.that the slope is -Hm/R.O返回 Le chatelier原理，增加压力，反应向体积减小的方向进行。这里可以用压力对平衡常数的影响从本质上对原理加以说明。BBB()ppKp $BppyBB)(Bppky)()ln(lnlnlnppkKBByTpK)ln

22、(Typk)ln(pB= 0O返回Typk)ln(pB00BaA=bB0abab O返回 对于分子数增加的反应，加入水气或氮气，会使反应物转化率提高，使产物的含量增加。)ln(lnlnlnppkKBByBBBByBBBnnykBnknpTnpTnknK,)ln()ln(0nBO返回 在一个反应体系中，如果同时发生几个反应，当到达平衡态时，这种情况称为同时平衡。 在处理同时平衡的问题时，要考虑每个物质的数量在各个反应中的变化，并在各个平衡方程式中同一物质的数量应保持一致。O返回例题：600 K时， 与 发生反应生成，继而又生成，同时存在两个平衡：3CH Cl(g)2H O(g)3CH OH32(

23、CH ) O32333 22(1) CH Cl(g)H O(g)CH OH(g)HCl(g)(2) 2CH OH(g)(CH ) O(g)H O(g)已知在该温度下， 。今以等量的 和开始，求 的平衡转化率。,1,20.00154,10.6ppKK$3CH Cl2H O3CH ClO返回解：设开始时 和 的量为1.0，到达平衡时，生成HCl的量为x， 生成 的量为y，则在平衡时各物的量为：3CH Cl2H O32(CH ) O323(1) CH Cl(g)H O(g)CH OH(g)HCl(g)112xyxxyx3322(2) 2CH OH(g)(CH ) O(g)H O(g) -2 1- x

24、yxyyO返回因为两个反应的 都等于零，所以pxKK$BB,1,22(2 )0.00154(1)(1)(1)10.6(2 )ppxy xKxxyyxyKxy $将两个方程联立，解得 。 的转化率为0.048或4.8 。0.048, 0.009xy3CH ClO返回 It can be seen quite often that certain reactions are not spontaneous i.e. G 0. But if the same reactions are coupled to other reactions, which are spontaneous, the or

25、iginal reactions become feasible. This can only be possible if the total free energy change I.e., sum of the free energies of the second and the original reaction is negative.O返回例如：在298.15 K时：-12242rm,1 (1) TiO (s)2Cl (g)TiCl (l)O (g) 161.94 kJ mol G$反应(1)、(2)耦合，使反应(3)得以顺利进行。-122rm,2(2) C(s)O (g)CO

26、(g) 394.38 kJ molG $-1rm2242,3 (3) TiO (s)C(s)2Cl (g)TiCl (l)CO (g) 232.44 kJ mol G 则$O返回The standard Gibbs free energy for the forward reaction is related to the equilibrium constant by the expression: The H can be obtained from the variation of lnK with temperature:The S is then calculable as O返回1

27、 的估算rm( )GT$r(298.15 K)T rrrmrmrmrmrmrrmrmr( )( )( )( )()d( )()dTpTTpTGTHTTSTHTHTCTCSTSTTT $当 不大，或不要作精确计算时，设 ，则：pC0pCrmrmrrmr( )()() GTHTTSTabT $这里实际上设焓和熵变化值与温度无关，从298.15 K的表值求出任意温度时的 值。rmG$O返回For an equilibrium between a liquid A and a gas A (the vaporization and condensation of A):)()(glAAAt P ,T;

28、lglgmrG)ln(PPRTAggplgPgg)ln(PPRTAO返回2估计反应的有利温度rmrmrm( )( )( )GTHTTST $ 通常焓变与熵变在化学反应中的符号是相同的。要使反应顺利进行，则 越小越好。rm( )GT$rmrm(1) ( )0, ( )0HTST$提高温度对反应有利。rmrm(2) ( )0, ( )0HTST$降低温度对反应有利。The endO返回JACOBUS HENRICUS VANT HOFF (1852-1911) Dutch physical chemist,received the first Nobel Prize in chemistry in

29、 1901 for “the discovery of the laws of chemical dynamics and of osmotic pressure.” Vant Hoff was one of the early developers of the laws of chemical kinetics,developing mehtods for determining the order of a reaction;he deduced the relation between temperature and the equilbrium constant of a chemi

30、cal reaction. O返回In 1874, vant Hoff (and also J.A. Le Bel, independently) proposed what must be considered one of the most important ideas in the history of chemistry, namely the tetrahedral carbon bond. Vant Hoff carried Pasteurs ideas on asymmetry to the molecular level , and asymmetry required bo

31、nds tetrahedrally distributed about a central carbon atom. Structural organic chemistry was born.O返回JOHN DALTON (1766-1844)English chemist, physicist, and meteorologist, is considered by many to be the “father of the atomic theory of matter,although grandfather is perhaps a more he suffered. In 1803

32、, he published his paper “Absorption of Gases by Water and Other Liquids,” in which he presented what is now known as Daltons law of partial pressures. O返回He was led to his theory of atomism by his studies of gases. In one of his papers published in 1805, he said, “Why does not water admit its bulk

33、of every kind of gas alike?The circumstance depends on the weight and number of the ultimate particles of the several gases.” John Dalton was led to the atom by reflecting that different gases had different values of the Henrys law constant.忿鏅獉酆奌壳貫蚇洉垢骜匧铬嘧靿螒翌堪椐姬椸卝炊沦浂鯰霄輢鏹箆挚熙丄庳榠浝鋌铉傜瞰啷疰棊澗搡竽粵耒坎馪鸞倬幰騡澈苹鎯蒪氙

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