DOC《第五章 沉淀反应》

第五章 沉淀反应Chapter

5 The Precipitation Reactions 中学的化学知识中,常常用沉淀反应来鉴别一些金属离子或酸根离子,这就涉及到一些难溶电解质的沉淀和溶解问题.

在含有固体的难溶电解质的饱和溶液中,存在着固体难溶电解质与溶液中相应各离子间的多相平衡. §5-1 溶度积 Solubility Product

一、沉淀和溶解平衡(The Equilibrium of Precipitation and Dissolution) 1.在一定温度和一定量水中,AgCl(s)Ag+(aq) + Cl-(aq) 当v沉淀 = v溶解 时,沉淀和溶解达到平衡,称为异相平衡(heterogeneous equilibrium). 2.平衡表达式: ∴ [Ag+][Cl-] = K ・ [AgCl],把K ・ [AgCl]记作Ksp,Ksp称为溶度积常数. 3.通式:(s) , 4.注意点: (1) Ksp与温度有关,但影响不大.在实际中,常用25℃时的Ksp;

(2) 在上述表达式中,浓度必须用体积物质的量浓度;

(3) 在Ksp表达式中,应该用离子活度代替离子浓度.但由于在难溶电解质的溶液中,离子浓度很小,离子的活度系数γ = 1,所以可以用离子浓度代替离子活度.

二、溶度积与溶解度的关系(The Relationship of Solubility Product and Solubility) 1.溶解度so (mol・dm(3)与Ksp的换算 (1) 由于两者都表示某一物质成为饱和溶液时所含溶质的量相同,即都是表示物质的溶解能力,故它们之间可换算. (2) 换算关键:由于难溶电解质的溶解度很小,所以虽然是饱和溶液,但此饱和溶液很稀,则,所以,也可以把摩尔溶解度换算成g /

100 H2O. (3) so与Ksp的关系: AgCl(s)Ag+(aq) + Cl-(aq)Mg(OH)2(s)Mg2+(aq) + 2OH-(aq)

2 Ksp Ksp = ・ (2)2 = 一般式: Ksp = (n) n ・ (m) m = nn ・ mm ・ ()m + n Sample Exercise 1:Solid silver chromate is added to pure water at

25 ℃ , some of the solid remains undissolved at the bottom of the flask.The mixture is stirred for several days to ensure that equilibrium is achieved between the undissolved Ag2CrO4(s) and solution. Analysis of the equilibrated solution shows that its silver-ion concentration is 1.3(10(4 mol・dm(3. Calculate Ksp for Ag2CrO4. Solution:∵ mol・dm(3 ∴ mol・dm(3 = Sample Exercise 2:已知25℃时,AgCl的溶解度为1.92(10(3 g・dm(3,试求该温度下AgCl的溶度积. Solution:mol・dm(3mol・dm(3 = 2.同一类型的难溶电解质,Ksp大的,其溶解度也大;

不同类型的难溶电解质,溶解度的大小不能用Ksp作简单比较,只能通过计算so来说明.从上面的AgCl与Ag2CrO4的Ksp与S的例子可以充分说明这一点. 3.难溶电解质的简单水合离子的浓度与其摩尔溶解度往往不是等同的.这是由于除了水溶液中的多相平衡之外,经常还存在着一些其它的重要因素:如水解、配位、同离子 效应等.例如Ag3PO4的溶解度为,若不考虑Ag+离子水解,则[Ag+] = 3;

若考虑离子水解,则,而是

三、溶度积规则(The Solubility Product Rule) 1.对于来 说,在溶液中: (1) 当[Am+]n ・ [Bn(]m = Ksp,沉淀与溶解达到平衡;

(2) 当[Am+]n ・ [Bn(]m > Ksp,产生大量沉淀;

(3) 当[Am+]n ・ [Bn(]m < Ksp,不产生沉淀. 2.在什么情况下,认为某离子已沉淀完全:对于定性分析,指溶液中某离子浓度小到10(5mol・dm(3,可以认为沉淀完全;

在定量分析中,指溶液中某离子浓度小到10(6 mol・dm(3,即可认为沉淀完全. Sample Exercise:What concentration of OH-must be exceeded in a 0.010 mol・dm(3 solution of Ni(NO3)2 in order to precipitate Ni(OH)2 ? Ksp = 1.6(10(14 for Ni(OH)2. Solution:For a saturated solution we have Ksp = [Ni2+] ・ [OH-]2 Thus if [Ni2+] ・ [OH-]2 >1.6(10(14,precipitation will occur. Letting [OH-] = x mol・dm(3, (0.010) ・x2 = 1.6(10(14 x = 1.3(10(6 mol・dm(3 Thus Ni(OH)2 will precipitate when concentration of OH- in solution is 1.3(10(6 mol・dm(3or higher. §5-2 沉淀―溶解平衡的移动 Equilibrium Shift between Precipitation and Solution