大学物理化学电化学省公共课一等奖全国赛课获奖课件.pptx

上一内容下一内容回主目录77.10.10 分解电压及极化作用分解电压及极化作用1.1.分解电压分解电压 :大气压力下两个铂电极电解大气压力下两个铂电极电解 l moldm3盐酸溶液，如图：盐酸溶液，如图：分解电压分解电压/10/10第1页上一内容下一内容回主目录77.10.10 分解电压及极化作用分解电压及极化作用电解池电极反应电解池电极反应:阴极阴极 2 H+2 e-H2(g)阳极阳极 2Cl-2 e-Cl2(g)总电解反应总电解反应 2 H+2Cl-H2(g)+Cl2(g)为何存在分解电压？为何存在分解电压？当当H2(g)、Cl2(g)压力压力等于大气压力时，气泡等于大气压力时，气泡才能逸出才能逸出/10/10第2页上一内容下一内容回主目录77.10.10 分解电压及极化作用分解电压及极化作用过程分析：在开始外加一定电压时，电极表面上产生了过程分析：在开始外加一定电压时，电极表面上产生了少许氢气和氯气，其压力很小，被吸附在少许氢气和氯气，其压力很小，被吸附在Pt电极上。电极上。因为氢气因为氢气H2(g)和氧气和氧气Cl2(g)可发生可发生氧化还原反应，当它们被吸附在氧化还原反应，当它们被吸附在惰性电极上时，就形成两个第一惰性电极上时，就形成两个第一类电极酸性氢电极和酸性氧电类电极酸性氢电极和酸性氧电极，组成原电池极，组成原电池Pt H2(g)HCl(0.1 moldm3)Cl2g)Pt 自发电自发电池，氢电极为负极，氯电极为正池，氢电极为负极，氯电极为正极；极；电池电动势恰好和电解时外电池电动势恰好和电解时外加电压相反，加电压相反，负极对负极、正极负极对负极、正极对正极，对正极，称为称为反电动势反电动势正极正极负极负极/10/10第3页上一内容下一内容回主目录77.10.10 分解电压及极化作用分解电压及极化作用原电池原电池 Pt H2(g)HCl(0.1 molkg1)Cl2(g)Pt这是一个自发电池，电池氢电极为负极这是一个自发电池，电池氢电极为负极(阳极阳极)氯电极氯电极为正极为正极(阳极阳极)；电池电极反应电池电极反应:阴极阴极 H2(g)-2 e-2 H阳极阳极 Cl2(g)+2 e-2Cl-电池反应电池反应 H2(g)+Cl2(g)2 H+2Cl-/10/10第4页上一内容下一内容回主目录77.10.10 分解电压及极化作用分解电压及极化作用原电池原电池 Pt H2(g)HCl(0.1 molkg1)Cl2(g)Pt 这是这是一个自发电池，电池氢电极为负极一个自发电池，电池氢电极为负极(阳极阳极)氯电极为正氯电极为正极极(阳极阳极)；外加电压小于分解电压时，形外加电压小于分解电压时，形成反电势与外加电压抵消；同成反电势与外加电压抵消；同时电解产物时电解产物H2(g)和和Cl2(g)会扩会扩散而损失，因而在电极上仍有散而损失，因而在电极上仍有微小电流经过，使得电解产物微小电流经过，使得电解产物得到补充，以维持一定压力；得到补充，以维持一定压力；从而使原电池电动势保持一定从而使原电池电动势保持一定大小，以抵消外加电动势大小，以抵消外加电动势/10/10第5页上一内容下一内容回主目录77.10.10 分解电压及极化作用分解电压及极化作用原电池原电池 Pt H2(g)HCl(0.1 moldm3)Cl2(g)Pt外加电压外加电压，PH2(g)和和 PCl2(g)，反电动势，反电动势PH2(g)和和 PCl2(g)等于外界气压等于外界气压而逸出，即气体压力达最大值，反而逸出，即气体压力达最大值，反电动势也达最大值，此时电动势也达最大值，此时外加电压外加电压等于分解电压。等于分解电压。/10/10第6页上一内容下一内容回主目录77.10.10 分解电压及极化作用分解电压及极化作用原电池原电池 Pt H2(g)HCl(0.1 moldm3)Cl2(g)Pt所以，理论所以，理论分解电压应等于分解电压应等于原电池最大可逆反电动势原电池最大可逆反电动势实际分解电压不等于理论分解电压实际分解电压不等于理论分解电压如如H2SO4、H3PO4和和NaOH等溶液分等溶液分解电压很相近，在解电压很相近，在1.70V左右左右因为电解这些溶液就是电解水，而因为电解这些溶液就是电解水，而电解水理论分解电压为电解水理论分解电压为1.229V。E(分解分解)E(理论理论)这是因为电极上存在极化作用，不再是可逆电极这是因为电极上存在极化作用，不再是可逆电极/10/10第7页上一内容下一内容回主目录2.2.电极极化电极极化 电流经过电极时，电极电势偏离平衡电极电势现象电流经过电极时，电极电势偏离平衡电极电势现象称为电极极化。称为电极极化。超电势超电势 =|EE平平|电极极化程度与经过电极电流密度相关；所以电极电电极极化程度与经过电极电流密度相关；所以电极电极电势也与电流密度相关。伴随电极上电流密度增加，极电势也与电流密度相关。伴随电极上电流密度增加，电极过程不可逆程度越来越大，电极电势对平衡电极电极过程不可逆程度越来越大，电极电势对平衡电极电势偏离也就越来越远电势偏离也就越来越远/10/10第8页上一内容下一内容回主目录2.2.电极极化电极极化(1)浓差极化浓差极化 以以Zn2+阴极还原为例阴极还原为例 在电流经过电极时，在电流经过电极时，Zn2+沉积到电极上，电极附近沉积到电极上，电极附近浓度降低，低于它在本体溶液中浓度。就好像是将电浓度降低，低于它在本体溶液中浓度。就好像是将电极插入了一个浓度较小极插入了一个浓度较小Zn+溶液中一样，从而使电极溶液中一样，从而使电极电势偏离平衡电极电势电势偏离平衡电极电势E E平平,搅拌可减小浓差极化。搅拌可减小浓差极化。浓差极化使阴极电极电势更负浓差极化使阴极电极电势更负(减小减小);阳极电极电势更正阳极电极电势更正(增大增大)/10/10第9页上一内容下一内容回主目录2.2.电极极化电极极化（2）电化学极化）电化学极化 当电流经过电极时，由电极反应速率限制，外电源当电流经过电极时，由电极反应速率限制，外电源供给电子供给电子Zn2+来不及消耗，电极上电子比平衡态时电来不及消耗，电极上电子比平衡态时电子数多，阴极表面上积累了多出电子。因为电子带负子数多，阴极表面上积累了多出电子。因为电子带负电荷，所以阴极表面上积累多出电子电荷，所以阴极表面上积累多出电子,使其电极电势就使其电极电势就更负更负 E E平平。在阳极上正电荷来不及被完全消耗，而在阳极上积累在阳极上正电荷来不及被完全消耗，而在阳极上积累多出正电荷，使得阳极电势更正多出正电荷，使得阳极电势更正 因为电化学反应本身迟缓性而引发极化称为电化学因为电化学反应本身迟缓性而引发极化称为电化学极化；使阴极电极电势更负极化；使阴极电极电势更负(减小减小);阳极电极电势阳极电极电势更正更正(增大增大)/10/10第10页上一内容下一内容回主目录2.2.电极极化电极极化/10/10第11页上一内容下一内容回主目录2.2.电极极化电极极化对于整个电池来说，极化作用结果是不一样对于整个电池来说，极化作用结果是不一样/10/10第12页上一内容下一内容回主目录2.2.电极极化电极极化/10/10第13页上一内容下一内容回主目录77.1.11 1 电解时电极反应电解时电极反应/10/10第14页上一内容下一内容回主目录77.1.11 1 电解时电极反应电解时电极反应电解时，在阳极、阴极都有各种反应能够发主情况下：电解时，在阳极、阴极都有各种反应能够发主情况下：阳极阳极上总是上总是极化电极电势最低极化电极电势最低氧化反应优先进行；氧化反应优先进行；阴极阴极上总是上总是极化电极电势最高极化电极电势最高还原反应优先进行。还原反应优先进行。EE阴阴,2E阴阴,1E阳阳,2E阳阳,1 阴阴=E阴阴,平平 E阴阴 故故 E阴阴=E阴阴,平平 阴阴 阳阳=E阳阳 E阳阳,平平 E阳阳=E阳阳,平平+阳阳/10/10第15页上一内容下一内容回主目录77.1.11 1 电解时电极反应电解时电极反应 阴极阴极上总是上总是极化电极电势最高极化电极电势最高还原反应优先进行。还原反应优先进行。/10/10第16页上一内容下一内容回主目录77.1.11 1 电解时电极反应电解时电极反应/10/10第17页上一内容下一内容回主目录NERNSTWALTHER NERNST(1864-1941),German physical chemist,did much of the early important work in electrochemistry,studying the thermodynamics of galvanic cells and the diffusion of ions in solution.Besides his scientific researches,he developed the Nernst lamp,which used a ceramic body.This lamp never achieved commercial importance since the tungsten lamp was developed soon afterwards./10/10第18页上一内容下一内容回主目录NERNSTHis electrical piano,which used radio amplifiers instead of a sounding board,was totally rejected by musicians.Nernst was the first to enunciate the third law of thermodynamics,and received the Nobel Prize in chemistry in 1920 for his thermochemical work./10/10第19页上一内容下一内容回主目录MICHAEL FARADAYMICHAEL FARADAY(1791-1867)English chemist and physicistwas a completely selftaught man.In 1812,while still a bookbinders apprentice,Faraday was drawn to chemistry by attending Davys lectures at the Royal Institute.His life was changed by an accident when Davy was temporarily blinded by an explosion and took on Faraday as his secretary./10/10第20页上一内容下一内容回主目录MICHAEL FARADAYFaraday presented Davy with the careful notes he had taken at his lectures,and Faraday became a laboratory assistant when his predecessor was fired for brawling.Faradays first experiment consisted in constructing a voltaic pile using copper halfpenny pieces and zinc discs separated by paper soaked in salt solution./10/10第21页上一内容下一内容回主目录MICHAEL FARADAYHe decomposed magnesium sulfate with the pile.He produced the first known chlorides of carbon,C2Cl6 and C2Cl4,in 1820,and discovered benzene in 1825.He investigated alloy steels and optical glass.During this latter work,he discovered the rotation of the plane of polarization of light in a magnetic field.He discovered diamagnetism and coined the words paramagnetic and diamagnetic./10/10第22页上一内容下一内容回主目录FRIEDRICH WILHELM GEORG KOHLARUSCHFRIEDRICH WILHELM GEORG KOHLARUSCHFRIEDRICH WILHELM GEORG KOHLARUSCH(1840-1910),German chemist and physicist,is best known for his work on the electrical conductivity of solutions.His work is characterized by a high degree of precision,as exemplified in his determination of the electrochemical equivalent of silver.His main work on electrolyte conduction was mad possible by the realization that polarization at the electrodes could be eliminated by using ac instead of dc currents for conductivity measurements./10/10第23页上一内容下一内容回主目录FRIEDRICH WILHELM GEORG KOHLARUSCHFRIEDRICH WILHELM GEORG KOHLARUSCHIn 1876,following the work of Hittorf on ion migrations,he stated,“in a dilute solution every electrochemical element has a perfectly definite resistance pertaining to it,independent of the compound to publish an instructional manual on laboratory physics.The manual,Leitfaden der Praktischen Physik(1870),was widely used and translated into several languages,including English./10/10第24页上一内容下一内容回主目录GILBERT NEWTON LEWISGILBERT NEWTON LEWIS(1875-1946),American chemist,began his career as a superintendent of weights and measures in the Philippines in 1904,after receiving the PhD degree from Harvard.His book Thermodynamics and the Free Energy of Chemical Substances,first published in 1923 in collaboration with M.Randall,is still in use in a new edition revised by K.S.Pitzer and L.Brewer./10/10第25页上一内容下一内容回主目录GILBERT NEWTON LEWISIn1916,Lewis observed that of the hundreds of thousands of known chemical compounds,less than ten contained an odd number of electrons,and he proposed the“electron pair”chemical bond./10/10第26页上一内容下一内容回主目录JACOBUS HENRICUS VANT HOFFJACOBUS HENRICUS VANT HOFF(1852-1911)Dutch physical chemist,received the first Nobel Prize in chemistry in 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 chemical reaction./10/10第27页上一内容下一内容回主目录JACOBUS HENRICUS VANT HOFFIn 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 bonds tetrahedrally distributed about a central carbon atom.Structural organic chemistry was born./10/10第28页上一内容下一内容回主目录PETER JOSEPH WILLIAM DEBYEPETER JOSEPH WILLIAM DEBYE(1884-1966)Dutch-born physical chemist,made extraordinary contributions to physical chemistry in various subject areas.He took his first degree in electrical engineering and received the Ph.D.degree in physics under Arnold Sommerfeld in Munich.At the age of 27,he succeeded Einstein as professor of theoretical physics at the University of Zurich./10/10第29页上一内容下一内容回主目录PETER JOSEPH WILLIAM DEBYEIn a five-year period starting in 1911,Debye produced three important results his theory of specific heats,his theory of permanent molecular dipole moments,and his theory of anomalous dielectric dispersion.In collaboration with Paul Scherrer,he developed the powder method of X-ray crystallography,an important tool for determining the structure of crystals./10/10第30页上一内容下一内容回主目录PETER JOSEPH WILLIAM DEBYEFour of these five contributions are discussed in various sections of this book.After the outbreak of World War II,refusing to become a German citizen,he came to the United States.Debye received the Nobel Prize in chemistry in 1936“for his contributions to the study of molecular structure through his investigations on dipote moments and on the diffraction of X rays and electrons in gases.”/10/10第31页