电化学极化原电池电解池

1、7.10 分解电压及极化作用1.分解电压 :大气压力下两个铂电极电解 l moldm3盐酸溶液，如图： 分解电压2022/8/87.10 分解电压及极化作用电解池电极反应: 阴极 2 H+ + 2 e- H2(g) 阳极 2Cl- - 2 e- Cl2 (g) 总的电解反应 2 H+ + 2Cl- H2(g) + Cl2 (g) 为什么存在分解电压？当H2 (g)、Cl2 (g)的压力等于大气压力时，气泡才能逸出 2022/8/87.10 分解电压及极化作用过程分析：在开始外加一定电压时，电极表面上产生了少量的氢气和氯气，其压力很小，被吸附在Pt电极上。因为氢气H2(g)和氧气Cl2(g)可发

2、生氧化还原反应，当它们被吸附在惰性电极上时，就形成两个第一类电极酸性氢电极和酸性氧电极，构成原电池PtH2(g) HCl (0.1 moldm3) Cl2(g)Pt 自发电池，氢电极为负极，氯电极为正极；电池的电动势正好和电解时外加电压相反， 负极对负极、正极对正极，称为反电动势正极负极2022/8/87.10 分解电压及极化作用原电池 PtH2(g)HCl(0.1 molkg1) Cl2(g)Pt这是一个自发电池，电池的氢电极为负极(阳极) 氯电极为正极(阳极)； 电池电极反应: 阴极 H2(g) - 2 e- 2 H+ 阳极 Cl2 (g) + 2 e- 2Cl- 电池反应 H2(g) +

3、 Cl2 (g) 2 H+ + 2Cl-2022/8/87.10 分解电压及极化作用原电池 PtH2(g)HCl(0.1 molkg1) Cl2(g)Pt 这是一个自发电池，电池的氢电极为负极(阳极) 氯电极为正极(阳极)； 外加电压小于分解电压时，形成的反电势与外加电压抵消；同时电解产物H2(g)和Cl2(g)会扩散而损失，因而在电极上仍有微小电流通过，使得电解产物得到补充，以维持一定的压力；从而使原电池的电动势保持一定的大小，以抵消外加电动势 2022/8/87.10 分解电压及极化作用原电池 PtH2(g)HCl(0.1 moldm3) Cl2(g)Pt外加电压，PH2(g) 和 PCl

4、2(g) ，反电动势PH2(g) 和 PCl2(g) 等于外界气压而逸出，即气体压力达最大值，反电动势也达最大值，此时外加电压等于分解电压。2022/8/87.10 分解电压及极化作用原电池 PtH2(g)HCl(0.1 moldm3) Cl2(g)Pt因此，理论分解电压应等于原电池的最大可逆反电动势实际分解电压不等于理论分解电压如H2SO4 、H3PO4和NaOH等溶液的分解电压很相近，在1.70V左右因为电解这些溶液就是电解水，而电解水的理论分解电压为1.229V 。E (分解) E (理论) 这是因为电极上存在极化作用，不再是可逆电极2022/8/82. 电极的极化 电流通过电极时，电极

5、电势偏离平衡电极电势的现象称为电极的极化。 超电势 = | EE平 | 电极的极化程度与通过电极的电流密度有关；因此电极的电极电势也与电流密度有关。随着电极上电流密度的增加，电极过程的不可逆程度越来越大，电极电势对平衡电极电势的偏离也就越来越远 2022/8/82. 电极的极化(1)浓差极化 以Zn2+的阴极还原为例 在电流通过电极时，Zn2+沉积到电极上，电极附近浓度降低，低于它在本体溶液中的浓度。就好像是将电极插入了一个浓度较小的Zn+ 溶液中一样，从而使电极电势偏离平衡电极电势E E平 , 搅拌可减小浓差极化。 浓差极化使阴极的电极电势更负(减小);阳极的电极电势更正(增大) 2022/

6、8/82. 电极的极化（2）电化学极化 当电流通过电极时，由电极反应速率的限制，外电源供给的电子Zn2+来不及消耗，电极上的电子比平衡态时的电子数多，阴极表面上积累了多余的电子。由于电子带负电荷，所以阴极表面上积累多余电子,使其电极电势就更负 E E平。在阳极上正电荷来不及被完全消耗，而在阳极上积累多余的正电荷，使得阳极的电势更正 由于电化学反应本身的迟缓性而引起的极化称为电化学极化；使阴极的电极电势更负(减小);阳极的电极电势更正(增大) 2022/8/82. 电极的极化2022/8/82. 电极的极化对于整个电池来说，极化作用的结果是不一样的 2022/8/82. 电极的极化2022/8/

7、87.11 电解时的电极反应2022/8/87.11 电解时的电极反应电解时，在阳极、阴极均有多种反应可以发主的情况下： 阳极上总是极化电极电势最低的氧化反应优先进行； 阴极上总是极化电极电势最高的还原反应优先进行。EE阴,2E阴,1E阳,2E阳,1 阴 = E阴,平 E阴 故 E阴= E阴,平 阴 阳 = E阳 E阳,平 E阳 = E阳,平 + 阳 2022/8/87.11 电解时的电极反应 阴极上总是极化电极电势最高的还原反应优先进行。2022/8/87.11 电解时的电极反应2022/8/8NERNSTWALTHER NERNST (1864-1941), German physical

8、 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

9、tungsten lamp was developed soon afterwards. 2022/8/8NERNSTHis 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 the

10、rmochemical work.2022/8/8MICHAEL FARADAYMICHAEL FARADAY (1791-1867)English chemist and physicist, was 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 whe

11、n Davy was temporarily blinded by an explosion and took on Faraday as his secretary. 2022/8/8MICHAEL 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 con

12、sisted in constructing a voltaic pile using copper halfpenny pieces and zinc discs separated by paper soaked in salt solution. 2022/8/8MICHAEL FARADAYHe decomposed magnesium sulfate with the pile. He produced the first known chlorides of carbon, C2Cl6 and C2Cl4, in 1820, and discovered benzene in 18

13、25.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.2022/8/8FRIEDRICH WILHELM GEORG KOHLARUSCHFRIEDRICH WILHELM G

14、EORG 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 co

15、nduction was mad possible by the realization that polarization at the electrodes could be eliminated by using ac instead of dc currents for conductivity measurements.2022/8/8FRIEDRICH WILHELM GEORG KOHLARUSCHIn 1876,following the work of Hittorf on ion migrations, he stated, “in a dilute solution ev

16、ery 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.2022/8/

17、8GILBERT 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 i

18、n collaboration with M.Randall,is still in use in a new edition revised by K.S.Pitzer and L.Brewer. 2022/8/8GILBERT 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”

19、 chemical bond.2022/8/8JACOBUS 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 che

20、mical kinetics,developing mehtods for determining the order of a reaction;he deduced the relation between temperature and the equilbrium constant of a chemical reaction. 2022/8/8JACOBUS HENRICUS VANT HOFFIn 1874, vant Hoff (and also J.A. Le Bel, independently) proposed what must be considered one of

21、 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.2022/8/8PETER JO

22、SEPH 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. 2022/8/8PETER JOSEPH WILLIAM DEBYEIn a five-year period starting in 1911,Debye prod