電化學(xué)極化原電池電解池

1、7.10 分解電壓及極化作用1.分解電壓 :大氣壓力下兩個鉑電極電解 l moldm3鹽酸溶液，如圖： 分解電壓2022/8/87.10 分解電壓及極化作用電解池電極反應(yīng): 陰極 2 H+ + 2 e- H2(g) 陽極 2Cl- - 2 e- Cl2 (g) 總的電解反應(yīng) 2 H+ + 2Cl- H2(g) + Cl2 (g) 為什么存在分解電壓？當(dāng)H2 (g)、Cl2 (g)的壓力等于大氣壓力時，氣泡才能逸出 2022/8/87.10 分解電壓及極化作用過程分析：在開始外加一定電壓時，電極表面上產(chǎn)生了少量的氫氣和氯氣，其壓力很小，被吸附在Pt電極上。因?yàn)闅錃釮2(g)和氧氣Cl2(g)可發(fā)

2、生氧化還原反應(yīng)，當(dāng)它們被吸附在惰性電極上時，就形成兩個第一類電極酸性氫電極和酸性氧電極，構(gòu)成原電池PtH2(g) HCl (0.1 moldm3) Cl2(g)Pt 自發(fā)電池，氫電極為負(fù)極，氯電極為正極；電池的電動勢正好和電解時外加電壓相反， 負(fù)極對負(fù)極、正極對正極，稱為反電動勢正極負(fù)極2022/8/87.10 分解電壓及極化作用原電池 PtH2(g)HCl(0.1 molkg1) Cl2(g)Pt這是一個自發(fā)電池，電池的氫電極為負(fù)極(陽極) 氯電極為正極(陽極)； 電池電極反應(yīng): 陰極 H2(g) - 2 e- 2 H+ 陽極 Cl2 (g) + 2 e- 2Cl- 電池反應(yīng) H2(g) +

3、 Cl2 (g) 2 H+ + 2Cl-2022/8/87.10 分解電壓及極化作用原電池 PtH2(g)HCl(0.1 molkg1) Cl2(g)Pt 這是一個自發(fā)電池，電池的氫電極為負(fù)極(陽極) 氯電極為正極(陽極)； 外加電壓小于分解電壓時，形成的反電勢與外加電壓抵消；同時電解產(chǎn)物H2(g)和Cl2(g)會擴(kuò)散而損失，因而在電極上仍有微小電流通過，使得電解產(chǎn)物得到補(bǔ)充，以維持一定的壓力；從而使原電池的電動勢保持一定的大小，以抵消外加電動勢 2022/8/87.10 分解電壓及極化作用原電池 PtH2(g)HCl(0.1 moldm3) Cl2(g)Pt外加電壓，PH2(g) 和 PCl

4、2(g) ，反電動勢PH2(g) 和 PCl2(g) 等于外界氣壓而逸出，即氣體壓力達(dá)最大值，反電動勢也達(dá)最大值，此時外加電壓等于分解電壓。2022/8/87.10 分解電壓及極化作用原電池 PtH2(g)HCl(0.1 moldm3) Cl2(g)Pt因此，理論分解電壓應(yīng)等于原電池的最大可逆反電動勢實(shí)際分解電壓不等于理論分解電壓如H2SO4 、H3PO4和NaOH等溶液的分解電壓很相近，在1.70V左右因?yàn)殡娊膺@些溶液就是電解水，而電解水的理論分解電壓為1.229V 。E (分解) E (理論) 這是因?yàn)殡姌O上存在極化作用，不再是可逆電極2022/8/82. 電極的極化 電流通過電極時，電極

5、電勢偏離平衡電極電勢的現(xiàn)象稱為電極的極化。 超電勢 = | EE平 | 電極的極化程度與通過電極的電流密度有關(guān)；因此電極的電極電勢也與電流密度有關(guān)。隨著電極上電流密度的增加，電極過程的不可逆程度越來越大，電極電勢對平衡電極電勢的偏離也就越來越遠(yuǎn) 2022/8/82. 電極的極化(1)濃差極化 以Zn2+的陰極還原為例 在電流通過電極時，Zn2+沉積到電極上，電極附近濃度降低，低于它在本體溶液中的濃度。就好像是將電極插入了一個濃度較小的Zn+ 溶液中一樣，從而使電極電勢偏離平衡電極電勢E E平 , 攪拌可減小濃差極化。 濃差極化使陰極的電極電勢更負(fù)(減小);陽極的電極電勢更正(增大) 2022/

6、8/82. 電極的極化（2）電化學(xué)極化 當(dāng)電流通過電極時，由電極反應(yīng)速率的限制，外電源供給的電子Zn2+來不及消耗，電極上的電子比平衡態(tài)時的電子數(shù)多，陰極表面上積累了多余的電子。由于電子帶負(fù)電荷，所以陰極表面上積累多余電子,使其電極電勢就更負(fù) E E平。在陽極上正電荷來不及被完全消耗，而在陽極上積累多余的正電荷，使得陽極的電勢更正 由于電化學(xué)反應(yīng)本身的遲緩性而引起的極化稱為電化學(xué)極化；使陰極的電極電勢更負(fù)(減小);陽極的電極電勢更正(增大) 2022/8/82. 電極的極化2022/8/82. 電極的極化對于整個電池來說，極化作用的結(jié)果是不一樣的 2022/8/82. 電極的極化2022/8/

7、87.11 電解時的電極反應(yīng)2022/8/87.11 電解時的電極反應(yīng)電解時，在陽極、陰極均有多種反應(yīng)可以發(fā)主的情況下： 陽極上總是極化電極電勢最低的氧化反應(yīng)優(yōu)先進(jìn)行； 陰極上總是極化電極電勢最高的還原反應(yīng)優(yōu)先進(jìn)行。EE陰,2E陰,1E陽,2E陽,1 陰 = E陰,平 E陰 故 E陰= E陰,平 陰 陽 = E陽 E陽,平 E陽 = E陽,平 + 陽 2022/8/87.11 電解時的電極反應(yīng) 陰極上總是極化電極電勢最高的還原反應(yīng)優(yōu)先進(jìn)行。2022/8/87.11 電解時的電極反應(yīng)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