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[交流] Oxidation-Reduction Reactions 已有1人參與

Oxidation-Reduction Reactions
Oxidation is the removal of electrons from, and reduction is the addition of electrons to an atom. Consider the galvanizing of iron, that is coating from with zinc to prevent rusting. The two competing reactions are:
Fe2++ 2e==Fe    E°=-0.44volts Zn2++ 2e=Zn    E°=-0.76volts Since the zinc reaction in the forward reaction produces a larger negative potential, that is liberates more energy, the spontaneous reaction is
Zn+ Fe2+==Fe+Zn2+
氧化反應(yīng)是原子失去電子，還原反應(yīng)是原子得到電子。就鋅鍍鐵而言，就是一層鋅的涂層防止鐵生銹。兩個(gè)競(jìng)爭(zhēng)的反應(yīng)是
Fe2++ 2e==Fe    E°=-0.44volts Zn2++ 2e=Zn    E°=-0.76volts 由于鋅的正向反應(yīng)產(chǎn)生較大的負(fù)電勢(shì)，釋放更多的能量，這個(gè)自發(fā)反應(yīng)是
Zn+ Fe2+==Fe+Zn2+
The coating of zinc serves two purposes: firstly it covers the iron and prevents its oxidation  (rather like a coat of paint) and secondly it provides anodic protection. If the galvanized steel is scratched, allowing the air to oxidize some iron, the Fe2+ produced is immediately reduced to iron by the zinc, and rusting does not occur. Similar applications in which one metal is sacrificed to protect another are the attaching of sacrificial blocks of magnesium to underground steel pipelines and the hulls of ships to prevent the rusting of iron.
表面鍍鋅有兩個(gè)目的：一是覆蓋在鐵表面可以保護(hù)鐵不被氧氣，二是其提供陽(yáng)極保護(hù)。如果鍍鋅鋼鐵被劃破，讓空氣氧化一些鐵，F(xiàn)e2+在鋅下還原成鐵，以而生銹不會(huì)發(fā)生。犧牲一種金屬以保護(hù)另一種金屬的類似應(yīng)用是把一塊作犧牲用的鎂錠同下部的鋼管和船體聯(lián)接起來以防止鐵的生銹。 Reduction Potential Diagrams
The reactions and stability of the various oxidation states of an element can be shown by the appropriate half reactions and reduction potentials. Consider iron:
Fe2++ 2e = Fe                   E°=-0.44volts
Fe3++ 3e=Fe                   E°=-0.04 Fe3++ e = Fe2+                   E°=+0.77
FeO2-4 +3e+8H+ = Fe3+ + 4H2O    E°=+2.20 This information is consolidted into a single diagram, in which the highest oxidation state is written at the left, and the lowest state at the right。
還原電勢(shì)表
一種元素大量氧化態(tài)的反應(yīng)性和穩(wěn)定性可以從適當(dāng)?shù)囊话敕磻?yīng)和電勢(shì)的還原反應(yīng)表現(xiàn)出來，觀察鐵：
Fe2++ 2e = Fe                   E°=-0.44volts
Fe3++ 3e=Fe                   E°=-0.04 Fe3++ e = Fe2+                   E°=+0.77 FeO2-4 +3e+8H+ = Fe3+ + 4H2O    E°=+2.20
這些信息整理成一個(gè)單獨(dú)圖解，把最高氧化態(tài)寫在左邊，而把最低氧化態(tài)寫在右邊。
From the potentials it is apparent that reduction of FeO2-4 to Fe3+ releases a lot of energy, so FeO2-4 is a strong oxidizing agent. Similarly, Fe3+ is a weaker oxidizing agent going to Fe2+, but neither Fe3+nor Fe2+has any tendency to reduce to Fe.
Standard electrode potentials are measured on a scale with
H++e = H    E°=0.0volts
Since hydrogen is normally regarded as a reducing agent, reactions with negative values for E° are more strongly reducing than hydrogen, that is they are strongly reducing. Materials which  are generally accepted as oxidizing agents have E°values above +0.8 volts, those such as Fe3+→ Fe2+ of about 0.8 volts are stable (equally oxidizing and reducing), and those below +0.8 volts becoming increasingly reducing.
從電勢(shì)可以明顯看出FeO2-4 到Fe3+的反應(yīng)釋放出大量的能量，所以FeO2-4是強(qiáng)氧化劑。相似的，F(xiàn)e3+與Fe2+相比是較弱的氧化劑，但是不管是Fe3+還是Fe2+都沒有任何還原成Fe的趨勢(shì)。標(biāo)準(zhǔn)電極電勢(shì)是以（如下）標(biāo)度（為準(zhǔn)）來測(cè)量的。
H++e = H    E°=0.0volts
由于氫通常是被當(dāng)作還原劑，具有負(fù)電勢(shì)的反應(yīng)，它比H還原性強(qiáng)，從而說明它們是強(qiáng)還原性。元素材料的電勢(shì)超過0.8v通常被作為氧化劑所接受，那些像Fe3+    Fe2+有約0.8v的是穩(wěn)定的，而那些低于0.8v的則有逐漸增強(qiáng)還原性。
At first sight the potential of -0.04 V for Fe3+-Fe seems wrong since the potentials for Fe3+-Fe2+, and Fe2+-Fe are 0.77 V and -0.44V respectively. Potentials are not thermodynamic functions, and may not be added, but the potential may be "calculated from the free energy G, using the equation △G= —nFE0 where n is the number of electrons involved and F the Faraday.
起先看到的 Fe3+-Fe 的電勢(shì)是-0.04v時(shí)是錯(cuò)誤的，因?yàn)镕e3+-Fe2+, and Fe2+-Fe分別是0.77 V and -0.44V時(shí)，電勢(shì)不是熱力學(xué)函數(shù)，所以不能相加，但是電勢(shì)可能以自由能G計(jì)算出來，使用公式△G= —nFE0，這里n是相關(guān)的電子數(shù)，F(xiàn)是法垃第（常數(shù)）。
The reduction potential diagram for copper in acid solution is oxidation state. The potential, and hence the energy released when Cu2+is reduced to Cu+are both very small, hence Cu2+is stable. On moving from left to right the potentials Cu'+ - Cu+ - Cu become more positive. Whenever this is found, the species in the middle (Cu+ in this case) disproportionates, that is it behaves as both a self-oxidizing and a self-reducing agent because it is energetically favourable for the following two changes to occur together.Thus Cu+ disproportionates in solution, and is only found in the solid state. 酸溶液中銅的還原銅是氧化態(tài)。Cu2+被還原成Cu+時(shí)的電勢(shì)及后來釋放的能量都是很少的，因此Cu2+是穩(wěn)定的。Cu'+ - Cu+ - Cu以左側(cè)到右側(cè)電勢(shì)變得更正：當(dāng)存在這種情況時(shí)，處于中間的物質(zhì)便會(huì)發(fā)生歧化反應(yīng)，即它起自身氧化劑和自身還原劑的作用。因?yàn)橄铝袃煞N變化同時(shí)在能量上是有利的。因而Cu 能在溶液中歧化，(故)它只能在固態(tài)中找到。

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