(4)氧化还原反应方程式的配平 用氧化数升降的方法来配平,分三个步骤 1)根据实验现象确定生成物并注意反应条件; 2)确定有关元素氧化数的变化; 3)按氧化和还原同时发生,电子得失数目必须相等的原则进行配平。 水溶液中的反应根据实际情况用H、OH、H2O等配平H和O元 素。若写离子方程式时,还要注意电荷的配平
(4) 氧化还原反应方程式的配平 氧化还原反应方程式的配平 用氧化数升降的方法来配平，分三个步骤： 用氧化数升降的方法来配平，分三个步骤： 1) 根据实验现象确定生成物并注意反应条件； 根据实验现象确定生成物并注意反应条件； 2) 确定有关元素氧化数的变化； 确定有关元素氧化数的变化； 3) 按氧化和还原同时发生，电子得失数目必须相等的原则进行配平。 按氧化和还原同时发生，电子得失数目必须相等的原则进行配平。 水溶液中的反应根据实际情况用 水溶液中的反应根据实际情况用H+、OH−、H2O等配平H和O元 素。若写离子方程式时，还要注意电荷的配平。 素。若写离子方程式时，还要注意电荷的配平
例题】 The oxidation of fe2toFe?bycr2O2 in acid solution? 1. Write the unbalanced equation for the reaction in ionic form Fe2+ Cr,0-2 Fe3++ Cr3+ 2. Separate the equation into two half-reactions Oxidation: Fe2+-+Fe3+ Reduction: Cr202--+ Cr3+ 3, Balance the atoms other than o and h in each half-reaction Cr2O2→+2C
1. Write the unbalanced equation for the reaction in ionic form. Write the unbalanced equation for the reaction in ionic form. 【例题】The oxidation of Fe The oxidation of Fe2+ to Fe3+ by Cr2O72− in acid solution? in acid solution? Fe2+ + Cr2O72− Fe3+ + Cr3+ 2. Separate the equation into two half Separate the equation into two half-reactions. reactions. Oxidation: Oxidation: Cr2O72− Cr3+ +6 +3 Reduction: Reduction: Fe2+ Fe3+ +2 +3 3. Balance the atoms other than O and H in each half Balance the atoms other than O and H in each half-reaction. reaction. Cr2O72− 2Cr3+
4. For reactions in acid, add H2o to balance O atoms and H* to balance h atoms Cr2072-- 2Cr3++ 7H20 14H++ Cro →2cr3++7H2o 5. Add electrons to one side of each half-reaction to balance the charges on the half-reaction Fe Fe3++ 1e 6e-+14H+Cr2o2-2Cr3+7H2O 6. If necessary, equalize the number of electrons in the two half reactions by multiplying the half-reactions by appropriate coefficients 6Fe 6Fe3++6e 6e-+14H+Cr2O2-→2cr3++7H2o
4. For reactions in acid, add H For reactions in acid, add H2O to balance O atoms and H O to balance O atoms and H+ to balance H atoms. balance H atoms. Cr2O72− 2Cr3+ + 7H2O 14H+ + Cr2O72− 2Cr3+ + 7H2O 5. Add electrons to one side of each half Add electrons to one side of each half-reaction to balance the reaction to balance the charges on the half charges on the half-reaction. reaction. Fe2+ Fe3+ + 1e− 6e− + 14H+ + Cr2O72− 2Cr3+ + 7H2O 6. If necessary, equalize the number of electrons in the two half If necessary, equalize the number of electrons in the two halfreactions by multiplying the half reactions by multiplying the half-reactions by appropriate reactions by appropriate coefficients. coefficients. 6Fe2+ 6Fe3+ + 6e− 6e− + 14H+ + Cr2O72− 2Cr3+ + 7H2O
7. Add the two half-reactions together and balance the final equation by inspection. The number of electrons on both sides must cancel Oxidation: 6Fe2—→6Fe3++Ee Reduction: Se+ 14H*+ Cr202--2Cr3*+7H20 14H*+ Cr2 0, 2-+ 6Fe+-+6Fe3++ 2Cr++ 7H2O 8. Verify that the number of atoms and the charges are balanced 14×1-2+6×2=24=6×3+2×3 9. For reactions in basic solutions add oh- to both sides of the equation for every H* that appears in the final equation
7. Add the two half Add the two half-reactions together and balance the final reactions together and balance the final equation by inspection. The number of electrons on both equation by inspection. The number of electrons on both sides must cancel. sides must cancel. 6e− + 14H+ + Cr2O72− 2Cr3+ + 7H2O 6Fe2+ 6Fe3+ + 6e Oxidation: Oxidation: − Reduction: Reduction: 14H+ + Cr2O72− + 6Fe2+ 6Fe3+ + 2Cr3+ + 7H2O 8. Verify that the number of atoms and the charges are balanced. Verify that the number of atoms and the charges are balanced. 14 × 1 – 2 + 6 × 2 = 24 = 6 × 3 + 2 × 3 9. For reactions in basic solutions, add OH For reactions in basic solutions, add OH− to both sides of the to both sides of the equation for every H equation for every H+ that appears in the final equation. that appears in the final equation
72电池的电动势和电极电势 1)从氧化还原反应到化学电池 在溶液中发生的普通氧 Zn Cu2+ cu +zn 化还原反应不能产生定向移 Electron flow Salt bridge 动的电流,但可以通过适当Aod Circuit Cathode 的设计,使电流能够定向移 动,这种装置称为原电池或 Zn Cu 伽伐伲电池。 右图为锌铜电池(也叫 daniel电池,早期曾是普遍 实用的化学电源)的基本构成。 ZnSO溶液」CuS溶液 Zn→→Zn2++2eCu2++2 Cu
7.2 电池的电动势和电极电势 电池的电动势和电极电势 (1) 从氧化还原反应到化学电池 从氧化还原反应到化学电池 Zn Zn Zn Zn2+ + 2e− Cu2+ + 2e− Cu Zn + Cu Zn + Cu2+ ＝ Cu + Zn Cu + Zn2+ 在溶液中发生的普通氧 在溶液中发生的普通氧 化还原反应不能产生定向移 化还原反应不能产生定向移 动的电流，但可以通过适当 动的电流，但可以通过适当 的设计，使电流能够定向移 的设计，使电流能够定向移 动，这种装置称为原电池或 动，这种装置称为原电池或 伽伐伲电池。 右图为锌-铜电池(也叫 Daniel电池，早期曾是普遍 电池，早期曾是普遍 实用的化学电源 实用的化学电源)的基本构成。 Zn Cu ZnSO4溶液 CuSO4溶液