A-level化学两种氧化还原反应的配平方法！

氧化还原反应配平难度较大，普遍失分较多，现将常用的两种配平方法总结如下，供同学们参考：

一、半反应配平法

1、Balance Redox Equations Under Acidic Conditions

①Construct unbalanceed oxidation and reduction half -equations

②Balance all elements except H and O

③Balance O atoms by adding H₂O

④Balance H atoms by adding H⁺

⑤Balance charges by adding electrons to either LHS or RHS of the half-equations

⑥Balance electron transfer by multiplying appropriate integers

⑦Add the resulting half-equations and eliminate any common terms

⑧Check that there is mass and charge balance,with no electrons left in equation

2、Balance Redox Equations Under Alkaline Conditions after step 7

①Neutralisse the H⁺ ions by adding the required number of OH^- ions to both sides of half-equation and combine H+ions and OH^- ions to form water

②Re-balance the half-equation by eliminating any common terms

For example:

9701_w21_qp119

Balance the equation:Cr₂O₇^2-+Zn+H⁺→Cr³⁺+Zn²⁺+H₂O

①Cr₂O₇^2-+Zn+H⁺→Cr³⁺+Zn²⁺+H₂O

②Cr₂O₇^2-→2Cr³⁺

③Cr₂O₇^2-→2Cr³⁺+7H₂O

④14H⁺+Cr₂O₇^2-→2Cr³⁺+7H₂O

⑤6e^-+14H⁺+Cr₂O₇^2-→2Cr³⁺+7H₂O  Zn→Zn²⁺+2e^-

⑥3Zn→3Zn²⁺+6e^-

⑦=⑤+⑥ 3Zn+14H⁺+Cr₂O₇^2-→3Zn²⁺+2Cr³⁺+7H₂O

⑧Check the mass and charges of the overall equation.

二、氧化数配平法

①Find the elements whose valence has changed and mark the valence value

②Mark the directions and number of electrons transferred

③Find the least common multiple to determine the coefficients

④Check that there is mass and charge balance

For example:

9701_w21_qp119

Balance the equation:Cr₂O₇^2-+Zn+H⁺→Cr³⁺+Zn²⁺+H₂O

①Reactant Cr→+6;Product  Cr→+3;we know 1mol Cr₂O₇^2- gains 6mol e^-

Reactant Zn→0;Product  Zn→+2;we know 1mol Zn loses 2mol e^-

②Omitting

③6×1=6;2×3=6; 3 is the coefficient of Zn;1 is the coefficient of Cr₂O₇^2-

1Cr₂O₇^2-+3Zn+H⁺→Cr³⁺+Zn²⁺+H₂O

1Cr₂O₇^2-+3Zn+H⁺→2Cr³⁺+3Zn²⁺+H₂O

Balance the Charge

1Cr₂O₇^2-+3Zn+14H⁺→2Cr³⁺+3Zn²⁺+H₂O

Balance the water

1Cr₂O₇^2-+3Zn+14H⁺→2Cr³⁺+3Zn²⁺+7H₂O

以上为两种常见且效率很高的氧化还原配平方法，第一种适用于元素化合价不好确定的情形或是离子反应中，第二种方法较为常规，望同学们多加练习，熟能生巧。