A-level化學兩種氧化還原反應的配平方法！

氧化還原反應配平難度較大，普遍失分較多，現將常用的兩(liang) 種配平方法總結如下，供同學們(men) 參考：

一、半反應配平法

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、氧化數配平法

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

以上為(wei) 兩(liang) 種常見且效率很高的氧化還原配平方法，第一種適用於(yu) 元素化合價(jia) 不好確定的情形或是離子反應中，第二種方法較為(wei) 常規，望同學們(men) 多加練習(xi) ，熟能生巧。