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:Cr2O72-+Zn+H+→Cr3++Zn2++H2O

①Cr2O72-+Zn+H+→Cr3++Zn2++H2O

②Cr2O72-→2Cr3+

③Cr2O72-→2Cr3++7H2O

④14H++Cr2O72-→2Cr3++7H2O

⑤6e-+14H++Cr2O72-→2Cr3++7H2O  Zn→Zn2++2e-

⑥3Zn→3Zn2++6e-

⑦=⑤+⑥ 3Zn+14H++Cr2O72-→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:Cr2O72-+Zn+H+→Cr3++Zn2++H2O

①Reactant Cr→+6;Product  Cr→+3;we know 1mol Cr2O72- 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 Cr2O72-

1Cr2O72-+3Zn+H+→Cr3++Zn2++H2O

1Cr2O72-+3Zn+H+→2Cr3++3Zn2++H2O

Balance the Charge

1Cr2O72-+3Zn+14H+→2Cr3++3Zn2++H2O

Balance the water

1Cr2O72-+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。

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