Cân bằng các phản ứng oxi hóa – khử

a) NaBr + Cl₂ → NaCl + Br₂  

Na+1Br−1    +  Cl02  →  Na+1Cl−1  +  Br02

Br−1  →  Br0    +1eCl0    +1e  →  Cl−1  

1  × 1  × Br−1  →  Br0    +1eCl0    +1e  →  Cl−1 

 

⇒ Br−1  +  Cl0  →  Br0  +  Cl−1

2NaBr + Cl₂ → 2NaCl + Br₂

b) Fe₂O₃ + CO →  Fe + CO₂

Fe+32O−23  +  C+2O−2  →  Fe0  +  C+4O−22

Fe+3  +3e  →  Fe0C+2    →    C+4    +2e

2  × 3  × Fe+3  +3e  →  Fe0C+2    →    C+4    +2e

 

⇒ 2Fe+3  +  3C+2  →  2Fe0  +  3C+4

Fe₂O₃ + 3CO →  2Fe + 3CO₂

c) CO + I₂O₅ →  CO₂ + I₂

C+2O−2  +  I+52O−25  →  C+4O−22  +  I02

C+2   →   C+4  +2eI+5   +5e  →  I0

5  × 2  × C+2   →   C+4  +2eI+5   +5e  →  I0

 

⇒ 5C+2  +  2I+5  →  5C+4  +  2I0

5CO + I₂O₅ →  5CO₂ + I₂

d) Cr(OH)₃ + Br₂ + OH^- →  CrO₄^2- + Br^- + H₂O

Br0  +1e  →  Br−CrOH3+ 5OH−→   CrO42−+ 4H2O  +3e

3 × 1  ×Br0  +1e  →  Br−CrOH3+ 5OH−→   CrO42−+ 4H2O  +3e

 

⇒ CrOH3+ 3Br+ 5OH−→ CrO42−+ 3Br−+ 4H2O

2Cr(OH)₃ + 3Br₂ + 10OH^-   →  2CrO₄^2- + 6Br^- + 8H₂O

e) H⁺ + MnO­₄^- + HCOOH →  Mn²⁺ + H₂O + CO₂

H++ MnO4−+ H+1C+2O−2O−2H+1  → Mn2++ H2O + C+4O−22

H+1C+2O−2O−2H+1  →  C+4O−22  +2H+ +2e8H++ MnO4− +  5e→   Mn2++ 4H2O

5  × 2 × H+1C+2O−2O−2H+1  →  C+4O−22  +2H+ +2e8H++ MnO4− +  5e→   Mn2++ 4H2O

 

⇒ 16H++ 2MnO4− +  5H+1C+2O−2O−2H+1    →   2Mn2++ 8H2O  +  5 C+4O−22  +10H+

6H⁺ + 2MnO­₄^- + 5HCOOH →  2Mn²⁺ + 8H₂O + 5CO₂