Unit 2 ChemistryBalancing Complex Reduction and Oxidation Half Reactions Using KOHES

You can use the KOHES method to balance more complicated oxidation and reduction half equations involving compounds containing O and/or H, in acidic solutions.

The acronym KOHES stands for:Step 1 Key element (eg. Mn, V, Cl) (key element is NOT O or H)Step 2 Oxygen, balance by adding H2O Step 3 Hydrogen, balance by adding H+ ionsStep 4 Electrons, balance by adding e- to the more positive sideStep 5 States, add states (remember e- do NOT have states!)

Example

Permanganate ions MnO4- reduced to Mn2+ ions

Step 1 K MnO4- Mn2+ (Manganese Mn is the Key Element)

Step 2 O MnO4- Mn2+ + 4H2O (add 4H2O to RHS)

Step 3 H MnO4- + 8H+ Mn2+ + 4H2O (add 8 H+ to LHS)

Step 4 E MnO4- + 8H+ + 5e- Mn2+ + 4H2O (add 5e- to LHS)

Step 5 S MnO4-(aq) + 8H+

(aq) + 5e- Mn2+(aq) + 4H2O(l)

Problems:

Write ionic half equations for the following reduction half reactions

1. Dichromate ions Cr2O72- to Cr3+ ions

2. Dithionite ion S2O42- to thiosulfate ions S2O3

2- 3. Nitrate ions NO3

- to nitric oxide gas NO4. Sulfate ions SO4

2- to sulphur dioxide gas SO2

5. Hypochlorite ions OCl- to chloride ions Cl-

6. Oxygen gas to water7. Solid iodine I2 to iodide ions I-

Write ionic half equations for the following oxidation half reactions

1. Sulfite ions SO32- to sulphate ions SO4

2-

2. Hydrogen sulphide H2S to sulphur S3. Ethanol C2H5OH to ethanoic acid CH3COOH 4. Hydrogen peroxide H2O2 to O2

5. Carbon monoxide CO to carbon dioxide 6. Vitamin C (ascorbic acid) C6H8O6 to dehydroascorbic acid C6H6O6

7. Thiosulfate ion S2O32- to tetrathionate ion S4O6

2-

8. Oxalate ions (COO)22- to carbon dioxide

Challenge – balancing complex redox equations:

For each of the following, identify:

a) Each conjugate pair (half equation)b) The reductant, the oxidant and what has been oxidised and reduced. c) Then balance the equation using KOHES

1. Cl2(g) + S2O32-(aq) ® Cl-(aq) + SO42-(aq)

2. NO2– + MnO4

– ® NO3– + Mn2+

3. HNO3(aq) + H3AsO3(aq) ® NO(g) + H3AsO4(aq) + H2O(l)

4. Cr2O72−(aq) + HNO2(aq) ®  Cr3+(aq) + NO3

−(aq)

5. MnO4– + C2O4

2− ® Mn2+ + CO2–

6. Br2 + SO2 ® Br– + HSO4

–