ANSWERS

Unit 2 Chemistry

Balancing 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 1Key element (eg. Mn, V, Cl) (key element is NOT O or H)

Step 2Oxygen, balance by adding H2O

Step 3Hydrogen, balance by adding H+ ions

Step 4Electrons, balance by adding e- to the more positive side

Step 5States, add states (remember e- do NOT have states!)

Example

Permanganate ions MnO4- reduced to Mn2+ ions

Step 1 KMnO4-  Mn2+ (Manganese Mn is the Key Element)

Step 2OMnO4-  Mn2+ + 4H2O (add 4H2O to RHS)

Step 3HMnO4- + 8H+ Mn2+ + 4H2O (add 8 H+ to LHS)

Step 4EMnO4- + 8H+ + 5e-  Mn2+ + 4H2O (add 5e- to LHS)

Step 5SMnO4-(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 S2O32-
  3. Nitrate ions NO3- to nitric oxide gas NO
  4. Sulfate ions SO42- to sulphur dioxide gas SO2
  5. Hypochlorite ions OCl- to chloride ions Cl-
  6. Oxygen gas to water
  7. Solid iodine I2to iodide ions I-

Write ionic half equations for the following oxidation half reactions

  1. Sulfite ions SO32- to sulphate ions SO42-
  2. Hydrogen sulphide H2S to sulphur S
  3. 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- totetrathionate ion S4O62-
  8. Oxalate ions (COO)22-to carbon dioxide

Answers:

Write ionic half equations for the following reduction half reactions

  1. Dichromate ions Cr2O72- to Cr3+ ions

Step 1 KCr2O72-  2Cr3+ (Chromium Cr is the Key Element)

Step 2OCr2O72-  2Cr3+ + 7H2O (add 7H2O to RHS)

Step 3HCr2O72- + 14H+ 2Cr3+ + 7H2O (add 14 H+ to LHS)

Step 4ECr2O72- + 14H+ + 6e-  2Cr3+ + 7H2O (add 6e- to LHS)

Step 5SCr2O72-(aq) + 14H+(aq) + 6e-  2Cr3+(aq) + 7H2O(l)

  1. Dithionite ion S2O42- to thiosulfate ions S2O32-

Step 1 KS2O42- S2O32- (Sulfur S is the Key Element)

Step 2OS2O42- S2O32-+ H2O (add 1H2O to RHS)

Step 3HS2O42-+ 2H+ S2O32-+ H2O (add 2H+ to LHS)

Step 4ES2O42-+ 2H+ + 2e- S2O32-+ H2O (add 2e- to LHS)

Step 5SS2O42-(aq) + 2H+(aq) + 2e- S2O32-(aq) + H2O(l)

  1. Nitrate ions NO3- to nitric oxide gas NO

Step 1 KNO3- NO (Nitrogen N is the Key Element)

Step 2ONO3- NO + 2H2O (add 2H2O to RHS)

Step 3HNO3-+ 4H+ NO + 2H2O (add 4H+ to LHS)

Step 4ENO3-+ 4H+ + 3e-  NO + 2H2O (add 3e- to LHS)

Step 5SNO3-(aq)+ 4H+(aq) + 3e-  NO(g) + 2H2O(l)

  1. Sulfate ions SO42- to sulphur dioxide gas SO2

Step 1 KSO42- SO2 (Sulfur S is the Key Element)

Step 2OSO42- SO2+ 2H2O (add 2H2O to RHS)

Step 3HSO42-+ 4H+ SO2+ 2H2O (add 4H+ to LHS)

Step 4ESO42-+ 4H++ 2e- SO2+ 2H2O (add 2e- to LHS)

Step 5SSO42-(aq)+ 4H+(aq)+ 2e- SO2(g)+ 2H2O (l)

  1. Hypochlorite ions OCl- to chloride ions Cl-

Step 1 KOCl- Cl- (Chlorine Cl is the Key Element)

Step 2OOCl- Cl- + H2O(add 1H2O to RHS)

Step 3HOCl- + 2H+ Cl- + H2O (add 2H+ to LHS)

Step 4EOCl- + 2H+ + 2e-  Cl- + H2O (add 2e- to LHS)

Step 5SOCl-(aq)+ 2H+(aq) + 2e-  Cl-(aq)+ H2O(l)

6. Oxygen gas O2 to water H2O

Step 1 KO2 H2O (No Key Element)

Step 2OO2 2H2O (add 1H2O to RHS)

Step 3HO2+ 4H+ 2H2O (add 4H+ to LHS)

Step 4EO2+ 4H+ + 4e- 2H2O (add 4e- to LHS)

Step 5SO2(g) + 4H+(aq) + 4e- 2H2O(l)

7.Solid iodine I2 to iodide ions I-

Step 1 KI2 2I- (Iodine I is Key Element)

Step 2OI2 2I- (no need to add water)

Step 3HI2 2I- (no need to add H+)

Step 4EI2+ 2e- 2I- (add 2e- to LHS)

Step 5SI2 (s) + 2e- 2I-(aq)

Write ionic half equations for the following oxidation half reactions

  1. Sulfite ions SO32- to sulphate SO42-

Step 1 KSO32- SO42- (Sulfur S is the Key Element)

Step 2OSO32- + H2O SO42- ( 1H2O added to LHS)

Step 3HSO32- + H2O SO42- + 2H+ (2H+ added to RHS)

Step 4ESO32- + H2O SO42- + 2H+ + 2e-(add 2e- to RHS)

Step 5SSO32-(aq) + H2O (l) SO42-(aq) + 2H+(aq) + 2e-

  1. Hydrogen sulphide gas H2S to sulphur S

Step 1 KH2S  S (Sulfur S is the Key Element)

Step 2OH2S  S ( no O needs to be added)

Step 3HH2S  S + 2H+ (2H+ added to RHS)

Step 4EH2S  S + 2H+ + 2e-(add 2e- to RHS)

Step 5SH2S (g) S(s) + 2H+(aq) + 2e-

  1. Ethanol C2H5OH to ethanoic acid CH3COOH

Step 1 KC2H5OH  CH3COOH (Carbon C is the Key Element)

Step 2OC2H5OH + H2O CH3COOH ( H2O added to LHS)

Step 3HC2H5OH + H2O  CH3COOH + 4H+(4H+ added to RHS)

Step 4EC2H5OH + H2O  CH3COOH + 4H++ 4e-(add 4e- to RHS)

Step 5SC2H5OH(aq) + H2O(l) CH3COOH(aq) + 4H+(aq)+ 4e-

  1. Hydrogen peroxide H2O2 to O2

Step 1 KH2O2O2 (no Key Element)

Step 2OH2O2 O2( no water needs to be added)

Step 3HH2O2 2H+ + O2(2H+ added to RHS)

Step 4EH2O2 2H+ + O2 + 2e-(add 2e- to RHS)

Step 5SH2O2(aq) 2H+(aq) + O2(g) + 2e-

  1. Carbon monoxide CO to carbon dioxide CO2

Step 1 KCO  CO2 (Carbon C is Key Element)

Step 2OCO + H2O CO2( 1H2O added to LHS)

Step 3HCO + H2O CO2 + 2H+(2H+ added to RHS)

Step 4ECO + H2O CO2 + 2H+ + 2e- (add 2e- to RHS)

Step 5SCO(g) + H2O(l)  CO2(g) + 2H+(aq) + 2e-

  1. Vitamin C (ascorbic acid) C6H8O6 to C6H6O6

Step 1 KC6H8O6 C6H6O6 (Carbon C is Key Element)

Step 2OC6H8O6 C6H6O6( no water to be added)

Step 3HC6H8O6 C6H6O6 + 2H+(2H+ added to RHS)

Step 4EC6H8O6 C6H6O6 + 2H++ 2e- (add 2e- to RHS)

Step 5SC6H8O6(s)C6H6O6(aq) + 2H+(aq)+ 2e-

  1. Thiosulfate ion S2O32- to S4O62-

Step 1 K2S2O32- S4O62- (Sulfur S is Key Element)

Step 2O2S2O32- S4O62-(no water added)

Step 3H2S2O32- S4O62-(no H+ added)

Step 4E2S2O32- S4O62-+ 2e- (add 2e- to RHS)

Step 5S2S2O32-(aq) S4O62-(aq) + 2e-

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

Step 1 K(COO)22- 2CO2 (Carbon C is the Key Element)

Step 2O(COO)22- 2CO2 ( no water needs to be added)

Step 3H(COO)22- 2CO2 (no H+ need to be added)

Step 4E(COO)22- 2CO2+ 2e- (add 2e- to RHS)

Step 5S (COO)22-(aq)  2CO2(g) + 2e-