Redox Reactions

• Oxidation-reduction reactions (redox reactions):
• Reactions involving transfers or shifts of electrons 
• Oxidation:
• A loss of electrons which makes the oxidation number go up
• Occurs mainly in metals
• Occurs in some covalently bonded substances
• Does not require oxygen (that is not what oxidation means!)
• Reduction:
• A gain of electrons that makes the oxidation number go down (reduced)
• Occurs mainly in nonmetals that gain electrons by taking them from metals
• Review of oxidation numbers:
• Rule 1: free atoms = 0
• Rule 2: ion charge = oxidation number
• Rule 3: compound sum = 0
• Rule 4A: Group 1 = +1
• Rule 4B: Group 2 = +2
• Rule 4C: H = +1 or -1
• Rule 4D: O = -2 or -1
• Rule 4E: Group 17 = -1
• Rule 5: sum of ONs in polyatomic ion = charge
• Practice: Assign Oxidation Numbers
• H₂CO₃
• H: +1, O: -2, C: +4

• N2
• N: 0
• Zn(OH)₄^-2
• Zn: +2, H: +1, O: -2

Redox

• Short for reduction­-oxidation
• Pronounced “REE-docs”
• Must occur together (An element cannot take electrons without another one losing them.)
• LEO the GERm
• Lose Electrons Oxidation
• Gain Electrons Reduction
• Determine which element is oxidized and which is reduced?
• Zn + 2H⁺ ➝ Zn²⁺ + H₂
• Zn is oxidized (ON: 0 ➝ +2)
• H⁺ is reduced (ON: +1 ➝ 0)
• 3Hg²⁺ + 2 Fe(s) ➝ 3Hg + 2Fe³⁺
• Hg²⁺: reduced 
• Fe: oxidized
• Oxidizing and Reducing Agents:
• Reducing agent is a substance used to reduce another substance.
• If a substance is oxidized it is the reducing agent.
• Oxidizing agent is a substance used to oxidize another substance.
• If a substance is reduced it is the oxidizing agent.
• Example:
• Zn + 2H⁺ ➝ Zn²⁺ + H₂
• Zn is oxidized (ON: 0 ➝ +2) [REDUCING AGENT]
• H⁺ is reduced (ON: +1 ➝ 0) [OXIDIZING AGENT]
• 3Hg²⁺ + 2 Fe(s) ➝ 3Hg + 2Fe³⁺
• Hg²⁺: reduced [OXIDIZING AGENT]
• Fe: oxidized [REDUCING AGENT]

Balancing Redox Reactions

See practice for the procedure, but here are the steps:

1. Assign oxidation numbers
2. Make half reactions (oxidized reaction and reduced reaction) [forget everything else for now]
3. Balance electrons
4. Add everything back in and balance traditionally [see Chapter 8 for procedure]