Redox

REDOX REACTIONS

  • Definition: Redox reactions involve the transfer of electrons between substances.
  • Oxidation/Reduction:
    • The substance that loses electrons is oxidized.
    • The substance that gains electrons is reduced.
  • Mnemonic: OIL RIG (Oxidation Is Loss; Reduction Is Gain)
  • Oxidation States: Determine oxidation states to see the flow of electrons in a reaction.
  • Remember: Oxidation state is assigned per atom, not a sum across atoms.

OXIDATION

  • Concept:
    • The charge of an atom becomes more positive.
  • Reactions Involving Oxidation:
    • Example reaction for oxidation:
    • A^2
      ightarrow A^+ + e^-
    • A^+
      ightarrow A^{2+} + e^-
  • Reduction Concept:
    • The charge of an atom becomes more negative.

OXIDATION STATES

  • Rules for Assigning Oxidation States:
    • An atom in its elemental state has an oxidation number of 0.
    • A monatomic ion has an oxidation number equal to its charge.
    • Oxygen typically has an oxidation state of -2, except in peroxides (O22−{O_2}^{2-}) where it is -1.
    • For covalent compounds without O or H, the most electronegative element mimics the negative ion it creates.
    • The sum of oxidation numbers in a neutral compound is 0; in polyatomic ions, it equals the ion's charge.
    • Hydrogen is +1 with nonmetals and -1 with metals.

EXAMPLES OF OXIDATION STATES

  • Na2S:
    • Na = +1
    • S = -2
  • CuCO3:
    • Cu = +2
    • C = +4
    • O = -2
  • H2C2O4:
    • H = +1
    • C = +3
    • O = -2

PRACTICE ASSIGNING OXIDATION NUMBERS

  1. Na2CrO4:
    • Na = ?
    • Cr = ?
    • O = ?
  2. K2Cr2O7:
    • K = ?
    • Cr = ?
    • O = ?
  3. CO2:
    • C = ?
    • O = ?
  4. NaH:
    • Na = ?
    • H = ?
  5. HClO4:
    • H = ?
    • Cl = ?
    • O = ?

REACTIONS AND OXIDATION STATES

  • Identify changed oxidation states in reactions:
  • If an element is oxidized (loses electrons), another must be reduced (gains electrons).
  • Oxidation State Change:
    • If oxidation state decreases, the element is reduced.
    • If it increases, the element is oxidized.
  • Reactant Species: Always those undergoing oxidation or reduction.
  • Note: Redox reactions are not precipitation reactions but can be combustion, synthesis, or decomposition.

EXAMPLES OF OXIDIZED/REDUCED SPECIES

  1. 2 Mg(s) + O2(g) → 2 MgO(s)
    • Mg is oxidized (0 to +2)
    • O2 is reduced (0 to -2)
  2. Fe2O3 + 3 H2(g) → 2 Fe + 3 H2O
    • H2 is oxidized (0 to +1)
    • Fe3+ is reduced (+3 to 0)
  3. 4 NH3(g) + 5 O2 → 4 NO + 6 H2O
    • NH3 is oxidized (-3 to +2)
    • O2 is reduced (0 to -2)