Redox

• redox (reduction-oxidation, חִמצון־חיזור, חִמְזוּר) is a reaction in which the oxidation states of the reactants change.

• The oxidizing agent (or oxidant) (גורם מחמצן) is the species that:

  • oxidizes (מחמצן) (or causes oxidation to) the reductant
  • undergoes reduction (or being reduced (מתחזר)) which is to:
    • decrease in its oxidation number of one of its atoms
    • (historicaly: loss oxygen)

• The reducing agent (or reductant) (גורם מחזר) is the species that:

  • reduces (מחזר) (or causes reduction to) the oxidant
  • undergoes oxidation (or being oxidized (מתחמצן)) which is to:
    • one of its elements
    • increase in its oxidation number of one of its atoms
    • (historicaly: gain oxygen)

• mnemonic: OIL RIG

• disproportionation

  • ”A redox reaction in which a single element is simultaneously oxidized and reduced.” (Jones, 2016)
  • “the same substance is both oxidized and reduced” (Petrucci, 2016)
  • (e.g. $2\mathrm{Cu}{}^{+}(\mathrm{aq})\to \mathrm{Cu}(\mathrm{s})+\mathrm{Cu}{}^{2+}(\mathrm{aq})$)

• comproportionation

Oxidation state

• oxidation state (O.S.)
  • oxidation number
  • oxidation-state(species, atom)=n
  • ”the hypothetical charge of an atom if all of its bonds to other atoms are fully ionic” (Wikipedia)
  • List of oxidation states of the elements
  • An atom of a free element has an oxidation number of $0$.
    • ex. Each $\mathrm{Cl}$ atom in $\mathrm{Cl}{}_2$ has an oxidation number of $0$.
  • The sum of oxidation numbers of all atoms in a species equals its total charge.
    • examples:
      • $\mathrm{H}{}_2\mathrm{O}$:
        • $\mathrm{H}=+1$ (each), $\mathrm{O}=-2$
        • $2(+1)+(-2)=0$
      • $\mathrm{CO}{}_2$:
        • $\mathrm{O}=-2$, $\mathrm{C}=+4$
        • $(+4)+2(-2)=0$
      • $\mathrm{SO}{}_4{}^{2-}$
        • $\mathrm{O}=-2$, $\mathrm{S}=+6$
        • $(+6)+4(-2)=-2$
      • $\mathrm{Cr}{}_2\mathrm{O}{}_7{}^{2-}$
        • $\mathrm{O}=-2$, $\mathrm{Cr}=+6$
        • $2(+6)+7(-2)=-2$
      • $\mathrm{H}{}_2\mathrm{O}{}_2$
        • $\mathrm{H}=+1$, $\mathrm{O}=-1$
        • $2(+1)+2(-1)=0$
  • When combined with other elements:
    • (group 1) alkali metals are $+1$
    • (group 2) alkaline earth metals are $+2$.
  • halogens: $-1$, unless it is bonded to other halogen that is above it in the table
  • hydrogen:
    • $+1$ when bonded to non-metals
    • $-1$ when bonded to metals
  • oxygen
    • $-2$ in most of its compounds

Examples

• $\underset{\mathrm{oxidant}}{\mathrm{Cr}{}_2\mathrm{O}{}_7{}^{2-}}(\mathrm{aq})+6\underset{\mathrm{reductant}}{\mathrm{Fe}{}^{2+}}(\mathrm{aq})+14\mathrm{H}{}^{+}(\mathrm{aq})\to 2\mathrm{Cr}{}^{3+}(\mathrm{aq})+6\mathrm{Fe}{}^{3+}(\mathrm{aq})+7\mathrm{H}{}_2\mathrm{O}(\mathrm{l})$
• $\underset{\mathrm{reductant}}{2\mathrm{Al}}+\underset{\mathrm{oxidant}}{\mathrm{Fe}{}_2\mathrm{O}{}_3}\to \mathrm{Al}{}_2\mathrm{O}{}_3+2\mathrm{Fe}$ (thermite reaction)
• corrosion (קורוזיה, שיתוך)
  • rust
    • $\underset{\mathrm{reductant}}{4\mathrm{Fe}(\mathrm{s})}+\underset{\mathrm{oxidant}}{3\mathrm{O}{}_2(\mathrm{g})}\to 2\mathrm{Fe}{}_2\mathrm{O}{}_3(\mathrm{s})$ (rusting of iron)

Combustion

• combustion reaction: A substance reacts with oxygen gas, releasing energy in the form of heat and light.
• stoichiometric combustion of a hydrocarbon in oxygen:
  • $\underset{\mathrm{reductant}}{\mathrm{C}{}_x\mathrm{H}{}_y}+\left(x+\frac{y}{4}\right)\underset{\mathrm{oxidant}}{\mathrm{O}{}_2}\to x\mathrm{CO}{}_2+\frac{y}{2}\mathrm{H}{}_2\mathrm{O}$
    • $\mathrm{CH}{}_4+2\mathrm{O}{}_2\to \mathrm{CO}{}_2+2\mathrm{H}{}_2\mathrm{O}$

References

• Jones, Loretta (2016). Chemical Principles. W. H. Freeman.
• Petrucci, Ralph H. (2016). General Chemistry. Pearson Education Australia.