4.3 Redox

4.3 Redox

1. Oxidation Number

   • Can be thought of as the number of electrons involved in bonding to a different element
     • Helps when writing formulae and balancing equations as a check that all electrons are counted for

2. Oxidation Number RULES

   • Always zero for elements
   • Each atom in a compound has an oxidation number
   • An oxidation number has a sign which is placed before the number

3. Oxidation Numbers

   • Element
   • Oxidation Number
     • -2
       • O
       • CaO
       • +1
         • H2S
         • H
         • -1
           • HF
           • F
           • +2
             • MgCl2, CaO
             • Mg 2+, Ca2+
             • -1
               • HCl, KBr, CaI2
               • Cl- , Br- , I-
   • Example
   • Special Cases
     • Oxidation Numebr
       • -1
         • NaH, CaH2
         • H in metal hydrides
         • -1
           • H2O2
           • O in peroxides
           • +2
             • F2O
             • O bonded to F
     • Element
     • Example

4. Using Roman numerals in naming

   • The roman numeral shows the oxidation state (number) of the element, without a sign
     • E.g/ Iron(II) represents Fe2+ with oxidation number +2
       • E.g/ Iron (III) represents Fe3+ with oxidation number +3

5. Redox Reactions

   • Reduction and Oxidation
     • O.I.L R.I.G
       • Oxidation Is Loss (of electrons) Reduction Is Gain (of electrons)
     • Redox reactions involve reduction AND oxidation
       • If something is reduced, something else must be oxidised
   • Reduction is a decrease in oxidation number
   • Oxidation is an increase in oxidation number