Oxidation Number the charge an atom appears to have when electrons are distributed according to certain rules
Rules for assigning charges
- Free elements: 0 e.g. O2 – o
- Sum of oxidation numbers: 0 e.g. H2O – o
- Oxidation numbers equals charge on ion e.g. Fe3+ – +3
- Sum of oxidation numbers in a complex ion equal to charge e.g. NO3- = +5 +3(-2) = -1
- Hydrogen: +1 except hydrides: -1
- Oxygen: -2 except H2O2 – O: -1 and F2O – O:+2
- Group 1: +1, Group 2: +2…etc.
- Halogens: -1 (when bonded to less electronegative) e.g. NaCl
- Transition metal may have many oxidation numbers
What is the oxidation number of C in C6H12O6?
C6H12O6
6x+12(+1)+6(-1)=0
6x+12-12=0
x=0
Oxidation is an increase in oxidation number
Reduction is a decrease in oxidation number
What is oxidised and what is reduced in ZnS + 2O2 >>> ZnSO4
ZnS + 2O2 >>> ZnSO4
+2 -2 2(0)>>>+2 +6 4(-2)
S: -2>>-6 oxidised
O: 0>>-2 reduced
Balance the equation MnO4- + Fe2+ + H+ >>> Mn2+ + Fe3+ +H2O
MnO4- + Fe2+ + H+ >>> Mn2+ + Fe3+ +H2O
+7 4(-2) +2 +1 +2 +3 2(+1)-2
Mn: +7 >> +2 : +5 e-
Fe: +2 >> +3 : -1e-
Mn + 5e- >> Mn
5Fe – 5e- >> Fe
Mn + 5Fe Ratio 1:5
MnO4- + 5Fe2+ + H+ >>> Mn2+ + 5Fe3+ + H2O
MnO4- + 5Fe2+ + 8H+ >>> Mn2+ + 5Fe3+ + 4H2O
