this drove me crazy during my IGCSEs too..
S + 02 --> S02
S - oxidation number 0..
O2 - oxidation number 0
SO2 --> oxidation number of O = (-2*2) = -4 , thus oxidation number of S = +4
S--> 0 to 4 oxidation
O -> 0 to -4 reduction
thus redox.
2S02 + 02-->2 S03
2SO2 -> oxidation number of O = (-2*2) = -4, thus S = +4. O2 -> 0, final of O = -4+0 = -4
SO3--> Oxidation number of O = (-2*3) = -6, of S = +6
Final , S --> 4 to 6 oxidation
O --> -4 to -6 reduction
SO3 + H20--> H2SO4
SO3 --> O = (-3*2) = -6 , thus S = +6
H2O --> O = -2, thus H = +2
O in both compounds thus, final number of O = -6-2 = -8
H2SO4--> O = (-2*4) = -8, thus H = +2, as final oxidation number of the compound shud be 0, -8+2+S = 0
thus, S = +6
now the changes
S --> 6 to 6
H --> 2 to 2
O --> -8 to -8..
NOt redox.
I AM NOT SURE ABOUT THIS , NID PLZ CHECK!