redox reaction occur?

A.

most people will stare at "sulphur dioxide" and "conc. nitric acid",
and think that once they appear, there'll be redox.
WRONG! they also have acidic properties, not only redox prop.

however, if a substance in elemental form is involved, it's 99.9% sure that it'll be a redox (except interconversion of oxygen and ozone). reason is stated below.

2Na + 2H2O ------> 2NaOH + H2
oxidation number of an element must be zero, while O.N. of any species in a compound is almost always non-zero. thus there must be a change of O.N. and, a redox occured.

here, O.N. of sodium is 0, and sodium ion is +1. there's change in O.N., thus it's a redox.
(another species is hydrogen, from +1 to 0.)


B: SO2 + 2NaOH ------> Na2SO3 + H2O
O.N. of sulphur keeps at +4.
SO2 cannot reduce other species such as oxygen in hydroxide, as it's already at lowest possible O.N.

C: Cu(OH)2 -----> CuO + H2O
Cu remains at +2.

D: ZnO + 2HNO3 ------> Zn(NO3)2 + H2O
Zn remains at +2, N remains at +5.
nitric acid cannot oxidize zinc as Zn has attained it's maximum O.N., +2.

Tips: Some reactions such as neutralization, precipitation etc... the elements inside each of the reactants and products do not give a change in O.N. They are usually not redox.