Nitration by nitric acid alone is believed proceed by essentially the same mechanism as nitration in the presence of Sulphuric acid. Is there an equation for the generation of NO2+ (nitronium ion) from nitric acid alone?

The main purpose of concentrated sulfuric acid in nitration reactions is to absorb the water that is generated, Nitration will proceed without it, for a short time. You could use white fuming nitric acid, but the reaction stops when the water dilutes the nitric acid. As for red fuming nitric acid, I recall that if brown nitrogen dioxide fumes are seen rising from a reaction vessel, the entire batch is immediately dumped into a large reservoir of ice water to stop the reaction. So don’t try it with red fuming at home.

Well, it depends. Technically, potassium nitrate reacts with sulfuric acid to form nitric acid, so it can be used, but you will need a higher amount of sulfuric acid (and it might take longer). I recommend that you follow the procedure that can be found somewhere in the literature…

The reaction of phenol with nitric acid is a mono-nitration reaction i.e. Only one hydrogen atom gets replaced per molecule of phenol. The products formed are o-nitrophenol and p-nitrophenol.

HNO3 undergoes HETEROGENEOUS CLEAVAGE TO form NO2+

HNO3 → NO2+ +OH-

Why?

Phenol exhibits “activated” ortho/ para negative charges which induces HETEROGENEOUS CLEAVAGE of HNO3

Sulfuric acid is the most widely used inorganic compound in industry. It is used far more than nitric acid. I am guessing you submitted a question from your lab manual relating to a specific experiment that uses nitric acid. You need to provide the specific context; otherwise, your question is incoherent.

Sulfuric acid is a non-aqueous solvent. That is, the concentrated material has different properties from water. It also has different properties from HNO3. Concentrated sulfuric acid supports the formation of the nitronium ion NO2+ better than HNO3. The following equation is often given:
H2SO4 + HNO3 = HSO4- + NO2+ + H2O

(Effectively the water is removed by forming a hydrate with the excess sulfuric acid, driving the reaction to the right.)

The nitronium ion is the effective nitration agent.

If you mean common aromatic nitration, it is.

HNO3 is mixed with sulfuric acid to make the active reagent nitronium , NO2+, which goes on, as an electrophile, to attack the pi system of the ring.

Acids can also be classified by the Lewis Acid Base Concept.

In the Lewis theory of acid-base reactions, bases donate pairs of electrons and acids accept pairs of electrons. A Lewis acid is therefore any substance, such as the H + ion, that can accept a pair of nonbonding electrons. In other words, a Lewis acid is an electron-pair acceptor.

There is also a definition by a Russian author, whose name escapes me at this time, who pointed out that this concept could be stretched to include single electron donors and acceptors. This of course ties acids and bases with redox circles and implies every acid base reaction is also a sort of oxidation reduction process !

This is true because nitric acid forms an azeotrope with water, i.e. a constant boiling point mixture of 68% nitric acid and 32% water. You need another chemical to grab hold of water molecules, in this case conc. sulphuric acid.