The additive identity is 0.
There are no "the" 5 rational numbers. There are infinitely many of them and, while 0 and 1 have special status (as the additive and multiplicative identities), in mathematical terms the rest have the same status and so there are no others which are more important.
The additive identity for rational numbers is 0. It is the only rational number such that, for any rational number x, x + 0 = 0 + x = x
They are, respectively, the multiplicative and additive identities for the field of numbers.
Yes, it does.
Yes, it does.
It is the negative of the number.
No. It has a different additive inverses for each element.
There is only one set and it does have an additive identity.
It is the additive identity for integers, rational numbers, real numbers, complex numbers.
Yes. The additive identity is 0.
Yes.
Negative rational numbers are used in the same way that negative whole numbers are used: they are simply the additive inverses of their positive counterparts.