In the second part, for example, ZN$\ce{Zn}$ is 2+ and H$\ce{H}$ is -1 so. So using the criss-cross method, you get ZN(NO3)2$\ce{Zn(NO3)2}$ and water. I understand this.
I don't agree on the -1 for hydrogen, let's look again at that example:
$$\ce{2HNO3(aq) + ZnO(s) -> Zn(NO3)2(aq) + H2O}$$
Metal oxides, such as zinc oxide, are basic anhydrides.
Imagine that
- $\ce{ZnO}$ was made by heating $\ce{Zn(OH)2}$ until it loses water:
$$\ce{Zn(OH)2 ->[\Delta] ZnO(s) + H2O}$$
- $\ce{ZnO}$ is a salt of $\ce{Zn^2+}$ and $\ce{O^2-}$
The reaction between nitric acid and zinc oxide and just a neutralization reaction between two protons and $\ce{O^2-}$ (which, as such, isn't present in solution).
Now for the reaction with ammonia: For this you may want to remember that there are different acid-base theories: The one that helps here is the Bronsted-Lowry theory:
- Acids are proton donors
$\ce{HNO3}$ definitely is that.
- Bases are proton acceptors
How can something else other than the $\ce{OH-}$ that you knew and the $\ce{O^2-}$ from $\ce{ZnO}$ be a proton acceptor = form a bond to a (proton)?
Is that possible if this acceptor doesn't even have a negative charge?
(Yes, otherwise I wouldn't buld up the tension like in a suspense story :D)
If the proton can't provide the electrons for the bond, the acceptor has to deliver them! And that can happen in the form of an electron pair, a double filled, non-bonding orbital.
Therefore a reaction, such as
$$\ce{H+ + NH3 <=> NH4+}$$
is perfectly valid. The rest of the example is just arranging the counter ions.
