(Original post by TiTo20)
It's a typical question asking you how to write an ionic equation.
1) Start off with writing what you know:
NH3 + H2O ---> HNO3
2) Work out the oxidation states of the element that appears to be changing in oxidation state (This is the nitrogen atom).
The N in NH3 has an oxidation state of -3.
The N in HNO3 has an oxidation state of +5.
3) Add electrons to the side which allows the oxidation states to be balanced on both sides.
The left hand side has -3, the right hand side has +5, so if you add 8 electrons to the right hand side, then both sides will have -3.
NH3 + H2O ---> HNO3 + 8e-
4) Add hydrogen ions so the charges on both sides are the same.
The right hand side has a total charge of -8, the left hand side has a total charge of 0, so the right hand side needs 8H+ to make the charges on the right hand side equal to 0.
NH3 + H2O ---> HNO3 + 8e- + 8H+
5) Add water molecules to balance the number of oxygen atoms.
The right hand side has 3 oxygen atoms, the left hand side only has 1, so times the number of water molecules on the left hand side by three to get three water molecules.
NH3 + 3H2O ---> HNO3 + 8e- + 8H+
Hopefully this clears everything up =]
By the way, the equation on the bottom of the mark scheme is also correct, HNO3 is an acid and it dissociates into NO3- and H+, so the H+ ion can be added to the 8H+ to make 9H+.