Question

The standard heat of formation of solid ammonium nitrate. NH4NO3 is -330kJ/mol. The equation that represents this process is:

a) N2(g)+ 4H2(g)+3O2(g)->NH4NO3+ 330kJ
b) 2N(g)+ 4H(g)+ 3O(g)-> NH4NO3+ 330kJ
c) NH4(g)+ NO3(g)-> NH4NO3+ 330kJ
d) 0.5N2(g)+ 2H2(g)+ 1.5O2(g)-> NH4NO3+ 330kJ
e) N2(g)+ 2H2(g)+ 1.5O2(g)-> NH4NO3+ 330kJ

Can somebody help me out and explain this question. I think its between c and d because the rest aren't balanced.

Answer 1

Forgot to add that c) has a + ion on NH4
and a -ion on NO3

Answer 2

The heat of formation is the heat (released or absorbed) when a mole of substance is formed (in it's standard state) from its elements (in their standard states).
Therefore, c is eliminated immediately. It uses ions and not elements.
a is not balanced.
b is no go because neither H nor O are in their standard states (should be H2 and O2)
d is not balanced.
That leaves only e. Is it OK?
e is balanced. The elements are in their standard states (don't worry about the fraction for 3/2 O2. The 3/2 makes O atoms balance and O2 is in the standard state. The fraction is ok. Finally, the problem states delta H is -330 kJ/mol which means heat is released and the + 330 kJ/mol on the end means heat is released. So e is the correct answer.