{Note - for this example, I'm using very limited sig figs on purpose to make the math easier to follow.}
28g of nitrogen gas reacts with 7g of hydrogen gas to produce ammonia gas. How much heat is liberated by this reaction?
Approach it just like any other stoichiometry problem following the 4 steps (review here):
1. Write a balanced chemical equation. The only twist here is that since we're dealing with heat, we'll also need the {delta}H for the reaction as well.
N2(g) + 3 H2(g) --> 2 NH3(g)
{delta}Hrxn = 1(0 kJ/mol) + 3(0 kJ/mol) + 2(-46 kJ/mol) = -92 kJ/mol
2. Find moles. Again, there's a little twist here... this is a limiting reactant problem. So let's find moles of ammonia from each reactant:
(28g / 28g/mol N2) (2mols NH3(g) / 1mol N2(g)) (17g/mol NH3) = 34g NH3(g)
(7g / 2g/mol N2) (2mols NH3(g) / 3mol N2(g)) (17g/mol NH3) = 40.g NH3(g)
So we know that N2(g) is the limiting reactant.
3. Use the mole ratio from the balanced chemical equation to find "moles of interest". We already kind of blew past this in the last step... let's back up a bit now that we know the limiting reactant.
(28g / 28g/mol N2) (1mol of "reaction" / 1mol N2(g)) = 1 "mole of reaction"
4. Convert moles of interest to whatever you're looking for:
(28g / 28g/mol N2) (1mol of "reaction" / 1mol N2(g)) (92kJ/mole of reaction) = 92kJ liberated
