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You know how when you do calculations for the enhaloy change of a reaction you start sort of like this?:

NH3(aq) + HCl(aq) → NH4Cl(aq)
25 mL 25 mL
1.0 mol/L 1.0 mol/L

n = c x v
= 1 mol/L x 0.025L
= 0.025 mol

How come you only need the moles of one of the reactants? I just always did it but didn't really know why... I just know I'll remember better for a test or something if I understand, so I'd like to know. And for all our sample problems the concentrations and volumes of both reactants were conveniently the same. So we never had to choose which reactant to get the moles for. But what if the concentrations and volumes were different? Would that ever happen? Or maybe that's too advanced or something for right now? I'm just scared something like that will pop up on the quiz tomorrow and I'm just going to have to guess which reactant to use to find moles...

4 answers

The coefficents on a balanced equation are in mole ratios. If you know one, you know all.
So if you find the enthalpy change for one reactant, it's the enthalpy change for the other reactant (and products??), i.e. you know the enthalpy change for the entire reaction?

So enthalpy change of one reactant (or product also?) = enthalpy change of entire reaction?
No. Enthlapy change is from the total reaction. Remember Enthalpy is in kJ/molereactant
That's what I don't really understand. If it's of the total reaction, shouldn't you add, in my example, the moles of both reactants? i.e. 0.025 +0.025 = 0.05 mol. Then use that value when doing deltaH = -Q/n?

How come you only use The moles of one?