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...

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?

When calculating the enthalpy change of a reaction, you typically only need the moles of one of the reactants because enthalpy change is a measure of how much energy is transferred during a reaction, and this energy change depends on stoichiometry.

In the example you provided, NH3 and HCl react in a 1:1 ratio to form NH4Cl, so the moles of NH3 will be the same as the moles of NH4Cl produced. This allows us to conveniently use the moles of NH3 to calculate the enthalpy change for this reaction.

However, if the concentrations and volumes of the reactants were different, you would need to consider the stoichiometry of the reaction and choose the reactant that allows you to calculate the moles of the desired product. This involves using the balanced chemical equation for the reaction.

For example, let's consider the reaction:

2A + 3B → 4C

If you have 10 moles of A and want to calculate the enthalpy change, you would need to know the moles of C produced (since the ratio between A and C is 1:2) to determine the energy change accurately.

To find the moles of C, you would need to use the balanced chemical equation. In this case, you would need to know the concentration and volume of B to calculate the moles of C produced.

So, in summary, when the concentrations and volumes of reactants are different, you need to select the reactant that allows you to calculate the moles of the desired product using the stoichiometry of the reaction.