[brb725]

I have a question that has been bothering me, since I have a lab practical exam coming up in a few days.

My lab manual, as well as a few online resources that I've looked into, say that the ΔH_{neutralization} = q_{neutralization} / # moles of the limiting reactant. However, I know that when finding the ΔH_reaction you have to convert the number of moles of the limiting reactant into moles of reaction.

So, what would I do if I had a reaction between sulfuric acid and sodium hydroxide (H₂SO₄ + 2NaOH --> Na₂SO₄ + 2H₂O) where NaOH was the limiting reactant. Would I simply find the number of moles of NaOH and divide the q_{neutralization} by that value, or would I have to convert the number of moles of NaOH into moles of reaction (2 moles NaOH / 1 mole reaction)?

Thanks, I really appreciate any help.

[Valdorod]

ΔH_{neutralization} should be calculated on the moles of limiting reactant, however, at the end it is really based on the moles of water that you make.  The net ionic equation for all neutralization reactions is the formation of water.

H⁺ + OH^-  ---->  H₂O

So to be sure it is always better to divide by the moles of hydroxide or moles of hydrogen ion that reacted.

Valdo

[brb725]

Let me see if I understand. Say that I react 25 mL of 0.10 M NaOH with 25  mL of 0.10 M H₂SO₄ and find that the q_{neutralization} = -30 kJ (I'm completely making these numbers up, BTW). Since I would have 0.0025 moles of each,  H₂SO₄ would be my limiting reactant.

So, would I divide -30 kJ by 0.0025 moles, to find ΔH_{neutralization}, or would I divide -30 kJ by 0.00125 moles (since there would be 1 mole of reaction for every 2 moles of H₂SO₄)?

[Valdorod]

25 mL of 0.10 M NaOH = 0.0025 mol OH^-

25 mL of 0.10 M H₂SO₄  = 0.0050 mol H⁺

in this case NaOH is your limiting reagent and you divide -30 kJ by 0.0025 moles, to find ΔH_{neutralization}.

Valdo

[brb725]

I mean NaOH there  . But thank you so much, I definitely appreciate it.

If I were doing precipitation reaction, on the other hand, I would have to convert the number of moles of the limiting reactant into moles of reaction, correct? Or would I still just divide q_reaction by the number of moles of the limiting reactant?