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Stoichiometry help

Hi guys,

Here is the link to the specimen paper, http://www.ocr.org.uk/Images/67673-unit-f323-practical-skills-in-chemistry-1-evaluative-task-specimen.pdf

In the evaluative section, there is a question

The same method was used to measure the volume of carbon dioxide produced whenmagnesium carbonate reacted with hydrochloric acid.
Suggest, with reasons, any modifications you may need to make if the sulfuric acid used in
the procedure was replaced with the same concentration and volume of hydrochloric
acid.

And the answer is:

(e) explains that the stoichiometry means that with same quantity of HCl
the acid is not in excess
so double the volume of HCl is required

The equations for the reaction are:

Balanced symbol equations for the reactions of CaCO3

H2SO4 (aq) + CaCO3 à CaSO4 (aq) + CO2 (g) + H20 (l)
2HCl (aq) + CaCO3 (s) --> CaCl2 (aq) + CO2 (g) + H2O (l)

I don't understand why this would occur as with the 2HCL, the molar ratio would be 2:1 between the HCL and the CaCO3, hence, it would be in more excess as there is less of the CaCO3, why is it not in excess?
The equations tell you how much of each compound you need to react completely, not how much you have. If you have 1 mol HCl and 1 mol CaCO3, then only half of the CaCO3 will react as the equation dictates that you need twice as many moles of HCl as CaCO3. Essentially, the HCl donates only half as many protons as H2SO4 (it is half as strong), so you need twice as much.
Reply 2
Avatar for Rhy
Rhy
OP
4
Original post by morgan8002
The equations tell you how much of each compound you need to react completely, not how much you have. If you have 1 mol HCl and 1 mol CaCO3, then only half of the CaCO3 will react as the equation dictates that you need twice as many moles of HCl as CaCO3. Essentially, the HCl donates only half as many protons as H2SO4 (it is half as strong), so you need twice as much.


Thank you this made it extremely clear!
Reply 3
Avatar for Rhy
Rhy
OP
4
Original post by Rhy
Thank you this made it extremely clear!



But also, why would it not be in excess anymore?
Original post by Rhy
But also, why would it not be in excess anymore?


Well, if you need 2x moles of HCl, but only have 1x moles of HCl, it will not be in excess
Reply 5
Avatar for Borek
Borek
4
Sulfuric acid is diprotic, hydrochloric acid is monoportic - so the same amount in terms of moles is not the same amount in terms of equivalents. To have the same number of equivalents you need twice the number of moles of HCl.

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