A level chemistry help

Hi, stuck on q2d on this link. What I'm confused about is why there is no calculation of the limiting reagent in the markscheme, used for moles under the mcAT calculation.
https://www.physicsandmathstutor.com/pdf-pages/?pdf=https%3A%2F%2Fpmt.physicsandmathstutor.com%2Fdownload%2FChemistry%2FA-level%2FTopic-Qs%2FAQA%2FPhysical-I%2F1.4-Energetics%2FSet-F%2FApplication%20of%20Hess%60s%20Law%202%20QP.pdf

Reply 1

waveyyx

6

you wouldnt need to calculate the limiting reagent because in the equation for energy change q=mct the m refers to the mass in grams which in this case would be the 25g of water you wouldnt need to take into account the 25cm3.

Reply 2

UtterlyUseless69

12

Original post by mitostudent

Hi, stuck on q2d on this link. What I'm confused about is why there is no calculation of the limiting reagent in the markscheme, used for moles under the mcAT calculation.
https://www.physicsandmathstutor.com/pdf-pages/?pdf=https%3A%2F%2Fpmt.physicsandmathstutor.com%2Fdownload%2FChemistry%2FA-level%2FTopic-Qs%2FAQA%2FPhysical-I%2F1.4-Energetics%2FSet-F%2FApplication%20of%20Hess%60s%20Law%202%20QP.pdf

I wouldn’t bother with calculating moles of limiting reagents - the copper sulphate should be the limiting reagent in this case and they’ve told you how many moles of it there are (0.0210 mol).

25 cm^3 of water will be more than enough to completely hydrate it (assuming you hydrate each copper 5 times, you would only need 0.1050 mol of water - 25 cm^3 of water is waaaay more than that, at around 1.4 mol, given that its density is about 1 g/cm^3).

Reply 3

mitostudent

OP

11

Thanks for the replies, in which cases would limiting reagents need to be calculated then?

Reply 4

UtterlyUseless69

12

Original post by mitostudent

Thanks for the replies, in which cases would limiting reagents need to be calculated then?

If they tell you a reagent is in excess and they don’t explicitly give the moles of the limiting reagent, you only need to calculate the moles of the limiting reagent.

If they give you the means to calculate the moles of all reagents, it is well worth doing so and seeing if there is in fact a limiting reagent.

In the question you asked about, they gave you the moles of the limiting reagent. The salt is always a limiting reagent in problems regarding enthalpies of solution.

Reply 5

acetyl.chloride

4

Original post by mitostudent

Hi, stuck on q2d on this link. What I'm confused about is why there is no calculation of the limiting reagent in the markscheme, used for moles under the mcAT calculation.
https://www.physicsandmathstutor.com/pdf-pages/?pdf=https%3A%2F%2Fpmt.physicsandmathstutor.com%2Fdownload%2FChemistry%2FA-level%2FTopic-Qs%2FAQA%2FPhysical-I%2F1.4-Energetics%2FSet-F%2FApplication%20of%20Hess%60s%20Law%202%20QP.pdf

You do not have to calculate the limiting reagent. The difference in number of moles of water and anhydrous CuSO4(s) is extremely significant. This suggests water is in very very very large excess. In fact, water molecules used for hydrating CuSO4(s) and mass of CuSO4(s) are both negligible, so for any practical purpose it is safe to assume all the heat released by hydrating CuSO4(s) are used to heat up 25.0g of H2O(l) by 14.0°C.

A level chemistry help

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