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# deduce enthalpy change? watch

1. If given the following information, how can one deduce the enthalpy change?

S(s) + 1.5 O2 (g) --> SO3(g)
2. That's all the information youre given?
That's tricky
3. do you have to give a value or say if it is exo or endo
4. wait - I also have this information, but I'm still a little confused - I understand what they did, but not why they did it

S(s) + O2(g) <----> SO2 (g) kJ= -297
SO2(g) + 1/2 O2 <---->SO3(g) kJ= -92
5. don't you just add the numbers together?
because you just follow the steps to make so3
and its balanced so you don't need to change the numbers
6. Delta H= (-297)+(-92)
= -389 kJ
7. Does anybody know how to work this out? It was from the new Aqa Chem AS paper 2016 Thank You
8. what i would do is find the mass
you do this because you know the density is g / cm3
so just times it by the volume which is 0.5 dm3 (remember to convert it)
once you have the mass you can find the mols of ethanol
then times that by the enthalpy change
im not too sure if I'm right tho
(Original post by PBC Gyal)
Does anybody know how to work this out? It was from the new Aqa Chem AS paper 2016 Thank You
9. (Original post by CrystalSalvatore)
what i would do is find the mass
you do this because you know the density is g / cm3
so just times it by the volume which is 0.5 dm3 (remember to convert it)
once you have the mass you can find the mols of ethanol
then times that by the enthalpy change
im not too sure if I'm right tho
Yeah that's what I think too but I don't know how to calculate the mass, sorry for the mess in my working out, but with the converting and stuff it's all confusing 😳
10. (Original post by PBC Gyal)
Yeah that's what I think too but I don't know how to calculate the mass, sorry for the mess in my working out, but with the converting and stuff it's all confusing 😳
0.5 dm3 is 500 cm3 to times that by the density
11. Calculate mass of ethanol in grams
Calculate the moles of ethanol (moles=mass/mr)
Divide enthalpy change by the moles.
12. (Original post by Kyle03)
Calculate mass of ethanol in grams
Calculate the moles of ethanol (moles=mass/mr)
Divide enthalpy change by the moles.

Ohh okay I've just done that and I got 0.16KJ and I don't think that seems right 😮
13. (Original post by CrystalSalvatore)
0.5 dm3 is 500 cm3 to times that by the density
Ohh okay thanks and then I did that and got moles from it by dividing by Mr and then dividing it by 1000 to put it in KJ but I got 0.16KJ and it doesn't seem right 🤔
14. (Original post by PBC Gyal)
Ohh okay thanks and then I did that and got moles from it by dividing by Mr and then dividing it by 1000 to put it in KJ but I got 0.16KJ and it doesn't seem right 🤔
why did you divide the moles by 1000?
when you have the moles you times it by the enthalpy change because its Kj/Mol so if you times is by the mol it would leave the Kj
15. (Original post by CrystalSalvatore)
why did you divide the moles by 1000?
when you have the moles you times it by the enthalpy change because its Kj/Mol so if you times is by the mol it would leave the Kj
Ohh okay thanks that makes sense, so my answer would be 159.86KJ
16. (Original post by CrystalSalvatore)
what i would do is find the mass
you do this because you know the density is g / cm3
so just times it by the volume which is 0.5 dm3 (remember to convert it)
once you have the mass you can find the mols of ethanol
then times that by the enthalpy change
im not too sure if I'm right tho
I am so confused so how do you work it out i dont get it.
17. (Original post by Kyle03)
Calculate mass of ethanol in grams
Calculate the moles of ethanol (moles=mass/mr)
Divide enthalpy change by the moles.
once you have the moles, multiply them by the enthalpy of combustion.
18. (Original post by the bear)
once you have the moles, multiply them by the enthalpy of combustion.
Oh I divided the enthalpy of combustion by the moles not multiplied it?
19. (Original post by PBC Gyal)
Oh I divided the enthalpy of combustion by the moles not multiplied it?
20. (Original post by the bear)
😂😂😂but why would you multiply?

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Updated: January 8, 2017
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