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quick chemistry question

Hi
Regarding energetics, if a question says that a table gives some standard enthalpies of formation, and then it asked u to uses the data to calculate the enthalpy of combustion. Do I find the formation or combustion??? Because I did combustion in a test and I got it wrong cos apparently it was meant to be formation. Idk. Please help me, I'm rly confused. Thanks x

Reply 1

C_Yap

You calculate the enthalpy of combustion by making a Hess cycle with formation data

Reply 2

C_Yap

Hope it helped, but if still stuck then show me the question and I will help.

(I did this last year and was confused as well, so understand how you feel lol)

Reply 3

Yeolsi

Thanks, I think I've done it. I was just checking to make sure was finding the right value. But I do have a question regarding another question.

A enthalpy of formation equation:
N2 1/2O2=N2O

It gives enthalpy change of reaction:
C N2O=CO N2. = -193
C 1/2O2=CO. = -111

It says find formation so ik
formation=products - reactants
I'm not sure what to do tho. Pls could u help me by using the formation=products - reactants instead of drawing the cycle cos I get rly confused that
way :frown:

Thanks

(edited 6 years ago)

Reply 4

Yeolsi

**sorry it's meant to be
C+N2O=CO2+N2. and
C+1/2O2=CO2

I edited it but the adds are not showing up :frown:

Reply 5

C_Yap

Post a picture of question on this thread. I'm confused :/

cycles make the world go round

I think this is it

Original post by C_Yap

I think this is it

Thank you so much, that rly helped because I don't rly understand the Hess cycle. :smile:

Reply 10

C_Yap

No problem

By the way, try avoid doing the products minus reactants thing as it will only confuse you more. The ONLY time you should use it is in Gibbs free energy calculations (year 13 topic)

This book might be of interest to you. It helped me a great deal and now I'm on track to getting A*.

https://www.amazon.co.uk/gp/aw/d/0582411270/ref=mp_s_a_1_fkmr1_2?ie=UTF8&qid=1520884688&sr=8-2-fkmr1&pi=AC_SX236_SY340_FMwebp_QL65&keywords=calculations+in+asa+level+chemistry

Reply 11

Yeolsi

Omg, thank you so much cos I'm rly struggling with chem, it's such a challenging subject. And yh, i will definitely consider taking a look at the book. Thx