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Hess's Law Revision (Enthalpies of Combustion Confusion) Anyone Help

I am ok using Hess's Law to calculate the Enthalpy Change of Formation, at least in my worked examples as the Elements, Reactants and Products are all balanced, however this example: https://www.flickr.com/photos/140547341@N07/26683224012/in/dateposted-public/lightbox/ they are unbalanced.

I don't understand why the reactants are 2C + 3H2 +1/2O2, the product (ethanol) is C2H5OH (Both Balanced!) but then both route 1 and route 2 have + 3O2, whereas with Formation all the pathways had even numbers of elements and compounds, just arranged differently

P.S Chemistry is my worst subject
Reply 1
Avatar for Pigster
Pigster
20
The definition of DH of combustion involves the complete combustion of 1 mol of a substance in as much O2 as required. Quite how many are required really matters little.

What matters is you have the values for converting the substances in the reactants and products into their common combustion products (whatever goes at the bottom - which again, quite what goes at the bottom matters little as it will necessarily be common).

The arrows point downwards, unlike calcs done with DH of formation. If that matters to you - although some teachers teach it algebraically (like I used to).
Reply 2
Avatar for Sam00
Sam00
OP
19
Original post by Pigster
The definition of DH of combustion involves the complete combustion of 1 mol of a substance in as much O2 as required. Quite how many are required really matters little.

What matters is you have the values for converting the substances in the reactants and products into their common combustion products (whatever goes at the bottom - which again, quite what goes at the bottom matters little as it will necessarily be common).

The arrows point downwards, unlike calcs done with DH of formation. If that matters to you - although some teachers teach it algebraically (like I used to).


This makes sense.

However In my other examples, it says that when an element is in it's simple/individual state when calculating Enthalpy, it is given the value of 0, and only compounds and molecules etc are given values e.g. CO2, H2O etc

However, in the example on the Flickr link, both H2 and C have been given Enthalpy values, whereas in my other examples S was given the value 0, also in this example O2 has not been given an enthalpy value, so do I assume it is 0?

Please could you explain in layman's terms, I am not good with the Chem
(edited 7 years ago)
Reply 3
Avatar for Pigster
Pigster
20
DH of formation for an element is 0. The definition of DH of formation is the formation of 1 mol of a substance from its constituent elements. How much energy does it take to make 1 mol of H2 or C, if you are starting with 1 mol of H2 or C?

On the other hand, if you set fire to H2 (to make H2O) or C (to make CO2) there will be energy released.

Check what the values you're given represent, it is important whether they are DfH or DcH
Original post by Sam00
This makes sense.

However In my other examples, it says that when an element is in it's simple/individual state when calculating Enthalpy, it is given the value of 0, and only compounds and molecules etc are given values e.g. CO2, H2O etc

However, in the example on the Flickr link, both H2 and C have been given Enthalpy values, whereas in my other examples S was given the value 0, also in this example O2 has not been given an enthalpy value, so do I assume it is 0?

Please could you explain in layman's terms, I am not good with the Chem


From my probably inaccurate knowledge, The Standard Enthalpy Of Formation is The enthalpy change when one mole of a substance is formed from it's constituent elements (i.e. whatever you do, you should get 1 mole of that substance) under standard conditions with everything in their standard states. I think H2 and C have enthalpy values because you are considering the standard enthalpy change of combustion.

Hope that helps, i'm not good at chem either and i often find i am not great at explaining anything.
Reply 5
Avatar for Sam00
Sam00
OP
19
Original post by Pigster
DH of formation for an element is 0. The definition of DH of formation is the formation of 1 mol of a substance from its constituent elements. How much energy does it take to make 1 mol of H2 or C, if you are starting with 1 mol of H2 or C?

On the other hand, if you set fire to H2 (to make H2O) or C (to make CO2) there will be energy released.

Check what the values you're given represent, it is important whether they are DfH or DcH


Ok this is helpful, I am sorry to keep pestering you, but my last question then is why is Oxygen not given any Enthalpy values as it clearly forms bonds with the Hydrogen and the Carbon?
Reply 6
Avatar for Pigster
Pigster
20
I'm guessing you're asking why O2 doesn't have a DcH value.

Prolly 'cos you can't set fire to O2.
Reply 7
Avatar for Sam00
Sam00
OP
19
Original post by Pigster
I'm guessing you're asking why O2 doesn't have a DcH value.

Prolly 'cos you can't set fire to O2.


Ok thanks

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