[Lemos]

What is the enthalpy of formation for low-density polyethylene, knowing that to form 1 mol of LDPE you need 2000 mol of ethylene.

Some information:
E.f (gaseous ethylene) = 50kJ/mol
E.f (liquid water) = -290kJ/mol
E.f (gaseous carbon dioxide) = -390kJ/mol

I don't have the answer for this one and I'm not sure about the reaction to form LDPE (it is not given by the problem).
How do I solve this?

[Borek]

Start finding the reaction.

It should be just a matter of understanding what the LDPE is and what polymerization is.

[Lemos]

Could I use the (inverse)combustion reaction for ethylene?

2CO2 + 2H2O  ----->  C2H4 + 3O2

In that case, I would use 2000 mol for ethylene, resulting in:

4000CO2 + 4000H2O ----> 2000C2H4 + 6000O2

Then, for the enthalpy of formation, I'd do:

Ef = Ef(products) - Ef(reagents)

This would result in 2820000 kJ/mol.
But I don't think this is the correct answer.

I don't really know how to find the enthalpy of formation of LDPE using the polymerization, although I know the following reaction:

(C2H4=C2H4)n ---> -(C2H4-C2H4)-n

But the problem doesn't offer any values for the break of bonds for carbon. What do I do from here?
Thanks for the help so far.

[Sunil Simha]

Go back to the basics and think about what enthalpy of formation of LDPE means.

[Lemos]

I did my research on the subject but I'm still having problems.
I see that you can find the heat of formation by the bond energies, but the problem didn't offer any values for these (I found out that they vary).

The reaction in which I'm focusing on is the same one as before:

(C2H4=C2H4)n ---> -(C2H4-C2H4)-n

The double bond of carbon turns into 2 single bonds, is this right?

Am I still getting it wrong or is it really possible to determine the heat of formation of LDPE by having the heat of formation of CO2, H2O and C2H4?
CO2 and H2O doesn't go in the polymerization of C2H4... Would that change anything?

Thanks for the tips so far!

[Sunil Simha]

I thought "formation" of a substance is defined as the reaction between the substance's constituent elements to produce the substance.

Thus Enthalpy of formation of Ethylene is the enthalpy change for the reaction

2C + 2H₂  C₂H₄

For polythene, the enthalpy of formation (going by the definition of "formation") is the enthalpy change of the reaction

2nC +2nH₂  C_2nH_4n

Or is there some other definition of formation of a polymer?