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# Enthalpies of reactions! watch

1. I dont know whether I get this or not so I basically want my work verfied :P
So I need to work out the enthalpy for the reaction of propane decomposited to cyclopropane and hydrogen.
If the standard heat of combustion for propane is -2220,
and the standard heat of combustion for cyclopropane & hydrogen is -1966 and -286.
Would the enthalpy of reaction be -4472 kjmol-1?
It just seems way too high :s
2. (Original post by gemstar!)
I dont know whether I get this or not so I basically want my work verfied :P
So I need to work out the enthalpy for the reaction of propane decomposited to cyclopropane and hydrogen.
If the standard heat of combustion for propane is -2220,
and the standard heat of combustion for cyclopropane & hydrogen is -1966 and -286.
Would the enthalpy of reaction be -4472 kjmol-1?
It just seems way too high :s
There is no such word as 'decomposited'

3. (Original post by charco)
There is no such word as 'decomposited'

Explain..
4. So reaction you want is:

C3H8 ---> C3H6 + H2

you have:

C3H8 + 5O2 ---> 3CO2 + 4H2O = -2220
C3H6 + 4.5O2 ----> 3CO2 + 3H2O = -1966
H2 + 0.5O2 ----> H2O = -286

Use Hess's law
5. (Original post by EierVonSatan)
So reaction you want is:

C3H8 ---> C3H6 + H2

you have:

C3H8 + 5O2 ---> 3CO2 + 4H2O = -2220
C3H6 + 4.5O2 ----> 3CO2 + 3H2O = -1966
H2 + 0.5O2 ----> H2O = -286

Use Hess's law
ermm..can you possibly explain it in the context of hess's law :P We've only just started it and im slightly confused :L
6. (Original post by gemstar!)
ermm..can you possibly explain it in the context of hess's law :P We've only just started it and im slightly confused :L
Hess lets you go from A to B via any intermediate phase (eg Combustion products)

A --------------> combustion products <-------------------- B

To go from A to combustion products is the enthalpy

BUT to go from combustion products to B is the REVERSE of the combustion enthalpy

so you add the combution enthalpy of the reactants to the reverse of the combustion enthalpy of the products

does that help?
7. (Original post by charco)
Hess lets you go from A to B via any intermediate phase (eg Combustion products)

A --------------> combustion products <-------------------- B

To go from A to combustion products is the enthalpy

BUT to go from combustion products to B is the REVERSE of the combustion enthalpy

so you add the combution enthalpy of the reactants to the reverse of the combustion enthalpy of the products

does that help?
Thankyou, I get the basics now (:
However, how do you work it out with the reversion combustion? Would you just change the negative to a positive?
I now got an answer of 32kjmol-1??
8. (Original post by gemstar!)
Thankyou, I get the basics now (:
However, how do you work it out with the reversion combustion? Would you just change the negative to a positive?
I now got an answer of 32kjmol-1??
9. (Original post by charco)
so im guessing 32kjmol-1 is right? :P

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