[clevermartin]

estimate the molar enthalpy change for the hydrogenation below using the bong energy terms

H₂C=CH-CH=CH₂(g) + 2H₂(g) ---> CH₃CH₂CH₂CH₃(g)

I have already found out that the bond energy term of the left hand side is
4046 + 872 = 4918

and that of right hand side is
5168

what is the change?
5168-4918..or4918-5168..?
can somebody tell me how and why to do it..?
thanks a lot

[Borek]

Usually change is final minus start.

[clevermartin]

Usually change is final minus start.

final minus start = 250
however.. the experimental valued stated in part b was -239...

[enahs]

Change in something, is as Borek said is Final - Initial.

In this case you have to think though (not about change, but the results).

You are calculating the mean bond energy for each side, that represents the energy in the bonds, not the reaction.

So on the l.h.s you have 4918 KJ/mol and on the r.h.s you have 5168 kJ/mol, this is the energy in the bonds, not the reaction.

On the l.h.s you have the amount of energy needed to break the bonds (endothermic) and on the right you have the amount energy you 'get back' when new bonds are formed (exothermic) (going in the direction of the reaction, if you go the other direction it is different).

So, indeed the change is | (Final - Initial) | (that the absolute value in change is final-initial) but now you must determine the appropriate sign, is the reaction overall endothermic (+H) or exothermic (-H)?




To think of it in another way. Think of your laws of thermodynamics, more specifically conversation of energy.
You are told you have 4918 KJ/Mol on the left and 5168 KJ/mol on the right. Assuming an ideal world (which your question does), is energy conserved? Clearly no, but the reaction is telling you to go from left to right, so what on the right would have to be added to adhere to the conversation of energy (make them the same)?

[clevermartin]

I see~ Thanks a lot