Energetics questions URGENT Watch

annathecat
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#1
Report Thread starter 8 years ago
#1
hEY,
I'm having trouble with some enthalpy questions...please heeelp meeee!:confused:
Here r the Qs:

a) Name Formula S.Enthalpy formation
Ethene C2H4 +52.3
Hydrogen bromide HBr -36.2
Bromoethane C2H5Br -60.4

Use the data above to calculate the standard enthalpy change for the following reaction. C2H4+HBr arrow C2H5Br

I think I have to use Hess' law....rite?

b) Bond Average bond enthalpy
C=C +612
C-C +348
C-H +412
C-Br +276
H-Br +366

Use the data above to recalculate the enthalpy change for the same reaction, C2H4+HBr arrow C2H5Br.

c) Suggest why the value obtained in part b) is likely to be less accurate than that obtained in part a).

THANK YOU VERY MUCH !!
btw, all units are in kJ/mol
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charco
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#2
Report 8 years ago
#2
(Original post by annathecat)
hEY,
I'm having trouble with some enthalpy questions...please heeelp meeee!:confused:
Here r the Qs:

a) Name Formula S.Enthalpy formation
Ethene C2H4 +52.3
Hydrogen bromide HBr -36.2
Bromoethane C2H5Br -60.4

Use the data above to calculate the standard enthalpy change for the following reaction. C2H4+HBr arrow C2H5Br

I think I have to use Hess' law....rite?
b) Bond Average bond enthalpy
C=C +612
C-C +348
C-H +412
C-Br +276
H-Br +366

Use the data above to recalculate the enthalpy change for the same reaction, C2H4+HBr arrow C2H5Br.

c) Suggest why the value obtained in part b) is likely to be less accurate than that obtained in part a).

THANK YOU VERY MUCH !!
btw, all units are in kJ/mol
Yes, you have to use Hess' law...

In the first case you just go from reactants to products via the elements in their standard states:

Elements in their standard states --> Compound ............... = ΔH(formation)
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annathecat
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Report Thread starter 8 years ago
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Could u please guide me through the qs?
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charco
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(Original post by annathecat)
Could u please guide me through the qs?
The amount of energy required to change ethene into its constituent elements (in their standard states) is the reverse of the enthalpy of formation

This also same applies to HBr (and any other compound)

The energy required to form bromoethane from its elements (in their standard states) IS the enthalpy of formation..

Construct a Hess cycle going from the reactants to the products VIA the elements in their standard states
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