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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: carlee172 on December 01, 2008, 10:56:34 PM

Title: Enthalpy of Formation, CH2OH, radical?
Post by: carlee172 on December 01, 2008, 10:56:34 PM
Often it is impossible (or at least extremely difficult) to directly measure a thermochemical property of a molecule. For instance, the homolytic C-H bond dissociation enthalpy in CH3OH cannot be directly measured. Rather, one must use information from other experiments. This question is an example of such work.

a) The homolytic C-H bond dissociation in CH3OH refers to DH for this reaction:
CH3OH --> *CH2OH + H*
Assuming you cannot measure it directly, what thermochemical information do you need to evaluate it?
(Don't overcomplicate matters) [2]

b) Determine the homolytic C-H bond strength in CH3OH employing the following data [10].
DfH (CH3OH) = -201.6 kJ mol-1
DfH (H*) = 218 kJ mol-1
DfH (HBr) = -36 kJ mol-1
DfH (Br*) = 111.9 kJ mol-1
CH3OH + Br* --> *CH2OH + HBr
k(300K) = 1 x 106 M-1s-1; k(350K) = 4 x 108 M-1s-1

*CH2OH + HBr --> CH3OH + Br*
k(300K) = 1 x 107 M-1s-1; k(350K) = 7.7 x 108 M-1s-


OK, so for a) I know you need the deltaHf for products and reactants, but then for b), I don't know how to get them.
Title: Re: Enthalpy of Formation, CH2OH, radical?
Post by: opuktun on December 25, 2008, 12:57:50 PM
Just giving it a try:  ;D

Part (a)
The enthalpy formation of CH3OH, •CH2OH and H•

Part (b)
Reaction: CH3OH + Br• <=> •CH2OH + HBr
K = k1/k2
Combining :delta: G = -RT ln K and  :delta: G= :delta: H-T :delta: S gives  :delta: H=T :delta: S-RT ln K

Assume  :delta: H and  :delta: S are constant for T=300K and 350K,
300 :delta: S - 8.31451 x 300 ln (106/107) = 350 :delta: S - 8.31451 x 350 ln (4 x 108/7.7 x 108)
 :delta: S = 1/50 { 8.31451 x [350 ln (4x108/7.7x108) - 300 ln (1x106/1x107)] } = 126 J mol-1 K-1

 :delta: H (reaction) = 300 :delta: S - 8.31451 x 300 ln (1x106/1x107) = 43.5 kJ mol-1

 :delta: H (reaction)
=  :delta: H (•CH2OH)  + :delta: H (HBr)  - :delta: H (CH3OH)  - :delta: H (•Br)
=  :delta: H (•CH2OH)  + (-36) - (-201.6) - 111.9
= 43.5 kJ mol-1

:delta: H (•CH2OH) = -10.2 kJ mol-1

CH3OH  :rarrow: •CH2OH + •H
Bond strength of C-H in CH3OH =  :delta: H (•CH2OH) + :delta: H (•H) - :delta: H (CH3OH) = -10.2 + 218 - (-201.6) = 409.4 kJ mol-1