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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Rutherford on June 03, 2012, 10:06:28 AM

Title: Bond dissociation enthalpy
Post by: Rutherford on June 03, 2012, 10:06:28 AM: Found on the IChO, and it seemed very interesting to me:
Calculate the Si–Si bond dissociation enthalpy of Si₂H₆ from the following informations:
Bond dissociation enthalpy for H–H = 436 kJ/mol
Bond dissociation enthalpy for Si–H = 304 kJ/mol
ΔfH [Si_(g)] = 450 kJ/mol
ΔfH [Si₂H₆_(g)] = 80.3 kJ/mol

2Si_(g)+3H-H(g) :rarrow: H₃Si-SiH₃_(g)
I have two expresions to calculate ΔH:
ΔH=ΔfH [Si₂H₆_(g)] - 2ΔfH [Si_(g)]
ΔH=3Bond dissociation enthalpy(H-H) - 6Bond dissociation enthalpy(Si-H) - Bond dissociation enthalpy(Si-Si)
And I got that the bond dissociation enthalpy(Si-Si)=303.7kJ/mol, but I found somewhere on the net that Si-Si=around 50. Did I calculate this wrong or not?
Title: Re: Bond dissociation enthalpy
Post by: Schrödinger on June 04, 2012, 12:46:24 AM: 50 is most probably in kcal/mol units.
Title: Re: Bond dissociation enthalpy
Post by: Rutherford on June 04, 2012, 08:12:46 AM: Is my calculation right?
Title: Re: Bond dissociation enthalpy
Post by: AWK on June 05, 2012, 04:44:04 AM: Robin Walsh
Acc. Chem. Res. 1981,14, 246-252
(Table 4) 74 kcal/mol= 309 kJ/mol
Title: Re: Bond dissociation enthalpy
Post by: Schrödinger on June 05, 2012, 05:25:43 AM: I worked it out got the same result. I think we're right. Calculation seems legit to me
Title: Re: Bond dissociation enthalpy
Post by: Rutherford on June 05, 2012, 08:03:46 AM: Ok, thanks.