Bond dissociation enthalpies (or bond dissociation energies) are a measure of bond strength in chemi — Physical Chemistry — Thermodynamics Chemistry Question
Thermochemistry and Bond Dissociation Enthalpy
Bond dissociation enthalpies (or bond dissociation energies) are a measure of bond strength in chemical compounds. As such they can be useful in estimating whether a reaction is exo- or endothermic, that is, in estimating the enthalpy change occurring on reaction.
One use of dissociation enthalpies is to determine element–element bond strength, a parameter that can often not be measured directly. Here we wish to determine the Si–Si bond strength.
Silicon hydrides SinH2n+2 are called silanes. Most of them contain Si–Si bonds, but they become increasingly unstable as the number of silicon atoms increases.
Calculate the Si–Si bond dissociation enthalpy of Si2H6 from the following information:
Bond dissociation enthalpy for H–H = 436 kJ mol–1
Bond dissociation enthalpy for Si–H = 304 kJ mol–1
∆fH [Si(g)] = 450 kJ mol–1
∆fH [Si2H6(g)] = 80.3 kJ mol–1
Model Answer
2 Si(s) + 3 H2(g) → Si2H6(g)
2(450 kJ) 3(436 kJ) –6(304 kJ) – ∆H(Si-Si)
6 H(g) + 2 Si(g)
80.3 kJ
Solving for ∆H(Si–Si), we find 304 kJ mol–1. [The recent literature gives 305.0 kJ mol–1 for the Si–Si bond dissociation energy in Si2H6: M. Kaupp and S. Riedel, Inorg. Chim. Acta, 357, 1865–1872 (2004).] Be sure to note that the calculation is very sensitive to the value of the Si–H bond energy, and a range of values has been reported for this bond energy.
Compare the calculated Si–Si bond energy with that for the carbon-carbon single bond (bond dissociation enthalpy = 347 kJ/mol). What implications does this have for the thermodynamic stability of silanes with n = 2 or greater as compared to analogous alkanes?
Model Answer
The bond dissociation enthalpy for Si–Si is much less than that of C–C. The clear implication is that this low Si–Si bond strength contributes to the thermal and kinetic instability of compounds with Si–Si bonds.