The intermediate SiH2 is formed in the thermal decomposition of silicon hydrides. Calculate ΔH°f of SiH2 from the following reactions:
Si2H6(g) + (g) → 2SiH4(g); ΔH° = -11.7 kJ/mol
SiH4(g) → SiH

Question

The intermediate SiH2 is formed in the thermal decomposition of silicon hydrides. Calculate ΔH°f of SiH2 from the following reactions:
Si2H6(g) + $H_2$(g) → 2SiH4(g); ΔH° = -11.7 kJ/mol
SiH4(g) → SiH2(g) + $H_2$(g); ΔH° = +239.7 kJ/mol
ΔH°f, Si2H6(g) = +80.3 kJ mol^-1

Prashant Kotian · Accepted Answer

Correct answer: D. From the first reaction: Si2H6(g) + (g) → 2SiH4(g), ΔH° = -11.7 kJ/mol.
Using heats of formation:
ΔH° = 2 × ΔfH°(SiH4) - [ΔfH°(Si2H6) + ΔfH°()]
-11.7 = 2 × ΔfH°(SiH4) - 80.3 → 2 × ΔfH°(SiH4) = 68.6 → ΔfH°(SiH4) = +34.3 kJ/mol.

From the second reaction: SiH4(g) → SiH2(g) + (g), ΔH° = +239.7 kJ/mol.
ΔH° = ΔfH°(SiH2) - ΔfH°(SiH4)
239.7 = ΔfH°(SiH2) - 34.3 → ΔfH°(SiH2) = 239.7 + 34.3 = +274.0 kJ/mol.

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