ThermochemistryHard

Question

The intermediate SiH2 is formed in the thermal decomposition of silicon hydrides. Calculate ΔHf° of SiH2 from the following reactions.

Si2H6(g) + H2(g) → 2SiH4(g); ΔH° = −11.7 kJ/mol

SiH4(g) → SiH2(g) + H2(g): ΔH° = +239.7 kJ/mol;

ΔfH°, Si2H6(g) = +80.3 kJ mol–1

Options

A.353 kJ/mol
B.321 kJ/mol
C.198 kJ/mol
D.274 kJ/mol

Solution

$(a)Si_{2}H_{6}(g) + H_{2}(g) \rightarrow 2SiH_{4}(g);\Delta H = - 11.7\text{ kcal}$

$(b)SiH_{4}(g) \rightarrow SiH_{2}(g) + H_{2}(g);\Delta H = + 239.7\text{ kcal} $$$(c)2Si(s) + 3H_{2}(g) \rightarrow Si_{2}H_{6}(g);\Delta H = + 80.3\text{ kcal}$$

Required thermochemical equation is

$Si(s) + H_{2}(g) \rightarrow SiH_{2}(g) $$$\text{From }(b) + \frac{1}{2}(a) + \frac{1}{2}(c):\Delta H = + 274\text{ kcal/mol}$$

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