Question

The standard heat of combustion of solid boron is equal to :

• A. $\Delta$H⁰f (B2O3)
• B. 1/2 $\Delta$H⁰f (B2O3)
• C. 2$\Delta$H⁰f (B2O3)
• D. 1/2 $\Delta$H⁰f (B2O3)

Answer 1

Answer: (B)

1/2 $\Delta$H⁰f (B2O3)

Answer 2

Combustion reaction of solid boron

B(s) + $\frac{3}{4}$ O2(g) $\longrightarrow$ $\frac{1}{2}$B2O3 (s)

$\Delta$H°r = $\Delta$H°c$\frac{1}{2}$ = $\Delta$H°f (B2O3, s) – $\Delta$H°f (B, s) – $\frac{3}{4}\Delta$H°f (O2, g)

$\Delta$Hf° of element in stable state of aggregation is assumed to be zero.

$\Delta$H°C = $\frac{1}{2}$ $\Delta$H°f (B2O3, s)