Question

The standard heat of combustion of solid boron is equal to

• A. $\Delta H_f^0\left(B_2{O}_3\right)$
• B. $\frac{1}{2}\Delta H_f^0\left(B_2{O}_3\right)$
• C. $2\Delta H_f^0\left(B_2{O}_3\right)$
• D. $-\frac{1}{2}\Delta H_f^0\left(B_2{O}_3\right)$

Answer 1

The correct option is B

$\frac{1}{2}\Delta H_f^0\left(B_2{O}_3\right)$

$B\left(s\right)+\frac{3}{4}{O}_2\left(g\right)\to \frac{1}{2}{B}_2{O}_3$

$\Delta H_r^0=\Delta H_c^0=\frac{1}{2}{H}_f^0\left(B_2{O}_3\right)-\Delta H_f^0(B,S)-\frac{3}{4}$

$\Delta H_f^0(O_2,g)$

$\Delta H_f^0$ of elements is assumed to be zero.

$\Delta H_c^0=\frac{1}{2}\Delta H_f^0\left(B_2{O}_3\right)$