Question

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

• A. $△H_f^o\left(B_2{O}_3\right)$
• B. $1/2△H_f^o\left(B_2{O}_3\right)$
• C. $2△H_f^o\left(B_2{O}_3\right)$
• D. $3/2△H_f^o\left(B_2{O}_3\right)$

Answer 1

The correct option is B $1/2△H_f^o\left(B_2{O}_3\right)$

Combustion reaction of solid boron
$B\left(s\right)+\frac{3}{4}{O}_2\left(g\right)\to \frac{1}{2}{B}_2{O}_3\left(s\right)$
So, enthalpy,
$△H_r^o=△H_c^o=\frac{1}{2}△H_f^o\left(B_2{O}_3\right)-△H^{o}f\left(B\right)-\frac{3}{4}△H_f^o\left(O_2\right)$
$△H_f^o$ of element in stable state of aggregation is assumed to be zero.
$△H_C^o=\frac{1}{2}△H_f^o\left(B_2{O}_3\right)$


Theory:
Standard Enthalpy of Combustion ${\Delta }_c{H}^o$

Enthalpy change when one mole of a compound combines with the requisite amount of oxygen to give products in their stable forms at $1bar,298K$

• Always $-ve$;

• Always exothermic

For example

$C\left(s\right)+O_2\left(g\right)\to C{O}_2\left(g\right)$ ${\Delta }_c{H}^o={\Delta }_f{H}^o(C{O}_2,(g\left)\right)$