Question

When $1.0g$ of oxalic acid $\left(H_2{C}_2{O}_4\right)$ is burned in a bomb calorimeter whose heat capacity is $8.75kJ/K$, the temperature increases by $0.312K$. The enthalpy of combustion of oxalic acid at ${27}^{o}C$ is:

• A. $-300.7kJ/mol$
• B. $-280.452kJ/mol$
• C. $-241.947kJ/mol$
• D. None of these

Answer 1

The correct option is C $-241.947kJ/mol$

The reaction required is ;
$H_2{C}_2{O}_4\left(l\right)+\frac{1}{2}{O}_2\left(g\right)\to H_{2}O\left(l\right)+2C{O}_2\left(g\right);$

$△n_g=3/2$
$△U_c=q_v=-\frac{0.312\times 8.75}{1}\times 90$
$=-245.7kJ/mol$
negative sign indicates heat is released

$△H=△U+△n_{g}RT$
$=-245.7+\frac{3}{2}\times \frac{8.314\times 300}{1000}=-241.947kJ/mol$

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