Question

When $2$ moles of $C_2{H}_6$ are completely burnt, $3120kJ$ of heat is liberated. Calculate the heat of formation $\Delta H_f^o$ for $C_2{H}_6$.

Given: $\Delta H_f^o$ for ${CO}_2\left(g\right)$ and $H_{2}O\left(l\right)$ are $-395$ and $-286kJ$ respectively.

• A. $-88kJ/mol$
• B. $+88kJ/mol$
• C. $-58kJ/mol$
• D. None of the above

Answer 1

The correct option is A $-88kJ/mol$

$2C_2{H}_6+O_2⟶2{CO}_2+3H_{2}O$

$\Delta H_{rxn}=\frac{\Delta H}{mole}=\frac{3120}{2}=-1560kJ$

$\Delta H_{rxn}=2(-345)+3(-286)-{\left(\Delta H_f^o\right)}_{C_2{H}_6}$

$\Delta H_f^o=-790-858+1560$$=-88kJ/mol$