Question

The enthalpy of the formation of $C{O}_2$ and $H_{2}O$ are - $395kJ$ and - $285kJ$ respectively and the enthalpy of combustion of acetic acid is $869kJ.$ The enthalpy of formation of acetic acid is:

• A. 235 kJ
• B. 340 kJ
• C. 420 kJ
• D. 491 kJ

Answer 1

Answer: (D)

Combustion reaction for acetic acid is :

$C{H}_{3}COOH+2O_2⟶2C{O}_2+2H_{2}O$

Thus,

$H_R=\sum H_f^{\circ }$ (product) $-\sum H_f^{\circ }$ (reactant)

$-869=[2\times H_{f\left(C{O}_2\right)}^{\circ }$

$+2\times H_{f\left(H_{2}O\right)}^{\circ }]\left(H_{f\left(C{H}_{3}COOH\right)}^{\circ }+0\right)$

$-869=[2\times (-395)+2\times (-285)]$

$-\left[H_{f\left(C{H}_{3}COOH\right)}^{\circ }\right]$

$-869=-790-570-H_{f\left(C{H}_{3}COOH\right)}^{\circ }$

$\therefore H_{f\left(C{H}_{3}COOH\right)}^{\circ }=-1360+869=-491kJ$