Question

Using the following thermochemical data
$C\left(S\right)+O_2\left(g\right)\to C{O}_2\left(g\right)$, $\Delta H=-94.0Kcal$
$H_2\left(g\right)+\frac{1}{2}{O}_2\left(g\right)\to H_{2}O\left(l\right)$, $\Delta H=-68.0Kcal$
$C{H}_{3}COOH\left(l\right)+2O_2\left(g\right)\to 2C{O}_2\left(g\right)+2H_{2}O\left(l\right)$, $\Delta H=-210.0Kcal$
The heat of formation of acetic acid is

• A. $116.0Kcal$
• B. $-116.0Kcal$
• C. $-114.0Kcal$
• D. $+114.0Kcal$

Answer 1

Answer: (C)

$-114.0Kcal$

The thermochemical reactions are as given below.
$C\left(S\right)+O_2\left(g\right)\to C{O}_2\left(g\right),\Delta H=-94.0Kcal$ ......(1)
$H_2\left(g\right)+\frac{1}{2}{O}_2\left(g\right)\to H_{2}O\left(l\right),\Delta H=-68.0Kcal$ ......(2)
$C{H}_{3}COOH\left(l\right)+2O_2\left(g\right)\to 2C{O}_2\left(g\right)+2H_{2}O\left(l\right),\Delta H=-210.0Kcal$ ......(3)
The reaction (3) is reversed. The reactions (1) and (2) are multiplied by 2 to obtain the reaction for the formation of acetic acid which is

$2C\left(S\right)+2H_2\left(g\right)+O_2\left(g\right)\to C{H}_{3}COOH\left(l\right)$.
Hence, the heat of formation of acetic acid is $2(-94.0)+2(-68.0)+210.0$

$=534$

$=-114.0$ Kcal.