Question

Acetic acid has a heat of fusion of 10.8 kJ/mol and a heat of vaporization of 24.3 kJ/mol. What is the expected value for the heat of sublimation of acetic acid

Answer 1

Answer: 35.1 kJ/mol

Explanation:

According to Hess’s law of constant heat summation, the heat absorbed or evolved in a given chemical equation is the same whether the process occurs in one step or several steps.

According to this law, the chemical equation can be treated as ordinary algebraic expression and can be added or subtracted to yield the required equation. That means the enthalpy change of the overall reaction is the sum of the enthalpy changes of the intermediate reactions.

$CH_3COOH(s)\rightarrow CH_3COOH(l)$    $\Delta H_{fusion}=10.8kJ/mol$   (1)

$CH_3COOH(l)\rightarrow CH_3COOH(g)$    $\Delta H_{vap}=24.3kJ/mol$   (1)

The final reaction is:  

$CH_3COOH(s)\rightarrow CH_3COOH(g)$  $\Delta H_{sub}=?$   (3)

By adding (1) and (2)

$\Delta H_{sub}=\Delta H_{fusion}+\Delta H_{sub}=+10.8kJ+(+24.3kJ)=35.1kJ$

Hence the expected value for the heat of sublimation of acetic acid is 35.1 kJ/mol