The energy that must be supplied to the system to cause a phase change is called the enthalpy of formation. The energy required would depend upon the conditions, such as temperature and pressure at which the reaction occurs. The enthalpy change is determined by the difference in enthalpy between initial and final states.

The formula for enthalpy is,

Enthalpy change = Heat of reaction

$\Delta$ H$_{reaction}$ =H$_{2}$ - H$_{1}$.

The relation between enthalpy and heat of reaction is given by,

$\Delta$H = qp.

Some of the solved examples based on Enthalpy formula is given below:

Question 1: Calculate the enthalpy of formation for the following reaction.

C6H12O6 + 6O2 $\rightarrow$ 6CO2 + 6H2O
Given:
C6H12O6 = -393.5 kJ/mol
CO2 = 285.8 kJ/mol

Solution:

The enthalpy contribution from the products is calculated by using the enthalpies of formation of carbon dioxide multiplied by the relative numbers from the balanced equation.

$\Delta$ Hreaction = H2 - H
$\Delta$ H = 6 (-393.5) - 6 (285.8)
$\Delta$ H = - 4076 kJ/mol

Question 2: Calculate the enthalpies of formation of aluminium trioxide.

2Al + Fe2O3 $\rightarrow$ Al2O3 + 2Fe
Given:
$\Delta$H for Fe2O3 = -924 kJ/mol
$\Delta$H for Al2O3 = -1384 kJ/mol
$\Delta$H for Al = $\Delta$H for Fe = 0

Solution:
Substitute the values in the corresponding equation,

$\Delta$ Hreaction = H2 - H1 
$\Delta$ H = (-1384) - (-924)
$\Delta$ H = - 460 kJ/mol