Core Concept

Enthalpy of Formation ($\Delta H_f^\circ$​) is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions (298 K and 1 atm). Standard enthalpies of formation are essential for calculating the enthalpy changes of reactions.

Key Concepts

1. Standard Enthalpy of Formation ($\Delta H_f^\circ$​):

   • The enthalpy change when 1 mole of a compound is formed from its elements in their most stable forms at 1 atm pressure and 298 K.

   • Symbol: $\Delta H_f^\circ$​ (the degree symbol $\circ$∘ denotes standard conditions).

   • Measured in kilojoules per mole (kJ/mol).

2. Standard State:

   • Elements are in their most stable physical forms at 1 atm and 298 K (e.g., $\text{O}_2$​ for oxygen, $\text{N}_2$​ for nitrogen, and graphite for carbon).

3. Enthalpy of Formation of Elements:

   • The standard enthalpy of formation of any pure element in its standard state is zero.

   • Example: $\Delta H_f^\circ$​ for $\text{O}_2\text{(g)}$ and $\text{N}_2\text{(g)}$ is zero.

4. Applications:

   • Enthalpies of formation are used to calculate the enthalpy change for chemical reactions.

   • Hess’s Law and standard enthalpies of formation can be combined to find $\Delta H_{\text{reaction}}^\circ$​.

Calculating Enthalpy of a Reaction Using Enthalpies of Formation

The enthalpy change of a reaction can be calculated using the enthalpies of formation of the reactants and products:

$\Delta H_{\text{reaction}}^\circ = \sum \Delta H_f^\circ(\text{products}) - \sum \Delta H_f^\circ(\text{reactants})$

1. Identify the Products and Reactants:

   • Write the balanced chemical equation for the reaction.

2. Find the Enthalpies of Formation:

   • Look up the $\Delta H_f^\circ$​ values for each reactant and product in a table.

3. Multiply by Coefficients:

   • Multiply each $\Delta H_f^\circ$​ value by its coefficient in the balanced equation.

4. Subtract the Sum of Reactants from Products:

   • Sum the enthalpies of formation for the products and reactants separately, then subtract the total for the reactants from the total for the products.