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.

Key Concepts

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).

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).

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.

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 $\Delta H_{\text{reaction}}^\circ$​.