Enthalpy Change Calculation Examples

Introduction to Enthalpy Change

Enthalpy change (
ΔH
) is a key concept in thermochemistry that quantifies the heat absorbed or released during a chemical reaction at constant pressure. Understanding enthalpy changes is essential for predicting the energy dynamics of reactions, which can be invaluable in fields such as chemistry, materials science, and engineering. Below are three diverse examples that illustrate the calculation of enthalpy change for different types of reactions.

Example 1: Combustion of Methane

Context

The combustion of methane is a widely studied reaction due to its significance as a fuel source. This exothermic reaction releases energy, which is harnessed in various applications, including heating and electricity generation.

Example Calculation

1. The balanced chemical equation for the combustion of methane (
   CH₄
   ) is:

   CH₄(g) + 2 O₂(g) → CO₂(g) + 2 H₂O(g)

2. The standard enthalpy of formation (
   ΔH_f°
   ) values are as follows:

   • ΔH_f° (CH₄) = -74.8 kJ/mol

   • ΔH_f° (O₂) = 0 kJ/mol (elements in their standard state)

   • ΔH_f° (CO₂) = -393.5 kJ/mol

   • ΔH_f° (H₂O) = -241.8 kJ/mol

3. Calculate the enthalpy change using the formula:

   ΔH = ΣΔH_f°(products) - ΣΔH_f°(reactants)

4. Plug in the values:

   ΔH = [(-393.5) + 2(-241.8)] - [(-74.8) + 2(0)]
   ΔH = [-393.5 - 483.6] - [-74.8]
   ΔH = -877.1 + 74.8
   ΔH = -802.3 kJ/mol

Thus, the enthalpy change for the combustion of methane is -802.3 kJ/mol.

Notes

• This negative value indicates that the reaction is exothermic, meaning it releases energy to the surroundings.

Example 2: Formation of Ammonium Chloride

Context

The formation of ammonium chloride (
NH₄Cl
) from its elements is an important reaction in the production of fertilizers and other chemicals. This example highlights an endothermic process.

Example Calculation

1. The balanced chemical reaction is:

   N₂(g) + 2 H₂(g) + Cl₂(g) → 2 NH₄Cl(s)

2. The standard enthalpy of formation values are:

   • ΔH_f° (N₂) = 0 kJ/mol

   • ΔH_f° (H₂) = 0 kJ/mol

   • ΔH_f° (Cl₂) = 0 kJ/mol

   • ΔH_f° (NH₄Cl) = -314.4 kJ/mol

3. Now calculate the enthalpy change:

   ΔH = ΣΔH_f°(products) - ΣΔH_f°(reactants)

4. Plugging in the values gives:

   ΔH = [2(-314.4)] - [0 + 0 + 0]
   ΔH = -628.8 kJ/mol

The enthalpy change for the formation of ammonium chloride is -628.8 kJ/mol.

Notes

• A negative value indicates that the formation of NH₄Cl from its elements is an exothermic process.

Example 3: Dissolution of Sodium Chloride

Context

The dissolution of sodium chloride (
NaCl
) in water is a common endothermic reaction observed in everyday life, such as when adding salt to food or making saline solutions.

Example Calculation

1. The dissolution reaction can be represented as:

   NaCl(s) → Na⁺(aq) + Cl⁻(aq)

2. The enthalpy of dissolution is typically given in literature as:

   • ΔH = +3.88 kJ/mol for NaCl

3. This positive value indicates that energy is absorbed from the surroundings during the dissolution process.

Notes

• While the dissolution of sodium chloride is endothermic, the overall temperature of the solution may not decrease significantly due to the large heat capacity of water.

Conclusion

Understanding the calculation of enthalpy change for reactions is crucial for predicting energy flow in chemical processes. The examples provided illustrate different types of reactions, showcasing both exothermic and endothermic processes. By mastering these calculations, individuals can gain deeper insights into chemical behavior and energy dynamics.