Understanding the Concepts
* Ionization Energy: The energy required to remove one electron from an atom in its gaseous state.
* Electron Affinity: The energy change that occurs when an electron is added to a neutral atom in its gaseous state.
* Enthalpy of Formation: The enthalpy change when one mole of a compound is formed from its elements in their standard states.
Steps
1. Write the balanced chemical equation:
Na(g) + 1/2 Cl₂(g) → NaCl(g)
2. Identify the processes involved:
* Ionization of sodium: Na(g) → Na⁺(g) + e⁻ (requires +496 kJ/mol)
* Dissociation of chlorine: 1/2 Cl₂(g) → Cl(g) (this requires energy, but we don't need to calculate it for this specific problem)
* Electron affinity of chlorine: Cl(g) + e⁻ → Cl⁻(g) (releases -349 kJ/mol)
* Formation of NaCl: Na⁺(g) + Cl⁻(g) → NaCl(g) (this is the enthalpy of formation we want to find)
3. Apply Hess's Law:
Hess's Law states that the enthalpy change for an overall reaction is the sum of the enthalpy changes for each individual step.
Therefore, the enthalpy of formation of NaCl can be calculated as:
Enthalpy of formation (NaCl) = Ionization energy (Na) + Electron affinity (Cl)
4. Calculate:
Enthalpy of formation (NaCl) = +496 kJ/mol - 349 kJ/mol = +147 kJ/mol
Conclusion
The enthalpy of formation of sodium chloride (NaCl) is +147 kJ/mol. This means that the formation of one mole of NaCl from its elements in their standard states requires 147 kJ of energy.
