Identify the correct order of standard enthalpy of formation of halides.

Options

A. `NaI lt NaBr lt NaF lt NaCl`
B. `NaI lt NaBr lt NaCl lt NaF`
C. `NaCl lt NaF lt NaBr lt NaI`
D. `NaF lt NaCl lt NaBr lt NaI`

Answer

To determine the correct order of standard enthalpy of formation of sodium halides (NaX, where X is a halogen), we need to consider the size of the halide ions and the lattice energy associated with the formation of these compounds.

### Step-by-Step Solution:

1. **Understanding Standard Enthalpy of Formation**:
The standard enthalpy of formation (ΔH_f°) is defined as the change in enthalpy when one mole of a compound is formed from its elements in their standard states. For halides, we are looking at the formation of sodium halides (NaX).

2. **Identify the Halides**:
The halides we are considering are:
- NaF (sodium fluoride)
- NaCl (sodium chloride)
- NaBr (sodium bromide)
- NaI (sodium iodide)

3. **Consider the Size of Halide Ions**:
The size of the halide ions increases as we move down the group in the periodic table:
- F⁻ (fluoride) < Cl⁻ (chloride) < Br⁻ (bromide) < I⁻ (iodide)

4. **Lattice Energy and Enthalpy of Formation**:
The standard enthalpy of formation is influenced by the lattice energy, which is the energy released when gaseous ions combine to form an ionic solid. Smaller ions can get closer together, leading to stronger electrostatic attractions and higher lattice energy. Thus, the order of lattice energy (and hence the enthalpy of formation) is inversely related to the size of the ions:
- NaF has the highest lattice energy (most negative ΔH_f°)
- NaCl has a lower lattice energy than NaF
- NaBr has an even lower lattice energy
- NaI has the lowest lattice energy (least negative ΔH_f°)

5. **Order of Standard Enthalpy of Formation**:
Based on the above analysis, the order of standard enthalpy of formation for sodium halides from most negative to least negative is:
- ΔH_f° (NaF) < ΔH_f° (NaCl) < ΔH_f° (NaBr) < ΔH_f° (NaI)

6. **Conclusion**:
Therefore, the correct order of standard enthalpy of formation of sodium halides is:
- NaF < NaCl < NaBr < NaI