Question

Which of the following molecules has the highest standard enthalpy change of fusion (\( \Delta_{fus}H^\circ \)) (in kJ mol\(^{-1}\))?

• A. H\(_2\)O
• B. CO
• C. C\(_6\)H\(_6\)
• D. CCl\(_4\)
• E. NaCl

Hint:

Ionic compounds like NaCl tend to have the highest enthalpy change of fusion because their strong ionic bonds require more energy to break.

Answer 1

Answer: (E)

The standard enthalpy change of fusion (\( \Delta_{fus} H^\circ \)) refers to the heat absorbed when one mole of a substance undergoes fusion (melts) at constant pressure.
- H\(_2\)O: Water has a relatively moderate \( \Delta_{fus} H^\circ \), as its intermolecular hydrogen bonding makes it require a significant amount of heat to melt, but it is not the highest among the options listed.
- CO: Carbon monoxide has a low \( \Delta_{fus} H^\circ \), as its molecular forces are relatively weak, requiring less energy to melt. - C\(_6\)H\(_6\) : Benzene (C₆H₆) has a higher \( \Delta_{fus} H^\circ \) than CO but is still lower than that of ionic compounds.
- CCl\(_4\): Carbon tetrachloride has a moderate \( \Delta_{fus} H^\circ \), though not as high as ionic compounds.
- NaCl: Sodium chloride is an ionic compound, and ionic bonds are very strong. Therefore, NaCl requires a large amount of energy to overcome the lattice energy during fusion, resulting in a higher enthalpy change of fusion compared to the other molecules listed.
Thus, the correct answer is (E) NaCl.