The substance with the largest heat of vaporization is
A) Br2
B) Cl2
C) F2
D) 12
E) O2
The correct answer and explanation is :
The correct answer is D) I₂ (Iodine).
Explanation:
The heat of vaporization (also called enthalpy of vaporization) is the amount of energy required to convert a substance from a liquid (or solid, in the case of sublimation) to a gas at constant pressure. The magnitude of this value depends on the intermolecular forces within the substance. Stronger intermolecular forces result in a higher heat of vaporization.
1. Nature of Intermolecular Forces in the Given Substances
The substances listed in the options (Br₂, Cl₂, F₂, I₂, and O₂) are all diatomic molecules with nonpolar covalent bonds. Their intermolecular forces are London dispersion forces (LDFs), which arise due to temporary dipoles created by electron movement. The strength of these dispersion forces depends on the molecular size and polarizability—larger atoms/molecules have stronger dispersion forces because their electrons are more easily distorted.
2. Trends in Heat of Vaporization
- Fluorine (F₂) and Oxygen (O₂): These molecules are small, have weak dispersion forces, and therefore have low heats of vaporization.
- Chlorine (Cl₂): Larger than F₂ and O₂, meaning stronger dispersion forces and a higher heat of vaporization.
- Bromine (Br₂): Even larger than Cl₂, so it has stronger dispersion forces and a higher heat of vaporization.
- Iodine (I₂): The largest molecule in the group, with the strongest dispersion forces and the highest heat of vaporization.
Since I₂ has the largest molecular size and strongest intermolecular forces, it requires the most energy to transition into the gas phase. Therefore, I₂ has the largest heat of vaporization among the given choices.
