Questions 26-28 refer to the following information. The structure of haloacetic acids, XCH2COOH (whe — States of Matter Chemistry Question

STEPS:

1. Relate molecular structure to boiling point: The boiling point of a covalent molecular compound depends on the overall strength of the intermolecular forces (IMFs) holding its molecules together. The weaker these intermolecular attractions are, the less energy is required to separate the molecules, resulting in a lower boiling point.
2. Compare the overall structures and dipole moments: Both chloroacetic acid and iodoacetic acid share the same basic molecular skeleton ($\text{XCH}_2\text{COOH}$, where $\text{X}$ represents the halogen substituent). Because they have the same shape, their permanent molecular dipoles are highly similar, meaning their dipole-dipole forces are of comparable strength.
3. Compare the size and electron shell configuration of the halogen substituents: The chlorine atom ($\text{Cl}$) resides in period 3 of the periodic table, whereas the iodine atom ($\text{I}$) resides in period 5. Therefore, the chlorine atom has fewer occupied electron shells than the iodine atom.
4. Determine electron cloud polarizability: Because it has fewer shells, the chloroacetic acid molecule possesses a smaller, less polarizable electron cloud compared to the larger iodoacetic acid molecule.
5. Relate polarizability to the strength of London dispersion forces: Since London dispersion forces scale directly with electron cloud size and polarizability, the London dispersion forces among chloroacetic acid molecules are weaker than those among iodoacetic acid molecules.
6. Identify the compound with the lower boiling point: Because chloroacetic acid exhibits weaker London dispersion forces, its molecules are held together less tightly in the liquid state. This results in a lower boiling point for chloroacetic acid, which corresponds to Option A.

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WHY_OTHERS_WRONG:

• Option B is incorrect: While this option correctly identifies chloroacetic acid as having the lower boiling point, it incorrectly attributes this difference to weaker dipole-dipole forces. Because the two haloacetic acids share the same structural geometry and similar dipole moments, their dipole-dipole forces are virtually equal; the primary difference in their boiling points is instead governed by London dispersion forces.
• Option C is incorrect: This option falsely claims that iodoacetic acid has a lower boiling point than chloroacetic acid. Due to the larger number of occupied electron shells in iodine, iodoacetic acid has a larger, more polarizable electron cloud, resulting in stronger London dispersion forces and a *higher* boiling point.
• Option D is incorrect: This option incorrectly names iodoacetic acid as having the lower boiling point and incorrectly identifies dipole-dipole forces as the determining factor. As established, chloroacetic acid has the lower boiling point due to its weaker London dispersion forces.