The molecular formula and molar mass of two straight-chain hydrocarbons are listed in the table. [VI — Bonding Chemistry Question

STEPS:

1. Analyze the chemical structures of both compounds: Ethane ($\text{C}_2\text{H}_6$) and butane ($\text{C}_4\text{H}_{10}$) are both straight-chain hydrocarbons (alkanes). Because carbon and hydrogen have very similar electronegativities, all $\text{C}-\text{H}$ and $\text{C}-\text{C}$ covalent bonds are nonpolar. Therefore, both molecules are entirely nonpolar.
2. Identify the type of intermolecular forces (IMFs) present: Since both molecules are nonpolar, they lack permanent dipoles and cannot participate in dipole-dipole attractions or hydrogen bonding. The only intermolecular forces holding these molecules together in the liquid phase are London dispersion forces (LDFs).
3. Compare molecular sizes and electron count to determine polarizability:
* Ethane ($\text{C}_2\text{H}_6$) has a molar mass of $30\text{ g/mol}$ and contains 18 electrons.
* Butane ($\text{C}_4\text{H}_{10}$) has a molar mass of $58\text{ g/mol}$ and contains 34 electrons.
* London dispersion forces depend on the polarizability of a molecule's electron cloud. Because butane is a larger molecule with significantly more electrons, it has a larger, more easily distorted electron cloud. This greater polarizability leads to stronger temporary dipoles and thus stronger London dispersion forces.
4. Relate IMF strength to boiling point: The boiling point of a substance is a measure of the thermal energy required to overcome intermolecular attractions and separate the molecules during vaporization. Because butane's molecules are held together by stronger dispersion forces than those of ethane, butane requires more energy to boil, giving butane ($\text{C}_4\text{H}_{10}$) the higher boiling point, which corresponds to Option B.

*

WHY_OTHERS_WRONG:

• Option A is incorrect: Although butane has more hydrogen atoms than ethane, neither hydrocarbon is capable of hydrogen bonding. Hydrogen bonds only form when a hydrogen atom is covalently bonded to a highly electronegative atom ($\text{N}$, $\text{O}$, or $\text{F}$), none of which are present in these molecules.
• Option C is incorrect: Although ethane molecules are smaller, their smaller size and fewer electrons make their electron clouds much less polarizable. This results in weaker dispersion forces and a lower boiling point compared to butane, not a higher one.
• Option D is incorrect: This option incorrectly claims that ethane is polar. Both ethane and butane are symmetric, nonpolar hydrocarbons and do not possess any permanent dipole-dipole attractions.