AP化学真题

AP化学真题解析

The molecular formula and molar mass of two straight-chain hydrocarbons arelisted in the table above. Based on the information in the table, which compoundhas the higher boiling point, and why is that compounds boiling pointhigher?

(A) C4H10, because it has more hydrogen atoms resulting in more hydrogen bonding

(B) C4H10, because it has more electrons, resulting in greater polarizability and stronger dispersion forces

(C) C2H6, because its molecules are smaller and they can get closer to one another, resulting in stronger dispersion forces(D) C2H6, because its molecules are more polar resulting in stronger dipole-dipole attractions

(题目选自2015 AP Chemistry Practice Exam Q11)

解析如下：

本道题目主要考察如何比较有机小分子沸点的高低。

物质的熔沸点，蒸气压等物理性质都是由分子间作用力( IMF, Intermolecular Forces)决定的: IMF越强，熔沸点就越高，所以题目旨在比较两个有机小分子的IMF。

由于二者都是非极性(Non-polar)分子，所以二者分子间作用力都仅有dispersion force，而dispersion force取决于分子的极化率(polarizability)，电子数目越多越容易极化，所以dispersionforce的强弱与非极性分子的大小成正比，所以答案为B。