Which pair of molecules has the strongest dipole-dipole interactions? COz and COz CHa and CHa NHz and CH, COz and CH: NH, and NH;
The Correct Answer and Explanation is:
Correct Answer: NH₃ and NH₃
Explanation
Intermolecular forces (IMFs) are the attractive or repulsive forces that exist between neighboring molecules. The strength of these forces dictates many of a substance’s physical properties, such as boiling point and melting point. One of the principal types of IMFs is the dipole-dipole interaction, which occurs specifically between polar molecules.
A molecule is considered polar if it possesses a permanent net dipole moment. This arises from two conditions: the presence of polar covalent bonds (due to a significant difference in electronegativity between bonded atoms) and an asymmetrical molecular geometry that prevents these bond dipoles from canceling each other out.
Let’s analyze the molecules in question:
- CO₂ (Carbon Dioxide): The C=O bonds are polar. However, the molecule has a linear and symmetrical shape (O=C=O). The two bond dipoles are equal in magnitude and point in opposite directions, causing them to cancel out. Therefore, CO₂ is a nonpolar molecule.
- CH₄ (Methane): The C-H bonds have a very small electronegativity difference and are considered essentially nonpolar. Furthermore, the molecule has a perfectly symmetrical tetrahedral geometry. Thus, CH₄ is a nonpolar molecule.
- NH₃ (Ammonia): The N-H bonds are highly polar because nitrogen is significantly more electronegative than hydrogen. The molecule has a trigonal pyramidal shape due to the lone pair of electrons on the nitrogen atom. This asymmetry means the bond dipoles do not cancel, giving NH₃ a strong net dipole moment.
Evaluating the pairs, we find that only the NH₃ and NH₃ pair consists of two polar molecules. The partially negative nitrogen atom of one ammonia molecule is strongly attracted to the partially positive hydrogen atoms of a neighboring ammonia molecule. This attraction is the dipole-dipole interaction. In fact, because the interaction involves a hydrogen atom bonded to a highly electronegative atom (N, O, or F), this specific type of dipole-dipole force is known as hydrogen bonding, which is exceptionally strong. The other pairs, composed of nonpolar molecules, only exhibit weak London dispersion forces and thus lack dipole-dipole interactions entirely.thumb_upthumb_down
