A pentane (C5H12) molecule is nonpolar while water (H2O) is polar. According to this information: a. Pentane has a higher melting point and boiling point than water b. Water has a higher melting point than pentane, but pentane has a higher boiling point than water c. Water has a higher melting point and boiling point than pentane d. Pentane has a higher melting point than water, but water has a higher boiling point than pentane
The Correct Answer and Explanation is:
Correct Answer: c. Water has a higher melting point and boiling point than pentane
Explanation:
To understand why water has a higher melting point and boiling point than pentane, we must consider the nature of intermolecular forces.
1. Molecular Polarity and Intermolecular Forces:
- Water (H₂O) is a polar molecule. The oxygen atom has a partial negative charge, while the hydrogen atoms have partial positive charges, leading to a dipole. More importantly, water molecules can form hydrogen bonds, which are a strong type of dipole-dipole interaction.
- Pentane (C₅H₁₂) is a nonpolar hydrocarbon. Its molecules only exhibit London dispersion forces, which are the weakest type of intermolecular forces. These are temporary, induced dipoles that arise due to momentary shifts in electron clouds.
2. Boiling and Melting Points:
- Boiling Point: The stronger the intermolecular forces, the more energy is required to separate the molecules from each other and convert the substance into a gas.
- Water, due to hydrogen bonding, has a high boiling point of 100°C.
- Pentane has a boiling point of about 36°C, much lower because it only has weak London dispersion forces.
- Melting Point: Similarly, stronger intermolecular forces result in a higher melting point.
- Water has a melting point of 0°C, while pentane melts at about −130°C, again showing water’s stronger bonding.
3. Conclusion:
Water has both a higher melting point and boiling point than pentane because its molecules are held together by hydrogen bonds, which are significantly stronger than the London dispersion forces that hold pentane molecules together. These stronger forces require more energy to overcome, resulting in higher phase change temperatures for water.
Thus, the correct answer is c. Water has a higher melting point and boiling point than pentane.