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:
The correct answer is: c. Water has a higher melting point and boiling point than pentane.
Explanation:
The physical properties such as melting point and boiling point of substances are largely influenced by the intermolecular forces between their molecules. In this context, we are comparing water (H₂O) and pentane (C₅H₁₂).
1. Polarity and Intermolecular Forces:
- Water is a polar molecule due to the bent shape of the molecule and the difference in electronegativity between hydrogen and oxygen. Because of this polarity, water molecules are held together by hydrogen bonds, which are among the strongest types of intermolecular forces.
- Pentane, on the other hand, is a nonpolar molecule. The carbon-hydrogen bonds and the overall symmetrical arrangement of the molecule cause it to lack polarity. As a result, the only intermolecular forces present in pentane are London dispersion forces, which are much weaker than hydrogen bonds.
2. Melting and Boiling Points:
- Substances with stronger intermolecular forces generally require more energy to overcome these forces and undergo phase changes like melting or boiling. This leads to higher melting and boiling points.
- Since water forms hydrogen bonds and pentane only exhibits weak dispersion forces, water has both a higher melting point and a higher boiling point than pentane.
3. Empirical Data Supports This:
- Melting Point of Water: 0°C
- Boiling Point of Water: 100°C
- Melting Point of Pentane: ~−130°C
- Boiling Point of Pentane: ~36°C
This data clearly confirms the conceptual explanation above.
Conclusion:
Due to its strong hydrogen bonding, water has both a higher melting point and boiling point than the nonpolar molecule pentane. Thus, the correct answer is (c).