What is the decreasing order of boiling points for the following liquids: CO2, H2O, NH3, SO2?
A. CO2 > SO2 > NH3 > H2O
B. NH3 > CO2 > SO2 > H2O
C. H2O > SO2 > CO2 > NH3
D. H2O > NH3 > SO2 > CO2
The Correct Answer and Explanation is:
The correct answer is D. H2O > NH3 > SO2 > CO2.
To understand the decreasing order of boiling points for the given substances—carbon dioxide (CO₂), water (H₂O), ammonia (NH₃), and sulfur dioxide (SO₂)—we must consider the intermolecular forces present in each liquid.
- Water (H₂O) has the highest boiling point among these compounds at 100°C (at 1 atm pressure). This is due to its ability to form extensive hydrogen bonds between its molecules. Hydrogen bonding occurs when hydrogen is bonded to highly electronegative atoms like oxygen, resulting in strong attractions that require significant energy to overcome during the phase change from liquid to gas.
- Ammonia (NH₃) has a boiling point of -33°C. Like water, ammonia also exhibits hydrogen bonding; however, it is weaker than that in water due to the lower electronegativity of nitrogen compared to oxygen. Consequently, while NH₃ does have a higher boiling point than substances that exhibit only London dispersion forces, it is lower than that of water.
- Sulfur dioxide (SO₂) has a boiling point of -10°C. The intermolecular forces present in SO₂ are primarily dipole-dipole interactions, as the molecule is polar. While these interactions are stronger than the London dispersion forces present in nonpolar molecules, they are generally weaker than hydrogen bonds. Thus, SO₂ has a higher boiling point than CO₂ but lower than NH₃ and H₂O.
- Carbon dioxide (CO₂) has a boiling point of -78°C and is a nonpolar molecule. The intermolecular forces in CO₂ are limited to London dispersion forces, which are the weakest type of intermolecular interaction. This results in CO₂ having the lowest boiling point among the four substances.
In summary, the boiling points are primarily influenced by the type and strength of intermolecular forces present in each substance, leading to the order: H₂O > NH₃ > SO₂ > CO₂.