Answer/ explain the following in terms of intermolecular forces and properties of the substances referred to in each situation.
1. Why is it that if a glass is slightly filled above the brim with water, the liquid does not overflow?
2.Induced dipole forces or London force of attraction become very strong between large molecules.
3.The molecular weights of compound A and compound B differ only slightly. The molar heat of sublimation of A is much larger than that of B. How do the intermolecular interactions in A and B compare? Which would you expect to have the higher vapor pressure at room temperature?
4.Ordinary ice (solid water) melts at 0°C, while dry ice (solid carbon dioxide) melts at a much lower temperature.
5. The intermolecular forces in liquid methane is greater than those in liquid argon. Which substance has the HIGHER CRITICAL TEMPERATURE?
1. The reason why water does not overflow when a glass is slightly filled above the brim is due to the cohesive forces between water molecules, known as intermolecular forces. In this case, the dominant intermolecular force is hydrogen bonding. Hydrogen bonding is a strong intermolecular force in which the positively charged hydrogen atom in one water molecule is attracted to the negatively charged oxygen atom of a neighboring water molecule. This creates a network of hydrogen bonds that hold the water molecules together.
When water is slightly filled above the brim, the cohesion between water molecules is strong enough to overcome the force of gravity acting on the extra volume of water. As a result, the water forms a concave meniscus at the top of the glass, rather than overflowing. The surface tension, which is a result of the cohesive forces, helps maintain the water within the glass.
2. Induced dipole forces, also known as London dispersion forces, are weak intermolecular forces that arise from temporary fluctuations in electron distribution within molecules. These forces can become stronger between larger molecules because larger molecules have more electrons, resulting in larger and more polarizable electron clouds. When these large molecules come into close proximity, the temporary dipole moments that occur due to electron fluctuations induce temporary dipoles in neighboring molecules, leading to attractive forces between them.
3. The molar heat of sublimation refers to the amount of heat required to convert one mole of a substance from the solid phase to the gaseous phase, without passing through the liquid phase. The larger molar heat of sublimation in compound A compared to compound B suggests that compound A has stronger intermolecular interactions.
Stronger intermolecular interactions usually correspond to higher boiling points and lower vapor pressures at a given temperature. Therefore, compound A would be expected to have a higher boiling point and lower vapor pressure at room temperature compared to compound B.
4. The reason why ordinary ice (solid water) melts at 0°C, while dry ice (solid carbon dioxide) melts at a much lower temperature is due to the differences in intermolecular forces between the two substances.
Ordinary ice melts at 0°C because the intermolecular forces between water molecules, especially hydrogen bonding, are weakened as the temperature increases. At 0°C, the thermal energy (heat) supplied to the ice is sufficient to break the hydrogen bonds and convert the solid water into liquid water.
In contrast, dry ice melts at a much lower temperature of -78.5°C because the intermolecular forces in solid carbon dioxide are primarily composed of London dispersion forces. These forces are generally weaker than hydrogen bonding and require lower temperatures to be overcome. Therefore, dry ice can undergo sublimation (direct conversion from solid to gas) at much lower temperatures.
5. The intermolecular forces in liquid methane are greater than those in liquid argon. The strength of intermolecular forces generally increases with increasing molecular size and polarity. Methane (CH4) is a small polar molecule, while argon (Ar) is a larger nonpolar molecule.
The critical temperature is defined as the temperature above which a substance cannot exist in the liquid phase, regardless of the pressure applied. It reflects the strength of intermolecular forces in a substance. Because liquid methane has stronger intermolecular forces compared to liquid argon, it would have a higher critical temperature.