Prerequisite: Article on Intermolecular Forces
Welcome back! Now that we have explored Intermolecular forces, we can begin to apply these concepts to the daily behavior of these molecules. The strengths and types of intermolecular forces can determine many characteristics of the molecules, and these effects are what we are exploring today.
EFFECTS ON BOILING/MELTING POINT
A concept to keep clear before we move forward: phase changes such as boiling and melting merely overcome the intermolecular forces between molecules, not the individual bonds themselves. Many times students make the mistake that boiling a substance breaks the bonds that the substance is made up of. However, this is simply not true. For molecules, phase changes between solid, liquids, and gases only overcome the intermolecular forces that keep molecules together. The internal bonds remain intact.
Boiling a substance or melting a substance indicates imparting enough thermal energy to overcome the forces between molecules. How much energy is needed is completely dependent on the strength of intermolecular forces that exist for any given compound.
After we determine the types and strength of the intermolecular forces that a compound has, we can compare them with each other. Greater intermolecular forces indicate a stronger attraction between molecules. It follows that given a molecule with great intermolecular forces, the molecule would require more energy to reach equilibrium between a phase change; thus the molecule would need greater temperatures to reach the boiling points. Therefore:
Molecules with Greater Intermolecular Forces have Higher Boiling or Melting Points.
The liquids are being separated through distillation. The molecule with weaker intermolecular forces above will evaporate at a lower temperature than the other one. Thus we can separate liquids based upon differences in boiling point caused by IMFs.
EFFECTS ON VAPOR PRESSURE
Vapor Pressure can be defined as the pressure a gas exerts on its container when it exists in thermodynamic equilibrium with its more condensed form at a given temperature.
Different compounds have different vapor pressures, and this too is dependent upon the intermolecular forces that exist between molecules. When a system containing a a liquid is sealed, some of the liquid moleucles evaporate into their gas phase. However, in order to evaporate in this manner, they must overcome the intermolecular forces that exist between molecules.
If a molecule has greater intermolecular forces, then it would be more favorable for the molecule to remain in its more condensed, liquid state. However, if the molecule would experience very little intermolecular forces, then it would be much easier to overcome these forces between molecules, thus more molecules would evaporate, and raise the pressure of the container. Therefore:
Molecules with Greater Intermolecular Forces have Higher Vapor Pressures.
EFFECTS ON SURFACE TENSION AND VISCOSITY
Surface Tension can be defined as the extent to which liquids shrink to the minimum surface area possible, or the strength of the surface of the liquid.
Viscosity can be defined as the extent to which liquids resist flow.
Both of these properties are based on intermolecular forces. Intermolecular forces in liquids cause the molecules to be attracted to each other, thus leading to varying amounts of the two properties above.
Increasing intermolecular forces increases the attraction between particles throughout liquids. Near the surface, the tension that is experienced between particles is due to the forces between them, and increasing these forces increases the tension between these particles. This increases the ability of the surface to support suspended material, thus increasing surface tension
Furthermore, the increase in forces between molecules in liquids increase their tendency to interact with each other, thus decreasing the ease to which the molecules can flow past each other. If the molecules have greater difficulty flowing past each other, they have greater resistance to flow. Therefore:
Liquid Molecules with Greater Intermolecular Forces are More Viscous and have Greater Surface Tension
Water has hydrogen bonds, thus it has high intermolecular forces, so it tends to form rounded droplets on surfaces rather than spreading out.
We hope this article illuminates the common effects of intermolecular forces! If you have any comments or concerns, please leave them in the comments below. Check out our other articles. Happy Studying!
Note: The effects of intermolecular forces on deviation from ideal gas behaviour will be addressed in the future. Keep a lookout for gas articles.
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