Intermolecular Forces & Relative Boiling Points (bp) - Chemistry LibreTexts
Answering Questions about Intermolecular Force Strength If you are asked to rank molecules in order of melting point, boiling point, viscosity, surface tension. between intermolecular forces in a liquid and the liquid's boiling point and critical. It also depends on the strength of intermolecular forces of attraction within. Describe the types of intermolecular forces possible between atoms or molecules in Explain the relation between the intermolecular forces present within a substance .. So the ordering in terms of strength of IMFs, and thus boiling points .
In the gaseous phase, molecules are in random and constant motion. Each gas molecule moves independently of the others. In liquids, the molecules slide past each other freely.
In solids, the molecules vibrate about fixed positions. Heating Curves The transitions between the phases, phase changes, can be viewed in terms of a Heating Curve, like the one shown below, for water. It is a plot of time versus temperature. The time axis represents the addition of heat as a function of time.
The longer the time span, the more heat has been added to the system. In this Heating Curve, we are starting with ice at oC. As we add heat, we raise the temperature of the ice. In the solid phase, the allowed motions are in vibrational movements within the molecules. In the case of water, the O-H bonds are stretching and bending. The bond lengths and angles are oscillating around the predicted values. The amount of heat required to raise the temperature of the ice is determined by the heat capacity of ice, the heat required to change the temperature of 1 gram of ice by 1oC.
The heat capacity of each phase of each substance is unique, and depends on the chemical nature of the substance. When the temperature reaches 0oC, the melting point of ice, further addition of heat does not change the temperature.
At this phase transition temperature, the added energy goes to changing the Potential Energy of the system. It is coulombic in nature, arising from the attraction of charged species. In the case of H2O, it is the attraction between the partial positive charges on the H and the partial negative charges on the O.What Are Intermolecular Forces - Chemistry for All - FuseSchool
As we discussed earlier in the semester, these are hydrogen bonds, holding the water molecules in the crystalline structure of ice. At the phase transition temperature, 0oC, all of the ice will be converted to liquid water. The increase in temperature is, again, an increase in the KE of the system. The movement of the water molecules will increase in the liquid phase.
Effects of Intermolecular Forces - ChemistNATE | Lessons
There is still some degree of hydrogen bonding between molecules, but they are no longer in fixed positions in a crystal lattice. There is a second phase transition at oC. At this temperature, the water, at oC, is converted to steam at oC.
The remaining hydrogen bonds are broken, and all of the water molecules are now moving independently of each other, with no remaining hydrogen bonding.
The liquid water is converted to steam. As soon as this happens, addition of heat raises the temperature of the steam and increases the average kinetic energy of the gas molecules, as predicted by the Molecular Kinetic Theory.
- The Four Intermolecular Forces and How They Affect Boiling Points
Strength of IMF The heat of fusion heat required to melt a solid and heat of vaporization heat required to vaporize a liquid are determined by the strength of the Intermolecular Forces. Substances with high IMF will have higher melting and boiling points. It will require more energy to break the IMF. Think about an atom like argon. The fact that it forms a liquid it means that something is holding it together. Think about the electrons in the valence shell.
But at any given instant, there might be a mismatch between how many electrons are on one side and how many are on the other, which can lead to an instantaneous difference in charge. On average, every player is covered one-on-one, for an even distribution of players. The polarizability is the term we use to describe how readily atoms can form these instantaneous dipoles.
Polarizability increases with atomic size. For hydrocarbons and other non-polar molecules which lack strong dipoles, these dispersion forces are really the only attractive forces between molecules. Since the dipoles are weak and transient, they depend on contact between molecules — which means that the forces increase with surface area.
A small molecule like methane has very weak intermolecular forces, and has a low boiling point. However, as molecular weight increases, boiling point also goes up.
2.11: Intermolecular Forces & Relative Boiling Points (bp)
Therefore, dispersion forces increase with increasing molecular weight. How can a gecko lizard walk on walls? I talked about this in detail previously. The intermolecular forces increase with increasing polarization of bonds.