Lecture 07 intermolecular forces

Intermolecular Forces Intramolecular forces bonding forces exist within molecules and influence the chemical properties.

Lecture 07 intermolecular forces

Intermolecular Forces Intramolecular forces bonding forces exist within molecules and influence the chemical properties. Intermolecular forces exist between molecules and influence the physical properties.

We can think of H2O in its three forms, ice, water and steam. In all three cases, the bond angles are the same, the dipole moment is the same, the molecular shape is the same and the hybridization of the oxygen is the same.

However, the physical properties of H2O are very different in the three states. As solid ice, H2O possesses a definite shape and volume.

Liquid water possesses a definite volume, but will assume the shape of its container. It is slightly compressible. Steam will assume both the shape and volume of its container and is extremely compressible. Intermolecular forces IMF are the forces which cause real gases to deviate from ideal gas behavior.

They are also responsible for the formation of the condensed phases, solids and liquids. The IMF govern the motion of molecules as well. 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.

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. 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.

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.All blood vessels leading from the heart are called arteries or the blood vessels which carry blood from the heart to various body organs are called arteries.

Nanomechanics of Materials and Biomaterials Thursday 03/08/07 Prof. C. Ortiz, MIT-DMSE LAST LECTURE: INTRODUCTION TO INTRA- AND INTERMOLECULAR FORCES. last lecture: introduction to intra- and intermolecular forces (within individual molecules) (between individual molecules)→ no real physical difference -Definitions: Interaction (more general), force (push or pull), bond (the attraction between atoms in a molecule or.

Lecture 07 intermolecular forces

Aims This meeting aims to gather key participants representing the full scientific scope of the topic, including the developers of methods and software, those developing the application of the methods and interested experimentalists who may benefit from advances in predictive computational methods.

 Intermolecular Forces By Lesley Myrick 1. Determine the electronegativity between the atoms of each molecule. Surface tension can be defined in terms of force or energy. In terms of force: surface tension γ of a liquid is the force per unit length. In the illustration on the right, the rectangular frame, composed of three unmovable sides (black) that form a "U" shape, and a fourth movable side (blue) that can slide to .

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