HS Chemistry - Intermolecular Forces

Unit Summary

States of Matter

• Intermolecular forces are the forces between molecules that keep them together. While intramolecular forces, the forces inside molecules, are stronger, intermolecular forces are still very important; many physical properties are determined by intermolecular forces. In fact, the greatest difference between the different states of matter is the strength of the intermolecular forces:

  • In a gas, the intermolecular forces are weakest. There is a lot of empty space between the molecules, and they expand to fill the shape and volume of their container. They move around very fast in random directions. The kinetic energies of the particles are very high, while the energies of attraction are very low.
  • In a liquid, the intermolecular forces are stronger than in gases, but weaker than in solids. There molecules are connected, though they can still move around and slide over each other. They take the shape, but do not fill the volume, of their container. The kinetic energies of the particles are close to the energies of attraction.
  • The intermolecular forces are strongest in a solid - so strong that they keep the molecules in fixed positions. Thus, the molecules only vibrate, and do not take the shape or fill the volume of their container. The kinetic energies of the particles are low, while the energies of attraction are high.
  • The kinetic energies of the particles also affect the rate of diffusion. If the particles can move fast, then diffusion can occur quickly.

Types of Intermolecular Forces

• There are four types of intermolecular forces:

  • Dispersion forces $aka London dispersion$ are induced-dipole forces between non-polar molecules caused by the random rotations of electrons. When the electrons get in the right position, they repel each other, and the sides of the molecules facing each other are temporarily given opposite charges. They thus attract, but this attraction is broken quickly as the electrons continue their orbits. Molecules that are bigger in size will cause more dispersion forces to form, as will molecules with more electrons.

  • Dipole-dipole forces are permanent dipole forces between polar molecules caused by the fact that both molecules have a polarity greater than 0.5, and thus have a significant dipole. They function the exact same as dispersion forces, differing only in that the molecules' ends will retain their charges even after the dipole-dipole bonds have been broken, as they were already polar.

    • If 2 molecules have roughly the same size and shape, dipole-dipole interactions will most likely be the stronger intermolecular force. If they don't, then it will be dispersion forces.

  • Hydrogen bonding is a special type of dipole-dipole force that's unusually strong. It forms when a Hydrogen atom bonded to a N, O, or F atom is brought near another electronegative atom in another molecule. The Hydrogen is attracted to this other electronegative atom.

    • The reason Hydrogen bonds are unusually strong is because the other atom is interacting with a practically bare nucleus. Hydrogen only needs two electrons to complete its outer shell, both of which are closer towards the N, O, or F atom to which Hydrogen is bonded. Hydrogen is left with a slightly positive charge, and no electrons near the end of its atom that's attracted to the other molecule. Thus, the atom from the other molecule is interacting with a practically bare nucleus.

  • Ion-dipole interactions are found in solutions of ionic compounds. When it dissolves, its ions split apart due to ion-dipole interactions. The solvent molecules, which contain some polarity, surround the ionic compound in groups. The negatively charged ions are pulled away towards the positively charged ends of the solvent molecules, and the positively charged ions are pulled away towards the negatively charged ends of the solvent molecules. The ionic compound dissolves.

Properties of Liquids

• There are several properties of liquids that are based on intermolecular forces:

  • Viscosity is how readily something flows. Something that flows less has greater viscosity, and stronger intermolecular forces.

  • Capillary action:

    • Cohesion is how strongly liquid molecules remain attracted to themselves. A more cohesive liquid has stronger intermolecular forces.
    • Adhesion is how strongly molecules remain attracted to another substance. A more adhesive liquid has stronger intermolecular forces.

  • Surface tension is the property of a liquid that allows it to resist an external force due to its cohesion. Stronger intermolecular forces increase surface tension.

  • Vapor pressure is the amount of pressure exerted by gases on a liquid in a closed system. The higher the vapor pressure is, the more frequently the gas particles will collide with the liquid particles. They’ll transfer some of their kinetic energy to the liquids, gradually causing some of the liquid to gain enough energy to become gases. A more volatile liquid will take a shorter amount of time to do this. Stronger intermolecular forces will increase the amount of time this takes and decrease the volatility of the liquid.
  • Miscibility is how easily two liquids mix together. Polar liquid molecules have high miscibility with other polar liquid molecules, as permanent dipole forces $dipole-dipole bonds$ are able to form between the molecules. Nonpolar liquid molecules have high miscibility with other nonpolar liquid molecules, as induced dipole forces $dispersion forces$ are able to form. However, polar liquid molecules do not have high miscibility with other polar liquid molecules, as neither induced dipole nor permanent dipole forces are able to form between them.

The Ideal Gas Law

• Ideal gases have five properties:

  1. A gas is a group of small particles moving randomly.
  1. There are no intermolecular forces of attraction between the particles.
  1. The particles are so small, and the distance between them so great, that the size of the particles can be ignored - we can assume it is 0.
  1. There is no energy lost when the particles collide with each other or the walls of the container.
  1. All particles have the exact same kinetic energy $all particles have the exact same thermal energy$/

• Gases that we see in reality (real gases) do not perfectly match all of these properties, but they come really close to doing so. Some do it better than others.

• The ideal gas law is PV = nRT

  • P = pressure $atm$
  • V = volume $Liters$
  • N = number of moles
  • R = Universal Gas Constant $0.0821 \* \(atm L$/$mol K$)
  • T = Temperature $K$

• To complete the equation, all the values must first be converted to the right units.

• There are two scenarios when the ideal gas law doesn't work:

  • When the pressure is very high.
  • When the temperature is very low.