Intermolecular Forces and Heating Curves

Intermolecular Forces

  • Intermolecular forces: forces that occur BETWEEN the molecules

  • Intramolecular forces: forces that occur INSIDE the molecules

Dipole-dipole: Attractive forces that occur between molecules lining up so that the positive end of one polar molecule is near the negative end of another, resulting in an electrostatic attraction.

  • becomes weaker as the distance between the dipoles increases (becomes unimportant in gaseous phase)

Hydrogen bonding: A special case of dipole-dipole interactions, hydrogen bonds are particularly strong and occur when hydrogen is bonded to highly electronegative elements such as oxygen, nitrogen, or fluorine.

  • boiling point of water is much higher than expected because the large electronegativity value of the oxygen atom compared with other group members causes the O-H bonds to be much more polar.

    • They tend to stay together in the liquid state, even at very high temperatures

London dispersion forces: These are the weakest type of intermolecular force and arise due to temporary dipoles that occur in atoms or non-polar molecules when electrons happen to be unevenly distributed.

  • forces that exist among noble-gas atoms and nonpolar molecules

  • motions of the atom must be greatly slowed down beforet he weak London dispersion forces can lock the atoms into place, forming a solid.

  • becomes more significant when sizes of atoms or molecules increases; larger size means there are more electrons available to form the dipoles.

*POLAR MOLECULES OCCUR WHEN ONE ATOM IS MORE ELECTRONEGATIVE COMPARED TO THE OTHER (partial positive charge on one end and a partial negative charge on the other end.)

  • Dipole - dipole attractions

  • Hydrogen bonding (only when a hydrogen is bonded to O, N, or F)

Water Properties (Solid/Liquid/Gas)

  • It takes more energy to change liquid water into steam at 100 degrees C compared to melting ice to form liquid water

    • Indicate that going from the liquid to the gaseous state involves a much greater change than going from the solid to the liquid. This indicates that solid and liquid states are more similar than liquid and gaseous states.

    • Liquid and solids have very similar densities compared to gases, which have much lower densities, showcasing the distinct differences in intermolecular interactions across the states of matter.

  • Ice has an unusual amount of empty space, so it is less dense than liquid water.

Heating Curves/Phase Changes

Phase Changes

  • 0 degrees C to 100 degrees C: between is liquid

  • 0 degrees: water is a solid (ice)

  • 100 degrees C: water vaporizes

Heating/Cooling Curves

  • Going left to right in the graph means energy is being added (heating)

  • Going from right to left means that energy is being removed (cooling)

DENSITY IS MASS/VOLUME

  • lower density of ice means that ice floats on the surface of lakes as they freeze, providing a layer of insulation that helps prevent lakes and rivers from freezing solid in the winter.