Comparing the Properties of Ionic and Molecular Compounds (2.3)
Melting Points and Boiling Points of Compounds
▪ reveals information about the strength of attractions that are holding the particles of the compound
together
Melting Point
▪ temperature at which it goes from solid to liquid at standard pressure
▪ in a solid the particles are so strongly attracted to one another that they cannot pull apart (think of
it like particles held by springs, fig 2.24)
▪ particles in a solid are always vibrating (kinetic energy)
▪ as energy enters a substance the kinetic energy increases, thus the temperature increases
▪ once the particles have enough kinetic energy to pull away from each other, the substance melts
▪ high mp means the attraction between particles must be strong (between ions in ionic compounds
and between molecules in molecular compounds)
Boiling Point
▪ temperature at which it goes from liquid to gas at standard pressure
▪ in a liquid the particles can pull away from each other, but will then be attracted by another particle
(slide past each other)
▪ at the boiling point the particles have enough kinetic energy to completely pull apart (they bounce
off each other rather than sticking)
▪ high bp means the attraction between the particles in the liquid must be strong
**Note that when a molecular compound boils or melts the covalent bonds stay intact
Forces Between Particles in a Compound
▪ table 2.12, 2.13
▪ intermolecular forces: forces between molecules
Compounds with High MP and BP
▪ ionic
▪ ionic bonds are very strong
▪ fig 2.25 shows the crystal structure of an ionic compound
▪ each ion is strongly attracted to the adjacent ions (6 of them)
▪ the formula is an indication of the ratio of ions
Compounds with Intermediate MP and BP
▪ molecular with one or more polar bonds
▪ the compounds may be overall polar (fig 2.26)
▪ dipole: molecule with a slightly positive end and a slightly negative end
▪ these molecules are held together by dipole-dipole forces (weaker than ionic), fig 2.27
Compounds with Low MP and BP
▪ non-polar (methane is non-polar because it is symmetrical)
▪ weak attractions between molecules
Solubility in Water
▪ very important property – for example, vitamins, minerals and waste products move throughout the
body by dissolving in water
▪ water is polar
▪ water will easily dissolve most ionic compounds and polar compounds (ie. the negative end of the
water will attract the positive end of the polar compound/positive ion)
▪ most non-polar compounds don’t dissolve in water because the water is more attracted to itself,
than the molecule
Electrical Conductivity
▪ ability of a substance (or object) to allow an electric current to exist within it
▪ the charges in the compound must be able to move independently
▪ solid ionic compounds can’t conduct electricity because the ions are held tightly together, but can
conduct when liquid or aqueous
▪ molecular compounds cannot conduct electricity
Classwork:
Page 82# 7
Chapter 2 Review page 89# 1, 2, 3, 4, 6, 9, 11, 12, 17, 18, 19, 20, 21, 22, 23, 28, 35, 37, 39