SCH 3UName: ______

Unit 1: Matter, Chemical Trends, Chemical BondingDate: ______

Intramolecular Bonding

  • bonding occurring within molecules (bonding between atoms)
  • is very strong and hard to break
  • Types:
    a) non-polar covalent bonds – occur between two non-metallic atoms with an
    electronegativity difference of zero
    b) polar covalent bonds – occur between two non-metallic atoms that generally have an
    electronegativity difference greater than zero less than or
    equal to 1.7, where the larger the value, the more polar the bond. Electronegativity differences from 0 to 0.4 generally correspond to bonds that are very slightly polar covalent.

Intermolecular Forces

  • These are forces of attraction between neighbouringmolecules. The neighboring molecules might be 2 identical molecules or they might be two non – identical molecules. Often, if the molecules are not identical, one of them is water.
  • These are the forces that are overcome when a molecular substance changes state
    (e.g. melting, boiling) and are also the forces involved in determining the solubility of the substance in a solvent.
  • These forces are MUCH weaker than covalent bonds.
  • The type of intermolecular force existing between neighbouring molecules depends on the
    polarity of those neighboring molecules.

Determining whether a MOLECULE is polar or non-polar
  • Molecules containing only non-polar covalent bonds are always non-polar molecules.
  • Molecules containing slightly polar covalent bonds or more polar covalent bonds may end up being polar molecules but may also end up being non-polar molecules. This is because sometimes the geometry of a molecule makes the bond dipoles “cancel” one another out. You will learn more about how this works in grade 12. For grade 11, assume polar molecules are as listed in Table 3 page 85 of Chemistry11, Nelson copied for you below.

  • Practice pg. 88 #8,9, Chemistry11.

8a)
H
H C O
H
H / d)

b) .. ..
: I – I:
.. .. / e)

c) ..
H : Br :
.. / f)

Types of Intermolecular Forces:
a) van der Waals Forces (also called London Dispersion Forces)
- weakest attractive force between molecules
- occur between any and all types of neighbouring molecules (both polar and non-polar) but are often
reasonably insignificant forces when dipole – dipole or H – bonding is around
- are the ONLY attractive force existing between neighbouring non-polar molecules

- will increase in strength as the size of the molecule or atom involved increases

- These forces are believed to occur when temporary displacements of the “electron clouds” around atoms in a molecule result in very short lived, temporary dipoles between neighbouring atoms. These temporary dipoles allow atoms in neighboring molecules to have a weak attraction for one another.

- Non-polar molecules for which this is the only force of attraction will end up being soft as solids and will have low melting and boiling points.

Example: F2(g) is a non-polar molecule (containing non-polar covalent bonds) yet there
is some force of attraction between molecules of F2(g)…Because the F2(g) molecule is smaller than the Cl2(g) molecule, these forces of attraction will involved more electrons in Cl2 and therefore, will be stronger in Cl2 than in F2. Similarly, CH4 and CH3CH2CH2CH3 are both non-polar molecules. The van der Waals forces between neighboring molecules of the larger molecule will be stronger than for the smaller molecule. The smaller molecule thus has a lower boiling point.

b) dipole-dipole attractions
- mid-strength attractive force between neighbouring molecules that have permanent dipoles
e.g. HCl molecules

c) hydrogen bonding
- a special type of dipole-dipole attraction that occurs between molecules containing the very
polar covalent bonds that exist between H and O; H and F and/or H and N

- molecules feeling this type of attractive force between them will have higher melting and
boiling points than those with regular dipole-dipole attractions or with only van der Waals
forces as hydrogen bonding is the strongest of the intermolecular forces

SCH 3UName: ______

Unit 1Date: ______

Topic: Intermolecular Forces

Strength of Intermolecular Forces:

- The relative strengths of Intermolecular Forces are:
van der Waals or London Forces < dipole-dipole attractions < hydrogen bonding

Intermolecular Forces and Properties of Molecules

  • The properties of molecules are much more variable than those of ionic compounds due to the various types of intermolecular forces present. Most ionic compounds are solids at room temperature, but molecules may be solids, liquids or gases, depending on the type(s) of intermolecular forces present.
  • The stronger the intermolecular forces, the higher the melting and boiling points of the molecule because more energy is required to overcome the attractive forces and change state.
  • Remember: The larger the molecules involved, the more electrons there are to displace, so the greater the van der Waals or London Force between them.
  • Note that covalent bonds WITHIN molecules are not affected by changes of state. Example: To make H2O(l) into H2O(g) one must overcome the strong hydrogen bonding attractive force between molecules. The H and O atoms remain covalently bonded together, however. This means that the covalent bonds in the H-O-H configuration do not break, spread out or change in any way during the change of state.

So, as a result of intermolecular forces:

  • Most molecules have relatively low or intermediate melting and boiling points compared to ionic compounds.
  • Most molecules do not conduct electricity under any conditions (so they are generally notelectrolytes Acids are an exception to this rule, though, because they ARE electrolytes). [An electrolyte is defined as any substance which produces ions in aqueous solution and can therefore conduct electricity in aqueous solution. Acids do this.]
  • Because of their lower boiling and melting points, most molecules are soft, waxy or flexible compared to ionic solids which are often brittle and crystalline.
  • Water is a polar molecule.Substances soluble in water are usually also polar molecules or they are ionic compounds. [This is called the “like dissolves like” rule.] This means that in terms of water solubility for molecules,: molecules with hydrogen bonds between them are the most water soluble followed by molecules with dipole-dipole attractions and finally by molecules with only London dispersion or van der Waals forces between neighbors.

In Comparison to Molecules,Here are Some Properties of Ionic Compounds

  • Simple ionic compounds are formed when a positively charged metal ion (cation) and a negatively charged non-metal ion (anion) are held together by the electrostatic attraction between + and -. This attraction is called an ionic bond. Generally, these ionic bonds occur between ions held together in a rigid repeating crystalline structure called a lattice structure.

/ In this diagram, you can “see” the ionic bonds between sodium and chloride ions as sticks. The overall repeating structure of the ionic crystal sodium chloride (table salt) and many other ionic solids make is called a crystal lattice structure. Because the ions are held in rigid alignment,
- they cannot move around to carry electric charges so the solid cannot conduct electricity;
- it takes a lot of energy to “free” the ions by melting the solid and breaking the ionic bonds in the lattice structure so the ionic solid has a very high melting point (compared to a molecule) and it will be able to conduct electricity in the molten (liquid) state once the crystal lattice is destroyed.
/ The end -of polar water molecules attracts the Na1+ ions and the + end of the water molecules attracts the Cl1- ions. We say thatsolid sodium chloride dissociates in water. Most ionic compounds dissociate in water to at least a certain extent, so we say that most have a reasonably high water solubility. The dissociation process “frees” the ions from the NaCl(s) so that they can move around and conduct electric charge. Because NaCl(s) dissociates in water to produce free ions in solution that can conduct an electric charge, NaCl(s) (and most other ionic solids) is called an electrolyte.
Practice
1. Complete the last column in your model building lab. Hand it in, please. Be sure you hand in the answers to the questions at the bottom of the back page, too.
/

7. Go back to your answers for #6 above.
a) Identify the molecules in the group of substances
from #6 above.
b) Divide the molecules you identified in 7a) into two
groups: non – polar molecules and polar molecules.
8. The differences in the properties of compounds often reflect the type of chemical bonding in the compounds. This is especially true for molecular compounds composed of similar atoms. Examine the following data for two such compounds and then answer the questions below.
Compound / Boiling Point (oC) / Water Solubility at 20oC
CH3Cl / – 24.2 / About 400 mL in 100 g of water
CH4 / – 161.5 / negligible
a)Draw a Lewis dot diagram for each molecule.
b)Compare the polarity of the C – H bond with the C – Cl bond.
c)Identify each molecule as either polar or non – polar.
d)Explain the differences in the properties of the two molecules as listed in the table above based on molecular polarity and intermolecular forces.

8a) Place the molecules in each of the following groups (group i and group ii) in order from the one with
the highest boiling point to the one with the lowest boiling point. Give an explanation for your
ranking.

i) F2, Cl2, Br2 ii)

b) Water is a very polar molecule. Ionic compounds will be attracted to the partial charges on the atoms in water and, as a result, will leave their crystal lattice and dissolve in water. The more polar a molecule is, the more likely its partial charges will be attracted to the partial charges on water and so the more soluble in water the molecules will be.

  • Go back to the diagrams in part ii) above and identify the molecule with the highest solubility in water. Explain your choice.
  • Is it expected that any of the molecules in 8ai) above will be water soluble? Explain.

9. Table salt (NaCl(s)) and sugar (sucrose, C12H22O11) are both white solids that dissolve in water. One is a molecular solid and one is an ionic compound. Describe two laboratory procedures you could use to identify unmarked samples of these substances. For each procedure, describe the results you would expect to observe. (Note that for safety reasons, “taste testing”, is not an acceptable lab procedure.)