Weak bonds range from about 10% as strong as a covalent or ionic bond to <1% as strong. Note the examples in the table below. The polar bonds (charge-dipole and dipole-dipole) are for your information. We will focus on van der Waals (* in table) and hydrogen bonds, returning to hydrogen bonding later when we look at water in depth.
-
Interaction Type Example Average Strength, kcal/mol (kJ/mol) Range** Dipole-dipole 
1/r3 Dipole-induced dipole* 
0.1-0.2 (0.4-4) 1/r6 -
Dispersion* (induced dipole-induced dipole)
0.1-0.2 (0.4-4) 1/r6 Hydrogen bond 
3-8 (12-30) van der Waals repulsion 1/r12 *van der Waals interactions, **from Zubay Biochemistry 3rd. Table 4.3, pg. 89. -
van der Waals bonds are strictly the dipole-induced dipole and dispersion types, but the term is also often used to refer to other weak bonds other than hydrogen bonds. Notice that the bonds are not only very weak (about 0.1 - 0.3% as strong as a covalent bond), they also do not act at a distance. Essentially they are contact bonds - they sort of act like "velcro" or weak tape. The corollary is that they increase in importance with increases in molecular size.
Thus for hydrocarbons, which are essentially completely non-polar, we see a very low boiling point for methane (CH4) of - 161°C and a fairly regular increase in boiling point as carbons are added (ethane, C2H6 - 88°C; butane, C4H10 - 0.5°C; hexane, C6H14 69°C; octane, C8H18 126°C; etc.) until very large molecules such as paraffin (about 100 C's) and polyethylene (>1,000 C's) are essentially non-volatile. Note also though, in these very large molecules the masses are very large making them non-voltile as well as the forces holding the substance together have become significant due to the very large contact areas.
Hydrogen bonds are a special case of weak bonds. Note that they are significantly stronger (>100 fold) than the other weak bonds at about 4-10% as strong as a covalent bond. Hydrogen bonds only occur when a hydrogen bound to a small, very electronegative atom is brought close to another small, very electronegative atom, such oxygen in water. Essentially this means that we only see hydrogen bonds between hydrogens bound to N, O, or F (second Period electronegative elements) and N, O, or F. So we can have O-H O, O-H N, O-H F, N-H O, N-H N etc. hydrogen bonds. This is because hydrogen bonds involve dipole-dipole interactions, but they also have covalent character (about 10% of the sharing we see in true covalent bonds) which requires that the participating atoms be small enough to get close enough to allow such partial sharing.
Hydrogen bonding accounts for much of the special properties of water, such as its very high boiling point (261°C higher than methane with only a 10% increase in MW), high viscosity (its 'thick'), high heat capacity etc. which in turn are due to the strong bonds between the individual molecules so they stick together.
Particles, Moles and Mass
Review Formulae, FW's, and MW's:
Recall that formula weights, atomic weights and molecular weights may all be expressed in two distinct ways:
- amu's/particle for a single atom, molecule or "formula unit," or
- they may be expressed in grams/mole for macroscopic quantities.
Recall also some important terms for describing substances:
- Empirical Formula: the smallest whole number ratio of atoms in a compound.
- Formula weight (FW): the sum of the atomic weights of the atoms in a formula. Note that no specific particle is implied. A formula weight could correspond to a molecule, or it could correspond to a crystal of ions, or it could correspond to an association of different particles such as water molecules hydrating ions or molecules etc., or it could correspond to the smallest repeating unit in a polymer.
- Molecular weight: the sum of the atomic weights in a single molecule of covalently bonded atoms.
- Examples:
- % quantities:
- What is the concentration of iron in iron(II) sulfate
- How many mg Fe in a 312 mg ferrous sulfate tablet)?
Masses and particles:
- What is the weight of 0.25 mole of:
- KBr?
- glucose (C6H12O6)?
- ethanol (CH3CH2OH)?
- How many moles are there in 23.5 g of:
- sucrose
- NaCl
- propane (C2H6)
- How many atoms are there in 1.00g of:
- KBr?
- glucose?
