CHEM10007 · Fundamentals Of Chemistry
Intermolecular Forces, Chemical Reactions & the Mole
Two big ideas meet in this chapter. First, intermolecular forces — dispersion, dipole–dipole and hydrogen bonding — explain bulk properties such as boiling point once you can judge a molecule's polarity. Second, the mole concept turns balanced equations into quantities you can weigh: you balance equations, write net-ionic equations for precipitation, and convert between mass, moles and number of particles, then derive empirical formulae from percentage composition.
What this chapter covers
- 01Polarity: polar bonds (electronegativity) vs polar molecules (geometry); why CO2 and CH4 are non-polar
- 02Intermolecular forces ranked weakest → strongest: dispersion < dipole–dipole < hydrogen bonding (H to N, O or F)
- 03IMF strength and bulk properties (boiling point, viscosity)
- 04Chemical vs physical change; writing and balancing equations with state symbols
- 05Precipitation reactions: molecular → complete-ionic → net-ionic; identifying spectator ions
- 06The mole: n = m/M, molar mass M, Avogadro's number to count particles
- 07Percentage composition: %X = (mass of X / mass of sample) × 100
- 08Empirical formula from % composition; molecular formula from M ÷ empirical-formula mass
Molar mass, moles and molecule count
- 1 mark — molar massM = 2 × 14.01 + 4 × 1.008 + 3 × 16.00 = 28.02 + 4.032 + 48.00 = 80.05 g mol−1.
- 1 mark — moles in 3.20 gn = m/M = 3.20 ÷ 80.05 = 0.0400 mol.
- 1 mark — mass of 0.250 molm = n × M = 0.250 × 80.05 = 20.0 g.
- 1 mark — particle countN = n × NA = 0.0400 × 6.0221 × 1023 = 2.41 × 1022 formula units.
Key terms
- Hydrogen bond
- The strongest intermolecular force, between an H atom bonded to N, O or F and a lone pair on N, O or F of a neighbouring molecule. Explains water's high boiling point.
- Dispersion (London) force
- The weakest, universal intermolecular force, arising from instantaneous induced dipoles; it grows with molecular size and is the only IMF in non-polar substances.
- Net-ionic equation
- A reaction equation showing only the species that change, with spectator ions cancelled — e.g. Pb2+(aq) + 2I−(aq) → PbI2(s).
- Spectator ion
- An ion present on both sides of a complete-ionic equation that does not take part in the reaction and is cancelled to give the net-ionic equation.
- Empirical formula
- The simplest whole-number ratio of atoms in a compound, found from percentage composition; the molecular formula is a whole-number multiple of it.
Intermolecular Forces, Chemical Reactions & the Mole FAQ
How do I find the strongest intermolecular force in a substance?
First decide if the molecule is polar. If it has H bonded to N, O or F it can hydrogen bond; if it is polar but cannot, the strongest force is dipole–dipole; if it is non-polar, only dispersion forces act. All molecules have dispersion forces underneath.
What is the difference between molecular, complete-ionic and net-ionic equations?
The molecular equation shows whole compounds; the complete-ionic equation splits all soluble strong electrolytes into ions; the net-ionic equation removes the spectator ions to show only the chemistry that actually happens.
How do I get an empirical formula from percentages?
Assume 100 g, convert each element's mass to moles (÷ molar mass), divide all by the smallest mole value, and scale to whole numbers. Multiply by the molecular-to-empirical mass ratio if a molecular formula is required.
Exam move
Build a reliable polarity-then-IMF decision tree, because IMF ranking is a frequent multiple-choice and short-answer item. Treat balancing and net-ionic equations as a writing skill to rehearse, not just read — examiners want correct state symbols and cancelled spectators. Above all, get fluent moving between mass, moles and particles, since the mole concept is the foundation for the entire stoichiometry chapter that follows.
