the shape of covalent molecules is determined by…
Valence shell electron pair repulsion (VSEPR) Theory
Describe the structure of LINEAR covalent molecules
Central atom forms 2 covalent bonds and no lone pairs.
Bond angles = 180°
Describe the structure of TRIGONAL PLANAR covalent molecules
Central atom forms 3 covalent bonds and no lone pairs.
Bond angles = 120°
Describe the structure of TETRAHEDRAL covalent molecules
Central atom forms 4 covalent bonds and no lone pairs.
Bond angles = 109°
Describe the structure of PYRAMIDAL covalent molecules
Central atom forms 3 covalent bonds and 1 lone pair.
Bond angles = 107°
Describe the structure of BENT covalent molecules
Central atom forms 2 covalent bonds and 2 lone pairs.
Bond angles = 104.5°
Explain how electronegativity and bond shape determine polarity of molecules.
Electronegativity differences cause uneven electron sharing: large differences create polar bonds (δ⁺ and δ⁻), while similar values give less/nonpolar bonds. A molecule’s overall polarity then depends on its shape, since bond dipoles can either cancel out (symmetry) or reinforce each other.
Define dispersion forces
a weak intermolecular force caused by temporary fluctuations in electron distribution in a molecule (e.g. more on one side), which create temporary “dipole moments” that induce dipoles in nearby molecules.
Define dipole-dipole attractions
an intermolecular attraction between the positive end of one polar molecule and the negative end of another polar molecule. Repulsion also occurs when like-charged ends meet.
Define hydrogen bonding (not actually a bond, but attraction)
the strongest form of dipole-dipole attraction that occurs when a hydrogen atom covalently bonded to a highly electronegative atom (N, O, or F) is attracted to a lone pair of electrons on another nearby electronegative atom. (e.g. water)
Define electronegativity
the tendency of an atom to attract shared electrons within a bond.
What three things occur when intermolecular forces in a substance are strong?
High M.P., high B.P., lower vapour pressure
Explain why alkanes (organic C & H molecules) have higher M.P. and B.P. with increased number of C atoms.
As the number of carbon atoms increases, alkanes gain more electrons and surface area. This strengthens dispersion forces, so more energy (higher temperature) is needed to melt or boil them. Though alkanes have a polar O-H end, differences in M.P./B.P. are not due to this since they all have it.
Explain the effect of intermolecular force strength on vapour pressure
Vapour pressure is how strongly a liquid “pushes” to become a gas—the pressure of gas particles above a liquid at a certain temperature. Liquids with stronger intermolecular forces (like hydrogen bonding) hold molecules together more tightly, so fewer escape into the gas phase, resulting in lower vapour pressure. Conversely, weaker intermolecular forces allow molecules to escape more easily, giving the liquid a higher vapour pressure and making it evaporate more readily.
Explain the effect of polarity/intermolecular forces on solubility.
-Polarity affects solubility: “like dissolves like.”
-Polar substances (e.g., water) dissolve other polar substances or ionic compounds because dipole–dipole and hydrogen-bonding forces can interact.
-Nonpolar molecules cannot break the strong dipole–dipole bonds between polar molecules, so they remain separate—e.g., oil and water don’t mix.
-Nonpolar substances can mix with each other easily because only weak dispersion forces need to be overcome.
What 3 things affect gas behaviour?
Pressure, volume, temperature
Explain Gay-Lussac’s Law, and the variables it relates to.
Temperature and pressure (with constant volume)
P1/T1 = P2/T2
P ∝ T (or vice versa)
Therefore, one/the-other = constant.
Explain Charles’ Law, and the variables it relates to.
Temperature and volume (with constant pressure)
V1/T1 = V2/T2
V ∝ T (or vice versa)
Therefore, one/the-other = constant
Explain Boyle’s Law, and the variables it relates to.
Pressure is inversely proportional to volume (with constant temperature).
P1V1 = P2V2
P ∝ 1/V (or vice versa, V∝ 1/P)
Therefore, P/T or T/P = Constant
Explain Avogadro’s Law.
Under equal temp. and pressure conditions, the equal volumes of gas contain the same number of gas particles.
n=V/Vm
Where:
n= no. of moles
V=volume (L)
Vm=molar volume, Vmol^-1
State the ideal gas equation.
PV = nRT
Where:
P = pressure (kPa)
V = volume (L)
n = no. of moles (n)
R = 8.31Jmol^-1K^-1
T = temperature (K)
The basis of collision theory is…
Particles must collide with sufficient energy (greater than activation energy) and correct orientation in order to react.
Explain how temperature, pressure, concentration, surface area and catalysts affect rate of reaction with reference to collision theory.
Temperature: Increasing temperature both gives particles more energy, so more have enough to overcome activation energy and increases movement, so more collisions with chance of correct orientation.
Pressure (gases): Increasing pressure packs particles closer, increasing the frequency of collisions.
Concentration: Higher concentration means more particles in the same space, increasing the frequency of collisions.
Surface area: Increasing surface area exposes more accessible particles to react, increasing the frequency of collisions.
Catalyst: lowers activation energy or aligns reactant orientation by providing alternate reaction pathways, so more collisions are successful.
Rate of reaction =
Δmass or concentration or volume / time (F&D book)
