Chem Unit 2

Intermolecular Force

An attractive force between molecules.

Intramolecular Force

Forces that hold atoms together within a molecule.

Three Main Intermolecular Forces

London dispersion forces, dipole–dipole attractions, and hydrogen bonding.

London Dispersion Forces

Weak attractions caused by temporary fluctuations in electron distribution that create temporary dipoles.

Do all molecules experience London dispersion forces?

Yes.

When do London dispersion forces become stronger?

As the number of electrons increases and as molecular surface area increases.

Why do larger molecules generally have stronger dispersion forces?

They have more electrons, making their electron clouds more easily distorted.

How does shape affect dispersion forces?

Molecules with greater surface contact generally have stronger dispersion forces.

Which has stronger dispersion forces: a long-chain or a highly branched molecule?

The long-chain molecule, due to greater surface contact.

Permanent Dipole

An unequal distribution of charge caused by differences in electronegativity in a polar molecule.

Dipole–Dipole Attractions

Attractions between the oppositely charged ends of neighbouring polar molecules.

Which molecules experience dipole–dipole attractions?

Polar molecules.

Hydrogen Bonding

A strong intermolecular attraction between H bonded to N, O or F and a lone pair on N, O or F in another molecule.

Conditions for Hydrogen Bonding

H covalently bonded to N, O or F, plus a lone pair on N, O or F of another molecule.

Strength Order of Intermolecular Forces

London dispersion < dipole–dipole < hydrogen bonding.

Effect of Stronger Intermolecular Forces on Boiling Point

Boiling point generally increases.

Effect of Stronger IMF on Vapour Pressure

Vapour pressure decreases.

What is volatility?

How readily a substance changes from liquid to gas.

Effect of IMF Strength on Melting Point

Stronger intermolecular attractions generally increase melting point.

What is a Lewis Structure?

A diagram showing valence electrons, bonding pairs and lone pairs in a molecule or ion.

Bonding Pair

A pair of electrons shared between two atoms in a covalent bond.

Lone Pair

A pair of valence electrons not involved in bonding.

VSEPR

Valence Shell Electron Pair Repulsion.

Bond Angle with 2 Electron Regions

Linear, 180°.

Shape with 3 Bonding Regions

Trigonal planar, 120°.

Shape with 4 Bonding Regions

Tetrahedral, 109.5°.

Shape with 3 Bonding Pairs and 1 Lone Pair

Trigonal pyramidal, approximately 107°.

Shape with 2 Bonding Pairs and 2 Lone Pairs

Bent, approximately 104.5°.

Electronegativity

The ability of an atom to attract shared electrons towards itself.

Polar Bond

A covalent bond in which electrons are shared unequally due to a difference in electronegativity.

Non-Polar Bond

A covalent bond in which electrons are shared equally or nearly equally.

What determines whether a molecule is polar?

Both the polarity of its bonds and its molecular geometry.

Can a molecule with polar bonds be non-polar?

Yes, if it is symmetrical, the bond dipoles can cancel.

Why is CO₂ non-polar?

It is linear and symmetrical, so the bond dipoles cancel.

Why is H₂O polar?

Its bent shape means the O–H bond dipoles do not cancel.

Kinetic Molecular Theory

Describes the behaviour of gases in terms of the motion and collisions of their particles.

Assumptions for an Ideal Gas

Particles have negligible volume; negligible intermolecular forces; move randomly; collisions are elastic; average kinetic energy depends on temperature.

What does gas temperature represent?

The average kinetic energy of the particles.

What causes gas pressure?

Collisions of gas particles with the walls of the container.

Why does pressure increase with temperature at constant volume?

Particles move faster, causing more frequent and energetic collisions.

Ideal Gas Equation

PV=nRT.

What does each symbol in PV=nRT represent?

P = pressure; V = volume; n = amount of gas; R = ideal gas constant; T = temperature in kelvin.

pH

A measure of the hydrogen ion concentration of an aqueous solution.

Net Ionic Equation for Neutralisation

H⁺ + OH⁻ → H₂O.

What happens when an acid and base react?

Neutralisation occurs, producing a salt and water.

Molar Concentration Equation

c = Vn.

What occurs during dilution?

The number of moles of solute remains constant; concentration decreases.

Solubility Definition

The maximum amount of a solute that can dissolve in a given amount of solvent at a specified temperature.

What does "like dissolves like" mean?

Polar substances tend to dissolve in polar solvents, while non-polar substances tend to dissolve in non-polar solvents.

Why does water dissolve ionic substances?

Water is polar, so its partial charges can attract and stabilise ions.

What is chromatography used for?

Separating and identifying components of a mixture.

What is Rf?

The ratio of the distance travelled by a substance to the distance travelled by the solvent front.

What does a larger Rf generally indicate?

The substance travelled further with the mobile phase and had a greater relative attraction to it than to the stationary phase.

What is a bond dipole?

A separation of partial positive and negative charge across a polar covalent bond caused by a difference in electronegativity.

What is a net dipole?

The overall dipole of a molecule after all individual bond dipoles are considered together.

What does it mean for a molecule to be symmetrical?

Its atoms and bond dipoles are arranged so that bond dipoles cancel.

What happens to net dipole in a symmetrical molecule?

The bond dipoles cancel, giving no net dipole.

If a molecule is non-symmetrical, what happens to its bond dipoles?

They do not completely cancel, resulting in a net dipole.