lecture one [imfs & liquids] | notes

Describe the particle arrangement and motion in a SOLID.

Particles are tightly packed and usually arranged in a pattern. They vibrate in fixed positions.

Describe the particle arrangement and motion in a LIQUID.

Particles are close together but have no arrangement. They move around in constant contact.

Describe the particle arrangement and motion in a GAS.

Particles are far apart with no arrangement. They move independently of each other unless there is a collision.

What two factors determine whether a substance is a solid, liquid, or gas?

Intermolecular forces (IMF) hold particles together; Kinetic energy (KE) overcomes attractive forces to increase distance between particles.

What happens if temperature is too low or pressure is too high for a liquid?

The kinetic energy of the particles cannot overcome their IMFs, so a solid forms.

What is the difference between "intra" and "inter" prefixes?

"Intra" means within or inside a group (intramolecular = bonds inside a molecule). "Inter" means between groups (intermolecular = forces between molecules).

What are Van der Waals forces (in the context of IMF)?

Attractive forces between neutral atoms and molecules (not for ionic/metallic bonding). Includes London dispersion forces and dipole-dipole interactions.

What causes London dispersion forces?

Constant electron motion creates an instantaneous (temporary) dipole, which can distort a neighbor's electron cloud, causing an induced dipole.

Using melting/boiling point data, which halogen (F₂, Cl₂, Br₂, I₂) has the strongest IMF, and why?

I₂. Larger atoms have more electrons farther from the nucleus that are less tightly held, making them easier to polarize.

How does atom/molecule size affect dispersion forces?

Larger atoms/molecules = more charge separation (higher polarizability) = stronger dispersion forces.

Why do boiling points of noble gases increase from He → Ne → Ar → Kr → Xe?

Size increases down the group, so dispersion forces increase (more charge separation), requiring more energy to overcome IMFs.

Why is CI₄ a solid at room temperature but CH₄ is a gas?

CI₄ has higher molar mass and iodine is larger than hydrogen, making CI₄ more polarizable. More instantaneous dipoles = stronger IMF = higher melting point.

How does molecular surface area affect dispersion forces?

Smaller surface area (more compact) = less intermolecular contact = weaker dispersion forces.

Why does octane have a higher boiling point than isooctane?

Octane is less compact (more surface area), meaning more attractions, so boiling point increases.

What are dipole-dipole interactions?

Attraction between the partially negative end of one polar molecule and the partially positive end of another polar molecule. Occurs only between polar molecules.

Which has a higher boiling point: N₂ or CO? (Same molar mass, size, atoms)

CO, because its polar bond causes dipole-dipole interactions, requiring more KE to overcome IMFs.

What is hydrogen bonding?

A "special force" — a very strong dipole-dipole interaction involving H (partially positive) bonded to O, N, or F (partially negative).

Which has stronger IMF: H₂O (18 g/mol) or ONF (49 g/mol)? Why?

H₂O. Even though ONF has higher molar mass, H₂O has hydrogen bonding, while ONF does not.

Put butane (-1°C bp), acetone (56.2°C), and 1-propanol (97.4°C) in increasing order of IMF strength.

Butane (dispersion only) < Acetone (dipole-dipole) < 1-propanol (hydrogen bonding).

Identify the IMF(s) in: HBr, CH₄, and SO₂.

HBr = dispersion + dipole-dipole; CH₄ = dispersion only; SO₂ = dispersion + dipole-dipole.

What is an ion-induced dipole interaction? Give an example.

Interaction between an ion and a nonpolar molecule. Example: Fe²⁺ and O₂.

What is an ion-dipole interaction? Give an example.

Interaction between an ion and a polar molecule. Example: Na⁺ and H₂O.

What is a dipole-induced dipole interaction? Give an example.

Interaction between a polar molecule and a nonpolar molecule. Example: HCl and CH₄.

What is the correct order of IMF strength from weakest to strongest?

Dispersion (weakest) < Dipole-dipole < Hydrogen bonding (strongest).

As IMF increases, what happens to boiling point, melting point, and viscosity?

Boiling point increases, melting point increases, viscosity increases.

As IMF increases, what happens to vapor pressure and volatility?

Vapor pressure decreases, volatility decreases.

What is viscosity, and what two factors affect it?

Viscosity is a liquid's resistance to flow. Affected by 1) IMF (shape/size) and 2) temperature (kinetic energy). Weaker IMFs and higher temperatures = lower viscosity.

What is the difference between cohesive and adhesive forces?

Cohesive forces = IMFs between identical molecules. Adhesive forces = IMFs between different molecules.

Why does water bead up on a waxed surface but spread on glass?

Wax: adhesive forces (water-wax) are weak, so cohesive forces dominate (beading). Glass: adhesive forces are strong compared to water's cohesive forces (spreading).

What is capillary action, and why does it occur?

Liquid flowing into a porous material (e.g., water in paper towel). Occurs when adhesive forces > cohesive forces.

What is surface tension?

The energy required to increase the surface area of a liquid, arising from cohesive forces.

How does vapor pressure change as temperature increases, and why?

Vapor pressure increases with temperature. Higher temperature gives molecules more kinetic energy to overcome IMFs and escape into the gas phase.

What does the Clausius-Clapeyron equation allow you to calculate?

It allows you to calculate the heat of vaporization (ΔH_vap) given temperature and vapor pressure data, or to find vapor pressure at a new temperature if ΔH_vap is known.

What are the three questions to ask when identifying the primary IMF (the "sloppy way")?

1) Is the molecule polar? If no → Dispersion only. If yes → 2) Is H directly bonded to N, O, or F? If yes → Hydrogen bonding. If no → Dipole-dipole.

List all physical properties that INCREASE as IMF increases.

Boiling point, melting point, heat of vaporization, heat of fusion, heat of sublimation, viscosity, surface tension.

List all physical properties that DECREASE as IMF increases.

Vapor pressure, volatility.

What is vapor pressure?

The pressure exerted by a vapor in equilibrium with its liquid (or solid) phase at a given temperature.

How does stronger IMF affect surface tension?

Stronger IMF leads to higher surface tension (more energy required to increase surface area).