Unit 4 Study Guide: Intermolecular Forces, Liquids & Solids, Thermochemistry, and Atomic Structure

Intermolecular Forces and Properties of Liquids & Solids

Types of Intermolecular Forces

Intermolecular forces are the attractive forces between molecules, which determine many physical properties of substances, such as boiling point, melting point, and solubility.

• London Dispersion Forces: Present in all molecules, especially nonpolar ones. Arise from temporary fluctuations in electron distribution.

• Dipole-Dipole Interactions: Occur between polar molecules due to permanent dipoles.

• Hydrogen Bonding: A strong type of dipole-dipole interaction occurring when H is bonded to N, O, or F.

Example: Water (H2O) exhibits hydrogen bonding, which accounts for its high boiling point compared to similar-sized molecules.

Boiling Point and Vapor Pressure

The boiling point of a liquid is the temperature at which its vapor pressure equals atmospheric pressure. Vapor pressure is influenced by the strength of intermolecular forces.

• Stronger intermolecular forces → Lower vapor pressure → Higher boiling point

• Weaker intermolecular forces → Higher vapor pressure → Lower boiling point

Example: At 298 K, H2O has a higher boiling point than CH3OH due to stronger hydrogen bonding.

Isomerism and Physical Properties

Isomers are compounds with the same molecular formula but different structures. Cis-trans isomerism affects physical properties due to differences in molecular polarity.

• Cis-isomer: Has a net dipole moment, leading to stronger dipole-dipole interactions and lower vapor pressure.

• Trans-isomer: More symmetrical, lower net dipole, higher vapor pressure.

Example: 1,2-dichloroethene: cis-isomer has stronger dipole-dipole forces than trans-isomer.

Thermochemistry and Phase Changes

Enthalpy of Vaporization

The enthalpy of vaporization ($riangle H_{vap}$) is the energy required to convert one mole of liquid to vapor at constant temperature.

• Formula: $q = n riangle H_{vap}$

• Higher $riangle H_{vap}$ indicates stronger intermolecular forces.

Liquid	$riangle H_{vap}$ (kJ/mol)
HF(l)	25.2
HBr(l)	17.3

Example: HF has a higher enthalpy of vaporization than HBr due to stronger hydrogen bonding.

Entropy Changes

Entropy ($S$) is a measure of disorder or randomness in a system. Processes that increase randomness (e.g., melting, vaporization) increase entropy.

• Decrease in entropy: Occurs when a system becomes more ordered (e.g., freezing, condensation).

• Increase in entropy: Occurs when a system becomes less ordered (e.g., boiling, dissolving gas into liquid).

Example: Freezing water decreases entropy; boiling water increases entropy.

Solubility and Molecular Structure

Solubility in Water

Solubility depends on the ability of a solute to interact with water molecules, often through hydrogen bonding or polarity.

• Methanol (CH3OH): Highly soluble due to hydrogen bonding.

• Methylamine (CH3NH2): Soluble due to hydrogen bonding and polarity.

• Methane (CH4): Insoluble; nonpolar, cannot hydrogen bond.

• Methyl Bromide (CH3Br): Moderately soluble; polar but does not hydrogen bond as strongly.

Example: Methanol is more soluble in water than methane.

Atomic Structure and Periodic Properties

Electron Configuration

The ground state electron configuration describes the arrangement of electrons in an atom's orbitals.

• Aluminum (Al): $1s^2 2s^2 2p^6 3s^2 3p^1$

• Aluminum Ion (Al3+): $1s^2 2s^2 2p^6$

Example: Al3+ has a smaller radius than Al due to loss of electrons and increased effective nuclear charge.

Periodic Trends

Atomic radius decreases across a period due to increased nuclear charge, and increases down a group due to added electron shells.

• Al vs. Al3+: Al3+ is smaller because it has lost its outer electrons, resulting in a higher effective nuclear charge per electron.

Lab Techniques and Calculations

Heating Curves

A heating curve shows the temperature change of a substance as heat is added, including phase changes.

• Calculate heat for temperature change: $q = m c riangle T$

• Calculate heat for phase change: $q = n riangle H_{fus}$ or $q = n riangle H_{vap}$

Example: To find the heat required to melt ice, use $q = n riangle H_{fus}$.

Lab Procedures: Precipitation Reactions

Precipitation reactions occur when two solutions are mixed and an insoluble product forms.

• Example: Mixing AgNO3 and NaCl forms solid AgCl.

Reactants	Products	Observations
Solid AgNO3 + Aqueous solution of silver ions	Solid AgCl (precipitate)	White solid forms

Additional info:

• Some content inferred from context and standard chemistry curriculum (e.g., details on entropy, electron configuration, and heating curves).

• Questions reference Chapters 12-14, which cover Liquids, Solids, Intermolecular Forces, Thermochemistry, and Atomic Structure.