Comprehensive AP Chemistry Study Guides for Units 3-5

Intermolecular Forces and States of Matter

Ever wonder why some substances are solid at room temperature while others are gases? It all comes down to intermolecular forces (IMFs). These attractive forces between molecules determine physical properties like melting point, boiling point, and solubility.

Different types of substances have different IMFs. Ionic compounds have strong attractions between ions, making them brittle with high melting points. Molecular compounds have weaker forces between molecules, resulting in lower melting points. Metallic substances contain delocalized electrons that allow them to conduct electricity and be malleable.

For molecular substances, the strength of attraction varies: hydrogen bonding (between H and N, O, or F) is strongest, followed by dipole-dipole forces (between polar molecules), and finally temporary dipoles (present in all molecules). Remember that stronger IMFs mean higher melting/boiling points and lower vapor pressures.

💡 The Ideal Gas Law (PV=nRT) helps predict gas behavior, but real gases deviate at high pressures and low temperatures because IMFs become more significant!

Gas behavior depends on molecular motion and kinetic energy. Heavier gas molecules move slower than lighter ones at the same temperature since kinetic energy (KE=½mv²) remains constant at a given temperature.

In chromatography, we separate substances based on their attractions to stationary and mobile phases. The retention factor (Rf) tells us how far a compound travels relative to the solvent. Remember: "like dissolves like" - polar substances dissolve better in polar solvents like water.

Chemical Reactions and Stoichiometry

Chemical reactions create new substances by breaking and forming bonds, unlike physical changes that just break IMFs. When writing ionic equations, remember to separate aqueous compounds into ions while keeping solids, liquids, and gases intact.

There are three ways to write chemical equations:

• Molecular equations show the complete formulas
• Complete ionic equations separate aqueous compounds into ions
• Net ionic equations eliminate spectator ions that don't participate in the reaction

Stoichiometry is your tool for calculating quantities in chemical reactions. The limiting reactant determines the theoretical yield because it's completely consumed first. Calculate percent yield by dividing actual yield by theoretical yield and multiplying by 100.

🧪 In acid-base reactions, acids donate H⁺ ions while bases accept them. When an acid reacts, it forms its conjugate base; when a base reacts, it forms its conjugate acid.

Redox reactions involve the transfer of electrons, with oxidation (losing electrons) and reduction (gaining electrons) occurring simultaneously. To identify what's being oxidized or reduced, track oxidation numbers using these rules:

• Free elements = 0
• Group 1 elements = +1 in compounds
• Group 2 elements = +2 in compounds
• Fluorine = -1 in compounds
• Hydrogen = +1 with non-metals, -1 with metals
• Oxygen usually = -2

Remember those diatomic molecules (Br₂, I₂, N₂, Cl₂, H₂, O₂, F₂) and polyatomic ions like sulfate (SO₄²⁻), nitrate (NO₃⁻), and ammonium (NH₄⁺) for writing correct formulas in reactions.