AP Chemistry — Topics by Unit

Topics by Unit

There are 9 units in AP Chemistry:

Atomic Structure and Properties
Compound Structure and Properties
Properties of Substances and Mixtures
Chemical Reactions
Kinetics
Thermochemistry
Equilibrium
Acids and Bases
Thermodynamics and Electrochemistry

Scroll down to see details on each unit

Unit 1: Atomic Structures and Properties

Unit 2: Compound Structures and Properties

Unit 3: Properties of Substances and Mixtures

Gases. All the named gas laws are one equation, PV = nRT, with something held constant. In the cylinder below the piston is free to move, so it settles wherever the gas pressure matches the applied pressure. Change one slider at a time: move only the pressure and you are doing Boyle's law; move only the temperature and you are doing Charles's law. Watch PV/nT — it never budges.

What to take away: temperature must be in kelvin — V ∝ T only works from absolute zero, which is why 20 °C to 40 °C does not double the volume. Boyle (PV constant) and Charles (V/T constant) are not separate rules to memorise; each is PV = nRT with one variable pinned.

Solutions. Diluting a solution adds solvent, never solute. That single fact is the whole of M₁V₁ = M₂V₂.

What to take away: the number of solute particles is identical before and after — only the volume they are spread through changes, so the concentration falls by exactly the factor the volume rose by. Take 25 mL of 1.00 M and dilute to 100 mL: the volume went up 4×, so the concentration drops 4× to 0.250 M, while the 25 mmol of solute stays 25 mmol throughout.

Unit 4: Chemical Reactions

Unit 5: Kinetics

Every reaction has to climb an energy barrier — the activation energy, Eₐ — before reactants can become products. A catalyst provides a different pathway with a lower barrier, which speeds the reaction up in both directions. What a catalyst cannot do is change how much energy the reaction releases or absorbs overall. Move the sliders and watch which parts of the diagram respond.

What to take away: the peak height above the reactants is Eₐ for the forward reaction; the peak height above the products is Eₐ for the reverse. The gap between the two flat levels is ΔH, and it depends only on where reactants and products sit — never on the barrier between them. So adding a catalyst lowers both activation energies and leaves ΔH untouched. A reaction that is exothermic (ΔH < 0) stays exothermic no matter how good the catalyst is.

Unit 6: Thermochemistry

Unit 7: Equilibrium

Equilibrium is not the reaction stopping — it is the forward and reverse reactions running at the same rate, so the amounts stop changing while the conversions keep going. The box below starts as pure colourless N₂O₄, which splits into brown NO₂. Nothing in the simulation pushes the mixture toward an answer: each particle is converted at random, at a rate set by the rate laws. Equilibrium is simply where those two rates happen to balance.

What to take away: compare Q (what the mixture is right now) with K (where it settles). Q wobbles around K instead of landing on it — with a countable number of particles that fluctuation is real, and it is what "dynamic" in dynamic equilibrium means. Q < K means the forward reaction is still winning; Q > K means the reverse is; Q = K is equilibrium. Adding N₂O₄, removing NO₂, or changing the volume all move Q, and the mixture drifts back until Q equals K again — that is Le Châtelier's principle. Temperature is the exception: it changes K itself. Because the forward reaction here is endothermic, heating raises K and drives the mixture browner, while cooling lowers K and drives it back toward colourless N₂O₄.

Unit 8: Acids and Bases

A titration follows how pH changes as titrant is added to an analyte. In a strong acid–strong base titration the only reaction that matters is H⁺ + OH⁻ → H₂O, so the pH at any point is decided by whichever ion is left over once neutralization is done. Drag the slider below to add 0.100 M HCl to 10.0 mL of 0.100 M NaOH and watch the curve build.

What to take away: the pH barely moves through most of the titration, because the leftover strong base is still concentrated enough to dominate. Near the equivalence point — where moles of added H⁺ equal the initial moles of OH⁻ — a single drop wipes out the last of the excess ion, and the pH falls almost vertically. For a strong acid–strong base pair at 25 °C the equivalence point sits at pH 7.00, because the only species left are water and the spectator ions Na⁺ and Cl⁻. That is not true for weak acid–strong base titrations, where the conjugate base makes the equivalence point basic.