Chemical Reactions

What this unit covers

The topics below follow the published Chemistry course framework for Unit 4. This unit is worth 7–9% of the exam, so budget your time against that rather than against how long the unit takes to teach.

Lessons in this unit

• Types of Reactions & Net Ionic Equations14 min · 3 objectivesDistinguish physical changes from chemical changes · Classify a reaction as synthesis, decomposition, combustion, acid–base, precipitation, or redox · Translate a molecular equation into complete-ionic and net ionic form using solubility rules
• Stoichiometry & Limiting Reactants15 min · 3 objectivesUse mole ratios from a balanced equation to convert between reactants and products · Identify the limiting reactant and calculate the theoretical yield · Compute percent yield from actual and theoretical yields
• Oxidation–Reduction (Redox) Basics13 min · 3 objectivesAssign oxidation numbers using the standard rules · Identify which species is oxidized and which is reduced in a redox reaction · Recognize the oxidizing agent and the reducing agent
• Solution Stoichiometry & Titrations15 min · 3 objectivesRelate molarity, moles, and volume, and calculate dilutions · Use titration data to find an unknown concentration · Apply the reaction mole ratio to titrations beyond the simple 1:1 case
• Balancing Redox by Half-Reactions14 min · 3 objectivesAssign oxidation states and split a redox reaction into oxidation and reduction half-reactions · Balance each half-reaction for atoms and then for charge with electrons, adding H₂O and H⁺ in acidic solution · Convert an acid-balanced half-reaction to basic solution by neutralizing H⁺ with OH⁻
• Advanced Stoichiometry15 min · 3 objectivesSolve combined limiting-reactant and percent-yield problems in a single chain · Use gravimetric analysis — the mass of a precipitate — to find the amount of an unknown · Work back-titration and multi-step solution-stoichiometry problems while tracking units throughout

Formulas in Unit 4

Every term in Unit 4

All 17 terms we publish for Chemical Reactions, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.

Net ionic equation

Shows only species that change, with spectator ions removed. Charge and mass must both balance.

Solubility rules

Group 1, ammonium and nitrate salts are soluble. Most carbonates, phosphates, hydroxides and sulfides are not, with the usual exceptions.

Precipitation reaction

Two soluble salts exchange partners and an insoluble product drops out. Predicted by the solubility rules.

Acid-base neutralization

Acid plus base gives salt and water; the net ionic equation for a strong acid and strong base is H⁺ + OH⁻ → H₂O.

Oxidation number rules

Free elements 0, monatomic ions equal their charge, oxygen usually −2, hydrogen usually +1. The sum must equal the species charge.

Redox identification

Oxidation is a rise in oxidation number and loss of electrons; reduction is a fall and a gain. The oxidizing agent is itself reduced.

Balancing redox in acid

Balance atoms, add H₂O for oxygen, H⁺ for hydrogen, then electrons for charge; equalize electrons between half-reactions before adding.

Limiting reactant

The reactant producing the least product. Found by converting each reactant to moles of product, not by comparing starting masses.

Percent yield

Actual over theoretical, times 100. Below 100% because of side reactions, incomplete reaction and loss during transfer and filtration.

Titration

Adding a known-concentration solution until stoichiometric equivalence. Moles of titrant times the mole ratio gives moles of analyte.

Equivalence vs endpoint

Equivalence is where stoichiometrically equivalent amounts have reacted; endpoint is where the indicator changes. A good indicator makes them nearly coincide.

Gravimetric analysis

Determining an amount by precipitating, filtering, drying and weighing a product of known formula.

Spectator ions

Ions unchanged on both sides of an equation. Removing them leaves the net ionic equation, which is what actually happened.

Identifying reaction type

Precipitation forms an insoluble solid, acid-base transfers a proton, and redox transfers electrons — check oxidation numbers to distinguish the third.

Stoichiometry road map

Grams → moles → mole ratio → moles → grams. The mole ratio comes from the balanced equation and is the only step that changes substance.

Titration calculation

Moles of titrant = M × V, then apply the mole ratio, then divide by the analyte volume to get its concentration.

Back titration

Add excess reagent, then titrate what is left over. Used when the analyte reacts too slowly for a direct titration.

What examiners penalize here

• AP free-response almost always asks for the *net ionic* equation, and it must be balanced for mass and charge — spectators earn no credit. Memorize the core solubility rules so you can decide instantly which species split and which stay solid.
• On limiting-reactant free-response, show the moles-of-product each reactant would make and explicitly state which is smaller. The reactant giving the *smaller* product amount is limiting, and that smaller number is your theoretical yield.
• To name agents, first find what is oxidized and what is reduced, then flip the labels: the reduced species is the oxidizing agent, the oxidized species is the reducing agent. Mixing these up is the single most common redox error on the AP exam.
• Every solution-stoichiometry problem runs the same loop: convert volume + molarity to moles, cross the balanced equation with the mole ratio, then convert back using the target’s volume or molar mass. Keep volumes in liters throughout and the arithmetic stays clean.
• On the AP exam a redox equation earns full credit only when it is balanced for mass *and* charge. Always end by summing the charges on each side — if they are not equal, you mis-scaled the electrons or lost an H⁺/H₂O when combining the halves.
• Multi-step AP problems reward a clear moles ledger: write moles of every species, label each with its source, and apply the balanced mole ratio at each junction. Do all conversions to grams or molarity only after the moles are settled — that keeps limiting-reactant, gravimetric, and back-titration problems on the same reliable track.

Practice Chemistry

Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.

Questions about this unit

All 9 units of AP Chemistry

1. Unit 1 · Atomic Structure & Properties
2. Unit 2 · Molecular and Ionic Compound Structure and Properties
3. Unit 3 · Intermolecular Forces and Properties
4. Unit 4 · Chemical Reactions
5. Unit 5 · Kinetics
6. Unit 6 · Thermodynamics
7. Unit 7 · Equilibrium
8. Unit 8 · Acids & Bases
9. Unit 9 · Applications of Thermodynamics