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AP Chemistry Unit 4 Visual ReviewA topic-by-topic visual walkthrough of Chemical Reactions — net ionic equations, stoichiometry, titration, reaction types, and redox.
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TOPIC 4.1
Introduction to Reactions
A chemical reaction rearranges atoms into new substances
In a reaction, bonds break and re-form so that REACTANTS become PRODUCTS with different properties. The atoms
themselves are conserved — never created or destroyed (Law of Conservation of Mass), just rearranged.
This is why every chemical equation must be BALANCED — the same atoms appear on both sides.
Macroscopic evidence a reaction occurred
• Color change
• Gas produced (bubbling)
• A precipitate (solid) forms
• Temperature change (heat released or absorbed)
• Light or odor produced
• A change that is difficult to reverse
Caution: some of these (e.g., bubbling, heat) can also occur in physical changes like boiling — see Topic 4.4.
Chemists describe reactions on three levels: MACROSCOPIC (what you observe), PARTICULATE (atoms/molecules),
and SYMBOLIC (the chemical equation). Connecting these three views is a core AP Chemistry skill.
A reaction rearranges atoms into new substances — atoms are conserved , so equations must balance.
The Review Hub · AP Chemistry Unit 4
TOPIC 4.2
Net Ionic Equations
THREE WAYS TO WRITE A PRECIPITATION REACTION
1 · Molecular equation
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
2 · Complete ionic
Ag⁺ + NO₃⁻ + Na⁺ + Cl⁻ → AgCl(s) + Na⁺ + NO₃⁻ (split all aqueous ionic compounds)
3 · Net ionic
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
Na⁺ and NO₃⁻ appear unchanged on both sides — they are SPECTATOR IONS and get cancelled out.
Spectator ions
Ions that are present but DON'T participate — they're
aqueous on both sides. Cancel them to reveal the actual
chemical change (the net ionic equation).
The net ionic equation shows only what really reacts.
Rules for splitting
Split (write as ions) only: strong electrolytes that are
AQUEOUS — soluble ionic compounds & strong acids/bases.
Keep together: solids (s), liquids (l), gases (g), and weak
electrolytes.
The net ionic equation keeps only the species that change — spectator ions cancel out.
The Review Hub · AP Chemistry Unit 4
TOPIC 4.3
Representations of Reactions
Macroscopic
What you observe with
your senses — bubbling,
color change, a solid.
The "real world" scale.
Measured in labs.
Particulate
→
Drawings of individual
atoms & molecules —
the "why" behind observations.
Symbolic
2 H₂ + O₂ → 2 H₂O
The chemical equation with
formulas & coefficients.
Coefficients show mole ratios;
must be balanced.
Balancing equations
Because atoms are conserved, the count of EACH element must be equal on both sides. Adjust only the
coefficients (the big numbers in front) — NEVER change the subscripts inside a formula.
Example: H₂ + O₂ → H₂O becomes 2 H₂ + O₂ → 2 H₂O
Now: 4 H and 2 O on each side. Coefficients also give the mole ratio: 2 mol H₂ react with 1 mol O₂.
A balanced equation is the foundation for all stoichiometry (Topic 4.5).
Reactions connect three views — macroscopic, particulate, symbolic — and equations must be balanced .
The Review Hub · AP Chemistry Unit 4
TOPIC 4.4
Physical & Chemical Changes
Physical change
The substance's identity STAYS THE SAME — only its
form or state changes. NO new bonds form.
Examples:
• Melting ice, boiling water (phase changes)
• Dissolving sugar in water
• Cutting, crushing, bending
Usually reversible; only IMFs (not bonds) are affected.
Chemical change
A NEW SUBSTANCE forms — bonds are broken and new
bonds are made. This is a chemical reaction.
Examples:
• Burning (combustion)
• Rusting (iron + oxygen)
• Digesting food, a precipitate forming
Usually hard to reverse; new chemical identity.
The decisive test: are bonds broken and formed?
Ask: "Is a new substance with new chemical properties produced?" If YES → chemical change. If the material is
still the same substance, just in a different form → physical change.
Tricky case: dissolving salt in water is PHYSICAL (you can evaporate the water to get the salt back), even though
the ions separate — no covalent bonds are broken or formed.
Chemical change makes a new substance (bonds break/form); physical change keeps the same identity.
The Review Hub · AP Chemistry Unit 4
TOPIC 4.5
Stoichiometry
THE STOICHIOMETRY ROAD MAP
grams A
→
moles A
→
moles B
→
grams B
÷M · mole ratio · ×M
the mole ratio is the key step
Mole ratio from coefficients
The balanced equation gives the ratio of reactants to
products in MOLES. For N₂ + 3 H₂ → 2 NH₃, the ratio of
H₂ to NH₃ is 3 : 2.
Always convert to moles before using a ratio — never use grams directly.
Limiting reactant & percent yield
The limiting reactant runs out first and caps how much
product forms (the theoretical yield). The other is in excess.
% yield = (actual ÷ theoretical) × 100Real reactions rarely give 100% — some product is always lost.
Worked example
How many grams of NH₃ form from 2.0 mol N₂ (with excess H₂)? For N₂ + 3 H₂ → 2 NH₃, ratio N₂:NH₃ = 1:2.
moles NH₃ = 2.0 × (2/1) = 4.0 mol → mass = 4.0 mol × 17.0 g/mol = 68 g NH₃
Stoichiometry: grams → moles → mole ratio → moles → grams; the limiting reactant caps the yield.
The Review Hub · AP Chemistry Unit 4
TOPIC 4.6
Introduction to Titration
TITRATION SETUP
burette
(known conc.
titrant)
flask (unknown)
+ indicator
What titration does
A titration finds an unknown concentration by slowly adding
a solution of KNOWN concentration (the titrant) until the
reaction is exactly complete.
The equivalence point
The point where moles of titrant exactly react with moles
of analyte (per the balanced equation). An indicator's color
change (the endpoint) signals it.
At equivalence: moles = M × V, so you can solve for the unknown M.
Worked example (1:1 acid–base)
25.0 mL of unknown HCl needs 30.0 mL of 0.100 M NaOH. At equivalence, moles acid = moles base:
M(HCl) = (0.100 × 30.0) ÷ 25.0 = 0.120 M HCl
Titration finds an unknown concentration; at the equivalence point , moles react in the balanced ratio.
The Review Hub · AP Chemistry Unit 4
TOPIC 4.7
Types of Chemical Reactions
Three big categories (AP focus)
1 · Precipitation
Two solutions mix → an insoluble solid (precipitate) forms.
2 · Acid–base (neutralization)
Proton (H⁺) transfer → water + a salt.
3 · Oxidation–reduction (redox)
Electron transfer → oxidation numbers change (Topic 4.9).
Other patterns you should recognize
Synthesis : A + B → AB (two combine)Decomposition : AB → A + B (one splits apart)Combustion : fuel + O₂ → CO₂ + H₂O (+ energy)Single replacement : A + BC → AC + BDouble replacement : AB + CD → AD + CBPredicting products
Recognizing the reaction TYPE lets you predict the products. Two clues appear constantly on the AP exam:
• Mix two ionic solutions → check a solubility table to see if a precipitate forms (precipitation)
• Mix an acid and a base → products are water + a salt (neutralization)
Combustion of a hydrocarbon ALWAYS produces carbon dioxide and water (with enough oxygen).
The three AP categories (precipitation, acid–base, redox) cover most reactions you'll be asked to analyze.
Three key types — precipitation, acid–base, and redox — let you predict a reaction's products.
The Review Hub · AP Chemistry Unit 4
TOPIC 4.8
Introduction to Acid-Base Reactions
Brønsted–Lowry definition
Acid = proton (H⁺) DONORBase = proton (H⁺) ACCEPTORA reaction is the TRANSFER of an H⁺ from acid to base.
(Arrhenius: acids make H⁺, bases make OH⁻ in water.)
Conjugate acid–base pairs
When an acid donates H⁺, what's left is its CONJUGATE BASE.
When a base accepts H⁺, it becomes its CONJUGATE ACID.
HCl + H₂O → Cl⁻ + H₃O⁺
HCl/Cl⁻ and H₂O/H₃O⁺ are conjugate pairs (differ by one H⁺).
Neutralization: acid + base → salt + water
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
The net ionic equation for any strong acid + strong base is simply:
H⁺(aq) + OH⁻(aq) → H₂O(l)
The H⁺ from the acid combines with the OH⁻ from the base to form water — the salt's ions are spectators.
This is the reaction that a titration (Topic 4.6) measures. Strong acids/bases fully ionize; weak ones don't (Unit 8).
An acid–base reaction transfers a proton (H⁺) ; strong acid + strong base → H₂O + a salt.
The Review Hub · AP Chemistry Unit 4
TOPIC 4.9
Oxidation-Reduction (Redox)
Redox = electron transfer
OIL RIG
Oxidation Is Loss · Reduction Is Gain (of e⁻)
Oxidation : loses electrons → oxidation number INCREASESReduction : gains electrons → oxidation number DECREASESAgents (they cause the other change)
The oxidizing agent is REDUCED (it takes electrons).
The reducing agent is OXIDIZED (it gives electrons).
Oxidation number rules (key ones):
Pure element = 0 · monatomic ion = its charge
O usually −2 · H usually +1 · sum = overall charge
Worked example
Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)
• Zn goes from 0 → +2: it LOSES 2 electrons = oxidized (Zn is the reducing agent)
• Cu goes from +2 → 0: it GAINS 2 electrons = reduced (Cu²⁺ is the oxidizing agent)
Half-reactions: Zn → Zn²⁺ + 2e⁻ (oxidation) and Cu²⁺ + 2e⁻ → Cu (reduction). Electrons lost = electrons gained.
Redox is the basis of batteries and electrochemistry (Unit 9).
Redox transfers electrons: OIL RIG — Oxidation Is Loss, Reduction Is Gain.
The Review Hub · AP Chemistry Unit 4
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How to use the visual review
Spend 30 seconds per slide before clicking next. Look at the diagram, then ask yourself: "Could I draw this from memory and explain it?"
Use the fullscreen button () on desktop for the best experience. Use arrow keys to navigate. Tap "Show all slides" to jump around.
This is great for review the night before the exam — fast, visual, and covers everything you need to remember about Unit 4's chemical reactions content.