Types of Chemical Reactions
Classify proton transfer, oxidation-number change and precipitation using chemical evidence.
AP Chemistry Topic 4.7 · CED printed p. 84, 85 / PDF p. 91, 92. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Moles and molarity
- Ionic charges and molecular formulas
- Intermolecular versus intramolecular interactions
- Ideal gas equation with kelvin temperature
An acid–base reaction moves a proton from one species to another. Follow the H+ transfer, rather than looking only for an OH group in a formula. NH3 can accept a proton even though it contains no hydroxide ion.
A redox reaction changes oxidation numbers. Complete hydrocarbon combustion produces CO2 and H2O while O2 is reduced. This is electron bookkeeping for changed bonding; it does not require isolating charged ions. In CH4 + 2O2 → CO2 + 2H2O, carbon goes from −4 to +4, while each oxygen atom goes from 0 to −2.
Oxidation is loss of electrons and increasing oxidation number; reduction is gain and decreasing oxidation number. They are coupled: the electrons lost by the oxidized species equal those gained by the reduced species. For methane combustion, one carbon’s increase of eight is balanced by four oxygen atoms each decreasing by two. Naming oxidizing/reducing agents is optional enrichment and is not assessed by this edition’s AP practice.
To assign oxidation numbers, begin with elemental species at zero and monatomic ions at their actual charge. Use familiar assignments such as oxygen usually −2 and hydrogen usually +1 with nonmetals, then require the sum to equal the species’ charge. Exceptions matter when supplied: peroxide oxygen is −1. An oxidation number in a covalent molecule is a formal accounting assignment, not a measured ionic charge.
Precipitation takes dissolved ions into a sparingly soluble solid lattice. Sodium, potassium, ammonium, and nitrate salts are soluble in the AP model; other solubilities are supplied where needed. You do not need a giant memorized solubility chart. Check species and oxidation numbers: Ag+ + Cl− → AgCl(s) is precipitation without oxidation-number change. Some reactions fit more than one description, so justify the process rather than forcing every equation into only one box.
Let's work through it
Classify NH3 + H2O ⇌ NH4+ + OH−, Zn + Cu2+ → Zn2+ + Cu, and Ag+ + Cl− → AgCl(s).
- NH3 gains a proton from water: acid–base. Nitrogen stays −3, so it is not redox.
- Zn changes 0 to +2 and Cu changes +2 to 0: redox, with two electrons lost and gained.
- AgCl is a new sparingly soluble solid; Ag stays +1 and Cl stays −1: precipitation without redox.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
Is combustion redox if the products are neutral molecules?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
Is combustion redox if the products are neutral molecules?
Worked answer and reason
Yes. Oxidation numbers can change inside neutral molecules; carbon oxidation and oxygen reduction still balance.
Oxidation number is formal electron bookkeeping, not overall molecular charge.
Retrieval 2
Which AP solubility patterns must be available without a long table?
Worked answer and reason
Sodium, potassium, ammonium, and nitrate salts are soluble. Use supplied information for other salts.
Practice must respect the CED’s explicit memorization exclusion.
Retrieval 3
In NH₄⁺ +OH⁻ →NH₃ +H₂O, what moves, and which species donates and accepts it?
Worked answer and reason
A proton moves from NH₄⁺ to OH⁻. NH₄⁺ is the acid and OH⁻ the base; the identities NH₃ and H₂O show that transfer.
Classifying acid–base reaction requires locating the actual proton transfer.
Retrieval 4
For Zn +Cu²⁺ →Zn²⁺ +Cu, how many electrons are transferred and which species loses them?
Worked answer and reason
Zn loses two electrons as its oxidation number rises 0→+2; Cu²⁺ gains those two as it falls +2→0.
Electron loss and gain must match in a coupled redox reaction.
Catch the tempting mistake
- An overall neutral molecule gives every atom oxidation number zero.
- Only elemental species have zero by that rule. The numbers inside a compound sum to its net charge.
- Precipitation is always redox.
- Compare oxidation numbers; ions may change phase without changing their numbers.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 4 problem 13 · ap synthesis · symbolic
- Unit 4 problem 14 · ap application · symbolic
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For the teacher
- Require the proton/electron/solid evidence alongside every classification.
- Do not test memorization of excluded agent terms or extended solubility rules.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 4.7.A
- Identify a reaction as acid-base, oxidation-reduction, or precipitation.
- 4.7.A.1
- Acid-base reactions involve transfer of one or more protons (H+ ions) between chemical species.
- 4.7.A.2
- Oxidation-reduction (redox) reactions involve transfer of one or more electrons between chemical species, as indicated by changes in oxidation numbers of the involved species. Combustion is an important subclass of oxidation-reduction reactions, in which a species reacts with oxygen gas. In the case of hydrocarbons, carbon dioxide and water are products of complete combustion.
- 4.7.A.3
- In a redox reaction, electrons are transferred from the species that is oxidized to the species that is reduced.
- 4.7.A.4
- Oxidation numbers may be assigned to each of the atoms in the reactants and products; this is often an effective way to identify the oxidized and reduced species in a redox reaction.
- 4.7.A.5
- Precipitation reactions frequently involve mixing ions in aqueous solution to produce an insoluble or sparingly soluble ionic compound. All sodium, potassium, ammonium, and nitrate salts are soluble in water.
- The meaning of the terms “reducing agent” and “oxidizing agent” will not be assessed on the AP Exam.
- Rote memorization of “solubility rules” other than those implied in 4.7.A.5 will not be assessed on the AP Exam.
Science references for this teaching
- College Board AP Chemistry Course and Exam Description, Fall 2024 — Exact unit learning requirements and exclusions; supplied PDF is pinned by the integration catalog.
- OpenStax Chemistry 2e: Classifying Chemical Reactions — Scientific check: precipitation, oxidation-number conventions, proton transfer; AP exclusions applied separately.
Glossary and deeper reading · 11 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Mole
A mole is an amount of substance containing exactly 6.02214076 × 10²³ specified entities. The entities can be atoms, molecules, ions, or formula units, so the counted entity must be named. For example, one mole of oxygen molecules contains two moles of oxygen atoms.
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Molarity
Molarity is the amount of a solute in moles divided by the final solution volume in liters. It describes concentration rather than the total amount of solute. For example, 0.10 mole dissolved to make 0.50 liter of solution gives a molarity of 0.20 mole per liter.
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Molecular formula
A molecular formula gives the actual number of each kind of atom in a molecule. It can be a whole-number multiple of the empirical formula. For example, glucose has molecular formula C₆H₁₂O₆ and empirical formula CH₂O.
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Ideal gas law
The ideal gas law relates pressure, volume, amount, and absolute temperature in an idealized gas model. It treats particles as having negligible volume and no intermolecular attractions. For example, at fixed amount and temperature, doubling the container volume halves the ideal gas pressure.
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Ion
An ion is an atom or group of atoms with a net electric charge. Ordinary ion formation changes electron count without changing the element's proton count. For example, a sodium atom losing one electron becomes a sodium ion with charge +1.
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Redox reaction
A redox reaction combines oxidation of one species with reduction of another. Electron loss and gain must balance in the net equation along with atoms and total charge. For example, zinc reducing copper ions involves zinc oxidation and copper-ion reduction.
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Oxidation number
An oxidation number is a formal electron-accounting value assigned using specified rules. Changes in oxidation number help identify oxidation and reduction but are not always actual atomic charges. For example, oxygen usually has oxidation number −2 in water while hydrogen has +1.
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Oxidation
Oxidation is electron loss, represented by an increase in oxidation number. It occurs together with reduction so that electrons are conserved in the overall reaction. For example, a zinc atom becoming a zinc ion with charge +2 undergoes oxidation.
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Reduction
Reduction is electron gain, represented by a decrease in oxidation number. It is paired with oxidation in an electron-transfer reaction. For example, a copper ion with charge +2 gaining two electrons to form copper metal undergoes reduction.
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Precipitate
A precipitate is a solid phase that forms from species in a solution. Whether it forms depends on the ion concentrations and the relevant equilibrium conditions. For example, silver ions and chloride ions can produce a silver chloride precipitate.
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Solubility
Solubility describes how much of a substance can dissolve in a specified solvent under stated conditions. It depends on the solute, solvent, temperature, and other relevant conditions and is distinct from how fast dissolution occurs. For example, two salts can have different equilibrium solubilities in water at the same temperature.