AP’s three classes: what gets transferred
Lesson 2.7.1 classified reactions by their form: synthesis, decomposition, single replacement, double replacement, and combustion. AP classifies them by what actually happens, in three classes:
| Class | What happens | How to tell | Example |
|---|---|---|---|
| Acid–base reaction酸碱反应 | Proton (H⁺) transfer | H⁺ moves from one particle to another; no oxidation number changes | HCl + NH₃ → NH₄Cl |
| Oxidation–reduction (redox) reaction氧化还原反应 | Electron transfer | Some element changes oxidation number | Zn + Cu²⁺ → Zn²⁺ + Cu |
| Precipitation reaction沉淀反应 | Ions combine into an insoluble solid | Ions in solution form an insoluble solid | Ag⁺ + Cl⁻ → AgCl(s) |
Identifying redox reactions with oxidation numbers
The usual rules for oxidation numbers (2.13.1): an element is 0 in its free form; a monatomic ion equals its charge; F is −1; Group 1 and Group 2 metals are +1 and +2; H is usually +1 and O usually −2; the oxidation numbers add up to the particle’s charge.
If any element changes oxidation number, the reaction is a redox reaction: the element whose oxidation number increases is oxidized (loses electrons), and the one whose oxidation number decreases is reduced (gains electrons). U4.9 goes on to count the electrons transferred.
The five basic types and the three classes
The two ways of classifying look at different things, so both can apply at once:
- Combustion and single replacement reactions are always redox.
- Synthesis and decomposition reactions may be redox (, ) or not (, ).
- Double replacement reactions are usually precipitation or acid–base reactions, and some are both: H₂SO₄(aq) + Ba(OH)₂(aq) → BaSO₄(s) + 2H₂O(l) forms a precipitate and also combines H⁺ with OH⁻ to form water.
Two cases deserve attention:
- In a disproportionation歧化反应, the same element is both oxidized and reduced. In , O goes from −1 to −2 and 0; in 3Br₂ + 6OH⁻ → 5Br⁻ + BrO₃⁻ + 3H₂O, Br goes from 0 to −1 and +5.
- Copper with dilute nitric acid: in 3Cu + 8H⁺ + 2NO₃⁻ → 3Cu²⁺ + 2NO + 4H₂O, what is reduced is the N in NO₃⁻ (+5 → +2), not H⁺. H keeps its oxidation number, so no H₂ forms.
Classify each reaction and look at the oxidation-number evidence:
Oxidation numbers: Mg: 0 → +2 (increases, oxidized); O: 0 → −2 (decreases, reduced)
Magnesium and oxygen combine into one substance, which looks like synthesis; but Mg loses electrons and O gains them, so it is also a redox reaction.
Oxidation numbers: No element changes oxidation number, so this is not a redox reaction.
H⁺ is transferred from HCl to OH⁻ (proton transfer).
Oxidation numbers: No element changes oxidation number, so this is not a redox reaction.
An insoluble solid, AgCl, forms.
Ag⁺ and Cl⁻ combine into insoluble AgCl, with no electron transfer and no proton transfer.
Oxidation numbers: Zn: 0 → +2 (increases, oxidized); Cu: +2 → 0 (decreases, reduced)
Zn transfers electrons to Cu²⁺.
Oxidation numbers: No element changes oxidation number, so this is not a redox reaction.
Two substances form one, which looks like synthesis, but what happens is that H⁺ moves from HCl to NH₃: an acid–base reaction.
Oxidation numbers: No element changes oxidation number, so this is not a redox reaction.
A gas forms, but no oxidation number changes: H⁺ is transferred to CO₃²⁻, and the H₂CO₃ formed breaks down into H₂O and CO₂.
Oxidation numbers: No element changes oxidation number, so this is not a redox reaction.
An insoluble solid, BaSO₄, forms.
Ba²⁺ and SO₄²⁻ combine into insoluble BaSO₄.
Oxidation numbers: O: −1 → −2 and 0 (both increases and decreases)
Some O goes from −1 up to 0 and some down to −2: a disproportionation, which is a redox reaction.
Oxidation numbers: Br: 0 → −1 and +5 (both increases and decreases)
Some Br goes from 0 down to −1 and some up to +5: a disproportionation.
Oxidation numbers: Cu: 0 → +2 (increases, oxidized); N: +5 → +2 (decreases, reduced)
What is reduced is the N in NO₃⁻ (+5 → +2), not H⁺: the oxidation number of H doesn’t change.
Oxidation numbers: C: −4 → +4 (increases, oxidized); O: 0 → −2 (decreases, reduced)
Every combustion is a redox reaction: C is oxidized and O is reduced.
Oxidation numbers: No element changes oxidation number, so this is not a redox reaction.
H⁺ is transferred from HC₂H₃O₂ to OH⁻. A weak acid still reacts completely with OH⁻.
Oxidation numbers: No element changes oxidation number, so this is not a redox reaction.
An insoluble solid, PbI₂, forms.
Pb²⁺ and I⁻ combine into insoluble, yellow PbI₂.
Oxidation numbers: Na: 0 → +1 (increases, oxidized); H: +1 → 0 (decreases, reduced)
Na loses electrons; some of the H in H₂O gains electrons, forming H₂.
Example 1: Classifying with reasons
Classify each reaction and justify your answer: (1) ; (2) ; (3) .
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Identify: what each type looks for.
- Acid–base: an (proton) is transferred
- Redox: oxidation numbers change, so electrons are transferred
- Precipitation: ions in solution combine into an insoluble solid
For each reaction, assign oxidation numbers first, then ask what is transferred or what forms.
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Reaction (1): . Oxidation numbers N −3, H +1, Cl −1 are the same on both sides, so it isn’t redox. moves from HCl to , forming : acid–base. It looks like a synthesis, but what happens is a proton transfer.
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Reaction (2): . Compare oxidation numbers:
Electrons move from Mg to O: redox (also a synthesis and a combustion).
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Reaction (3): . Pb stays +2 and I stays −1, with no electron or proton transfer; ions in solution combine into insoluble : precipitation.
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How to write the reason. For each, state what is transferred or what forms, and check with oxidation numbers.
Example 2: What is reduced?
Copper reacts with dilute nitric acid:
Identify the elements oxidized and reduced, and explain why no H₂ forms.
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Assign oxidation numbers before and after.
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Decide what is oxidized and reduced. Cu’s oxidation number rises as it loses electrons: oxidized. The N in drops as it gains electrons: reduced. H and O don’t change.
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Check that electrons balance.
Three Cu atoms each lose 2 and two N atoms each gain 3, so they match.
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Why no H₂ forms. Forming would need to gain electrons (+1 → 0), but hydrogen’s oxidation number doesn’t change: the N in takes the electrons. just provides the acidic conditions and ends up in water with the O.
- 2Al + 3Cl₂ → 2AlCl₃
Al 0 → +3, Cl 0 → −1.
- HNO₃ + KOH → KNO₃ + H₂O
H⁺ and OH⁻ combine into water.
- CaCl₂ + Na₂CO₃ → CaCO₃(s) + 2NaCl
Insoluble CaCO₃ forms.
- Fe + 2H⁺ → Fe²⁺ + H₂
Fe 0 → +2, H +1 → 0.
- NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
H⁺ moves from H₂O to NH₃.
- 2KClO₃ → 2KCl + 3O₂
Cl +5 → −1, O −2 → 0. A decomposition can be redox.
- Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
H⁺ is transferred to OH⁻.
- Sr²⁺ + SO₄²⁻ → SrSO₄(s)
Insoluble SrSO₄ forms.
- C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Every combustion is redox.
- Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O
Cl 0 → −1 and +1: a disproportionation.
- HCO₃⁻ + H⁺ → H₂O + CO₂
H⁺ is transferred to HCO₃⁻; no oxidation number changes.
- CuO + H₂ → Cu + H₂O
Cu +2 → 0, H 0 → +1.
Common mistakes
Mistake 1: “H atoms are transferred, so it’s an acid–base reaction.” An acid–base reaction transfers H⁺, with no change in oxidation numbers. In , H goes from 0 to +1: that’s electron transfer, so the reaction is redox.
Mistake 2: “Two reactants form one product, so it’s a precipitation reaction.” A precipitation reaction is ions in solution combining into an insoluble solid. forms one product but is acid–base; is redox.
Mistake 3: “Ions appear in the equation, so it’s double replacement.” contains ions, but oxidation numbers change, so it’s a redox reaction. Classify by what is transferred.
Quick check
Copper reacts with dilute nitric acid: 3Cu + 8H⁺ + 2NO₃⁻ → 3Cu²⁺ + 2NO + 4H₂O. Which statement is best?
Cu 0 → +2 and N +5 → +2. H keeps its oxidation number, so H⁺ isn't reduced; consuming H⁺ doesn't make it acid–base; and NO is a gas, not a precipitate.
2Mg + O₂ → 2MgO is a synthesis reaction. Is it also a redox reaction?
Whether a reaction is redox depends only on whether oxidation numbers change. Form-based types such as synthesis or decomposition can be redox too.
What class of reaction is NH₃(g) + HCl(g) → NH₄Cl(s), and why?
No oxidation number changes, but a proton is transferred. A solid forming isn't the same as precipitation, which is ions in solution combining into an insoluble solid.
Which statement about Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O is correct?
The same element goes both up and down: a disproportionation. O and H keep their oxidation numbers.
H₂SO₄ solution reacts with Ba(OH)₂ solution to form a BaSO₄ precipitate and water. What class of reaction is this?
H⁺ and OH⁻ combine into water (proton transfer), and Ba²⁺ and SO₄²⁻ combine into a precipitate. No oxidation number changes.
For 2H₂ + O₂ → 2H₂O, a student says, “H atoms move from H₂ to O, so it's an acid–base reaction.” Which evaluation is correct?
An acid–base reaction transfers H⁺ with no change in oxidation numbers; here oxidation numbers change, so electrons are transferred. Reactions without ions can still be redox.
AP-style free response
Consider these three reactions:
Reaction (1):
Reaction (2):
Reaction (3) (PbCl₂ is insoluble):
(a) Classify each reaction as acid–base, redox, or precipitation, and justify each answer.
(b) For reaction (1), give the oxidation numbers of the elements that change, and identify the oxidizing and reducing agents.
(c) A student claims, “Reaction (2) produces a gas, so it’s a redox reaction.” Evaluate the claim.
(d) Write the net ionic equation for reaction (3).
Model answer and scoring
(a) (1) Redox: Zn and H change oxidation numbers. (2) Acid–base: H⁺ is transferred to CO₃²⁻, and the H₂CO₃ formed breaks down into H₂O and CO₂, with no change in oxidation numbers. (3) Precipitation: Pb²⁺ and Cl⁻ combine into insoluble PbCl₂, with no electron or proton transfer.
(b) Zn: 0 → +2, oxidized; H: +1 → 0, reduced; Cl stays −1. The oxidizing agent is HCl (its H⁺), and the reducing agent is Zn.
(c) The claim is incorrect. Forming a gas doesn’t show electron transfer. In reaction (2), Na (+1), C (+4), O (−2), H (+1), and Cl (−1) all keep their oxidation numbers, so it isn’t redox.
(d)
Scoring: (a) each classification with a reason; (b) both elements’ changes and both agents; (c) uses oxidation numbers and checks each element; (d) net ionic equation.
Summary
- Acid–base reactions transfer protons; redox reactions transfer electrons (some element changes oxidation number); precipitation reactions form an insoluble solid.
- Combustion and single replacement are always redox; synthesis and decomposition may or may not be; double replacement is usually precipitation or acid–base, and can be both.
- In a disproportionation, one element is both oxidized and reduced.
- Justify a classification by what is transferred, checked with oxidation numbers; “H is transferred”, “one product forms”, and “ions appear” aren’t valid reasons.
AP tip: Multiple-choice options often pair a claim with a reason. First use oxidation numbers to decide whether the reaction is redox, then look for proton transfer or an insoluble product, and finally check that the reason given really describes what defines that class.