AP · Types of chemical reactions · 14 min read · Updated 2026-05-10

Types of chemical reactions — AP Chemistry Study Guide

For: AP Chemistry candidates sitting AP Chemistry.

Covers: Precipitation reactions, acid-base neutralization, oxidation-reduction (redox) reactions, net ionic equation writing, oxidation number assignment, and classification of combination, decomposition, single/double displacement, and combustion reactions.

You should already know: Balanced chemical equation writing, ionic compound solubility rules, charge assignment for monatomic ions.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Chemistry style for educational use. They are not reproductions of past College Board / Cambridge / IB papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official mark schemes for grading conventions.

1. What Is Types of chemical reactions?

Types of chemical reactions is a core classification framework in Unit 4 that organizes chemical transformations by the pattern of reactant rearrangement and the type of chemical change occurring. According to the AP Chemistry Course and Exam Description (CED), Unit 4 (Chemical Reactions) makes up 7-9% of total exam score, and reaction classification is assessed across both multiple-choice (MCQ) and free-response (FRQ) sections. This topic is not just a memorization exercise: the classification system allows you to predict unknown products, write accurate net ionic equations, identify the driving force for a reaction, and connect macroscopic observations to microscopic bond changes. Most AP exam questions will ask you to apply reaction classification to new contexts, for example identifying a reaction type from an unbalanced equation or predicting products for a given combination of reactants. You will also be expected to distinguish between reactions that involve proton transfer (acid-base) and electron transfer (redox), two core categories that appear repeatedly in later units including thermodynamics, kinetics, and electrochemistry.

2. Double Displacement Reactions: Precipitation and Acid-Base Neutralization

Double displacement reactions (also called metathesis reactions) follow the general pattern where two ionic reactants exchange anionic partners to form two new ionic products, with the general form: $A X + B Y \to A Y + B X$ This reaction type only proceeds to completion if one of the products is removed from the solution (as an insoluble solid, a gas, or a neutral weak electrolyte like water); otherwise all ions remain dissolved and no net reaction occurs. The two most common double displacement reactions tested on the AP exam are precipitation and acid-base neutralization. Precipitation reactions occur when at least one product is an insoluble ionic solid (called a precipitate) that falls out of solution. Using solubility rules, you can predict which product will be insoluble: for example, all nitrate salts are soluble, while most lead and barium sulfate salts are insoluble. Acid-base neutralization is a second type of double displacement where the reactants are an acid (proton donor) and a base (hydroxide or proton acceptor), producing a salt (ionic compound from the base’s cation and acid’s anion) and water. The driving force here is the formation of neutral, non-ionized water, which is a weak electrolyte. Writing net ionic equations for double displacement reactions requires removing spectator ions (ions that appear unchanged on both sides of the reaction equation) to show only the species that undergo chemical change.

Worked Example

Aqueous solutions of barium nitrate and sodium sulfate are mixed. Write the balanced net ionic equation and classify the reaction.

Write the full balanced molecular equation, using solubility rules to identify the precipitate: $B a (N O_{3})_{2} (a q) + N a_{2} S O_{4} (a q) \to B a S O_{4} (s) + 2 N a N O_{3} (a q)$ Barium sulfate is insoluble, so it is marked as solid.
Split all soluble strong electrolytes (aqueous ionic compounds) into their constituent ions to get the full ionic equation: $B a^{2 +} (a q) + 2 N O_{3}^{-} (a q) + 2 N a^{+} (a q) + S O_{4}^{2 -} (a q) \to B a S O_{4} (s) + 2 N a^{+} (a q) + 2 N O_{3}^{-} (a q)$
Cancel all spectator ions (ions that appear unchanged on both sides of the equation): $N a^{+}$ and $N O_{3}^{-}$ are spectators, so they are removed.
The remaining terms give the net ionic equation: $B a^{2 +} (a q) + S O_{4}^{2 -} (a q) \to B a S O_{4} (s)$ . This is classified as a double displacement precipitation reaction.

Exam tip: Always check solubility rules before writing net ionic equations; AP exam expects you to remember common rules (all nitrates are soluble, all group 1 salts are soluble, most sulfates are soluble except $B a S O_{4}$ , $P b S O_{4}$ , $C a S O_{4}$ ) and does not require memorization of obscure exceptions.

3. Oxidation-Reduction (Redox) Reactions and Oxidation Number Assignment

Oxidation-reduction (redox) reactions are defined as reactions that involve the transfer of one or more electrons between reactants, resulting in a change in oxidation number for the reacting species. Oxidation number (ON) is the hypothetical charge an atom would have if all bonds in the compound were fully ionic, and it follows a set of simple rules:

Any element in its elemental state has an oxidation number of 0
A monatomic ion has an oxidation number equal to its ionic charge
Oxygen is almost always -2 (exception: peroxides, where it is -1; bonded to fluorine it has a positive ON)
Hydrogen is +1 when bonded to nonmetals, -1 when bonded to metals
The sum of oxidation numbers in a neutral compound is 0, and equal to the overall charge for a polyatomic ion.

Oxidation is the loss of electrons, which causes an increase in oxidation number, while reduction is the gain of electrons, which causes a decrease in oxidation number. The common mnemonic OIL RIG (Oxidation Is Loss, Reduction Is Gain) helps remember this relationship. Any reaction with a change in oxidation number for at least one atom is classified as redox.

Worked Example

Assign oxidation numbers to all atoms in the reaction $Z n (s) + 2 H C l (a q) \to Z n C l_{2} (a q) + H_{2} (g)$ , identify if the reaction is redox, and label which species is oxidized and which is reduced.

Assign oxidation numbers per the rules: $Z n (s)$ is elemental, so ON = 0. H in HCl (bonded to nonmetal Cl) is +1, so Cl is -1. In $Z n C l_{2}$ , each Cl is -1, so Zn must be +2. $H_{2} (g)$ is elemental, so ON = 0.
Check for changes in oxidation number: Zn changes from 0 (reactant) to +2 (product), an increase of +2. H changes from +1 (reactant) to 0 (product), a decrease of -1 per H atom.
Apply OIL RIG: Zn lost electrons (oxidation number increased), so Zn is oxidized. H+ from HCl gained electrons (oxidation number decreased), so H+ is reduced.
Conclusion: This is a redox reaction.

Exam tip: Never assume a reaction is non-redox just because it is not single displacement; many combination and decomposition reactions are also redox. Always confirm by checking oxidation numbers.

4. Combination, Decomposition, and Combustion Reactions

These three reaction classes are grouped by the number of reactants and products, and most are redox reactions. Combination (or synthesis) reactions follow the general form $A + B \to A B$ , where two or more reactants combine to form a single product. For example, the reaction of solid sodium metal with chlorine gas to form sodium chloride is a combination redox reaction: sodium is oxidized, chlorine is reduced. Decomposition reactions are the reverse of combination: $A B \to A + B$ , where a single reactant breaks down into two or more simpler products. Many decomposition reactions are redox; for example, the decomposition of hydrogen peroxide into water and oxygen gas results in oxygen changing oxidation state from -1 to -2 (in water) and 0 (in oxygen gas). Combustion reactions are a specific class of redox reactions where a substance (usually a hydrocarbon or other organic fuel) reacts with oxygen gas. For complete combustion of a hydrocarbon, the only products are carbon dioxide and water vapor. Incomplete combustion produces carbon monoxide or solid carbon instead of carbon dioxide. All combustion reactions are redox, because oxygen goes from an oxidation state of 0 (as $O_{2}$ ) to -2 (in the product oxides).

Worked Example

Predict the products of complete combustion of propane ( $C_{3} H_{8}$ ), write the balanced chemical equation, and confirm it is a redox reaction.

Recall that complete combustion of a hydrocarbon only produces $C O_{2} (g)$ and $H_{2} O (g)$ , with $O_{2} (g)$ as the other reactant. Write the unbalanced equation: $C_{3} H_{8} (g) + O_{2} (g) \to C O_{2} (g) + H_{2} O (g)$ .
Balance C first: 3 C on the reactant side, so add a coefficient of 3 to $C O_{2}$ : $C_{3} H_{8} + O_{2} \to 3 C O_{2} + H_{2} O$ . Balance H next: 8 H on the reactant side, so add a coefficient of 4 to $H_{2} O$ : $C_{3} H_{8} + O_{2} \to 3 C O_{2} + 4 H_{2} O$ .
Balance O last: the product side has (3×2) + (4×1) = 10 O atoms, so add a coefficient of 5 to $O_{2}$ . The final balanced equation is: $C_{3} H_{8} (g) + 5 O_{2} (g) \to 3 C O_{2} (g) + 4 H_{2} O (g)$ .
Check oxidation numbers to confirm redox: average oxidation number of C in $C_{3} H_{8}$ is -8/3, and in $C O_{2}$ it is +4 (increase, so oxidized). O in $O_{2}$ is 0, and in products it is -2 (decrease, so reduced). The reaction is redox, as expected for combustion.

Exam tip: For incomplete combustion questions, the main carbon product is carbon monoxide (CO) instead of $C O_{2}$ ; always read the question carefully to confirm if it specifies complete or incomplete combustion.

5. Common Pitfalls (and how to avoid them)

Wrong move: Writing all ionic compounds as dissociated ions in net ionic equations, including insoluble precipitates and weak acids. Why: Students confuse all ionic compounds with strong electrolytes; only soluble strong electrolytes dissociate in net ionic equations. Correct move: Always check solubility for ionic compounds and strength for acids/bases before splitting into ions; leave insoluble solids, weak acids, and weak bases in their undissociated molecular form.
Wrong move: Calling all double displacement reactions non-redox, and misclassifying single displacement as double displacement. Why: Students forget that single displacement reactions always have one elemental reactant and are always redox, while double displacement almost never has elemental reactants. Correct move: Check for an elemental reactant or product first; an elemental species almost always indicates a redox single displacement reaction, not double displacement.
Wrong move: Assigning oxygen an oxidation number of -2 in hydrogen peroxide ( $H_{2} O_{2}$ ). Why: Students memorize the general rule for oxygen but forget the common peroxide exception tested on the AP exam. Correct move: Always check for the peroxide functional group when assigning oxidation numbers to oxygen; peroxides always have oxygen at -1.
Wrong move: Predicting solid carbon as a product of complete combustion of a hydrocarbon. Why: Students confuse complete and incomplete combustion. Correct move: Follow the rule that complete hydrocarbon combustion produces only $C O_{2}$ and $H_{2} O$ ; carbon or CO are only products of incomplete combustion.
Wrong move: Leaving spectator ions in the final net ionic equation. Why: Students rush through the canceling step and forget to remove all unchanged ions. Correct move: Always cross out every ion that appears in identical form (same state, same charge) on both sides of the full ionic equation before writing the net ionic.
Wrong move: Classifying the reaction $C a O (s) + H_{2} O (l) \to C a (O H)_{2} (a q)$ as double displacement. Why: Students see two reactants forming two products and misapply the double displacement definition. Correct move: Count the number of products vs reactants; any reaction with two reactants and one product is a combination (synthesis) reaction.

6. Practice Questions (AP Chemistry Style)

Question 1 (Multiple Choice)

Which of the following reactions is correctly classified and paired with its correct net ionic equation? A) Reaction between aqueous lead(II) nitrate and aqueous sodium chloride to form solid lead(II) chloride. Classification: redox. Net ionic: $P b^{2 +} (a q) + 2 C l^{-} (a q) \to P b C l_{2} (s)$ . B) Reaction between hydrofluoric acid (weak acid) and aqueous sodium hydroxide. Classification: acid-base neutralization. Net ionic: $H^{+} (a q) + O H^{-} (a q) \to H_{2} O (l)$ . C) Complete combustion of ethanol ( $C_{2} H_{5} O H$ ). Classification: redox. Balanced equation: $C_{2} H_{5} O H (l) + 3 O_{2} (g) \to 2 C O_{2} (g) + 3 H_{2} O (g)$ . D) Reaction between solid zinc and aqueous copper(II) sulfate. Classification: double displacement. Net ionic: $Z n (s) + C u^{2 +} (a q) \to Z n^{2 +} (a q) + C u (s)$ .

Worked Solution: Evaluate each option systematically: Option A: The reaction is a precipitation double displacement, with no change in oxidation numbers, so classification as redox is incorrect. Option B: Hydrofluoric acid is a weak acid, so it cannot be split into $H^{+}$ and $F^{-}$ in the net ionic equation; the given net ionic is incorrect. Option C: All combustion reactions are redox (oxygen changes from 0 to -2 oxidation state), and the equation is correctly balanced: 2 C, 6 H, and 7 O on both sides. This option is correct. Option D: The reaction is a single displacement redox reaction, not double displacement, so classification is incorrect. Correct answer: C.

Question 2 (Free Response)

A student mixes aqueous solutions of potassium phosphate and calcium nitrate, and observes a white precipitate form. (a) (i) Write the balanced molecular equation for this reaction. (ii) Classify the reaction by type. (b) Write the balanced net ionic equation for this reaction. Identify all spectator ions. (c) Is this reaction a redox reaction? Justify your answer with reference to oxidation numbers.

Worked Solution: (a) (i) Balanced molecular equation: $2 K_{3} P O_{4} (a q) + 3 C a (N O_{3})_{2} (a q) \to 6 K N O_{3} (a q) + C a_{3} (P O_{4})_{2} (s)$ (ii) This is a double displacement precipitation reaction. (b) Split all soluble strong electrolytes into ions to get the full ionic equation: $6 K^{+} (a q) + 2 P O_{4}^{3 -} (a q) + 3 C a^{2 +} (a q) + 6 N O_{3}^{-} (a q) \to 6 K^{+} (a q) + 6 N O_{3}^{-} (a q) + C a_{3} (P O_{4})_{2} (s)$ Cancel unchanged spectator ions. The spectator ions are $K^{+} (a q)$ and $N O_{3}^{-} (a q)$ . The net ionic equation is: $3 C a^{2 +} (a q) + 2 P O_{4}^{3 -} (a q) \to C a_{3} (P O_{4})_{2} (s)$ (c) Assign oxidation numbers to all atoms: K = +1, P = +5, O = -2, Ca = +2, N = +5 on both sides of the reaction. No species has a change in oxidation number, so this is not a redox reaction.

Question 3 (Application / Real-World Style)

Portland cement manufacturing produces large amounts of calcium carbonate ( $C a C O_{3}$ ) as an intermediate. Solid calcium carbonate is synthesized by mixing aqueous solutions of calcium chloride and sodium carbonate. Classify this reaction, write its balanced net ionic equation, and explain why the reaction proceeds spontaneously to form products rather than remaining as dissolved ions. Explain what this means for industrial processing.

Worked Solution: This is a double displacement precipitation reaction. The balanced molecular equation is $C a C l_{2} (a q) + N a_{2} C O_{3} (a q) \to C a C O_{3} (s) + 2 N a C l (a q)$ . Canceling spectator ions ( $N a^{+}$ and $C l^{-}$ ) gives the net ionic equation: $C a^{2 +} (a q) + C O_{3}^{2 -} (a q) \to C a C O_{3} (s)$ Calcium carbonate is highly insoluble in water, so it precipitates out of solution and is removed from the reaction equilibrium. This pulls the reaction forward to completion, so nearly all calcium ions are converted to solid calcium carbonate. For industrial processing, this means the solid product can be easily recovered by simple filtration, making the process low-cost and efficient.

7. Quick Reference Cheatsheet

Category	Formula / Rule	Notes
Double Displacement (Metathesis)	$A X + B Y \to A Y + B X$	Proceeds only if one product is insoluble, a gas, or neutral water
Precipitation Reaction	Double displacement forming insoluble solid	Requires solubility rules to identify the precipitate
Acid-Base Neutralization	Acid + Base → Salt + Water	Double displacement; driving force is formation of neutral water
Combination (Synthesis)	$A + B \to A B$	Two reactants form one product; often redox
Decomposition	$A B \to A + B$	One reactant breaks into multiple products; often redox
Single Displacement	$A + B X \to A X + B$	Element + ionic compound → new element + new ionic compound; always redox
Complete Combustion of Hydrocarbon	$C_{x} H_{y} + (x + \frac{y}{4}) O_{2} \to x C O_{2} + \frac{y}{2} H_{2} O$	Always redox; only $C O_{2}$ and $H_{2} O$ are products
Elemental State Oxidation Rule	ON = 0	Applies to all pure elements, including diatomics
Oxygen Oxidation Rule	ON = -2	Except peroxides (ON = -1) and F-O bonds (ON positive)
Hydrogen Oxidation Rule	ON = +1 with nonmetals, ON = -1 with metals	No common exceptions tested on AP
Redox Definition	Any reaction with a change in ON for at least one atom	OIL RIG: Oxidation Is Loss (e⁻, ON increases), Reduction Is Gain (e⁻, ON decreases)
Net Ionic Equation Rule	Remove spectator ions (unchanged ions)	Leave insoluble solids, weak acids/bases, gases in molecular form

8. What's Next

After mastering types of chemical reactions, you will immediately apply this classification framework to more advanced net ionic equation writing and reaction stoichiometry, the next core topics in Unit 4. This topic is the foundational prerequisite for nearly all subsequent units in AP Chemistry: you will use reaction classification to identify redox reactions for electrochemistry, predict acid-base behavior for equilibrium and titrations, and balance combustion reactions for thermochemistry enthalpy calculations. Without correctly classifying a reaction, you cannot reliably predict products, write accurate net ionic equations, or solve quantitative stoichiometry problems, which make up over 15% of the total AP exam score. Mastering reaction types now will simplify even complex reaction problems in later units.

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Types of chemical reactions — AP Chemistry Study Guide

1. What Is Types of chemical reactions?

2. Double Displacement Reactions: Precipitation and Acid-Base Neutralization

Worked Example

3. Oxidation-Reduction (Redox) Reactions and Oxidation Number Assignment

Worked Example

4. Combination, Decomposition, and Combustion Reactions

Worked Example

5. Common Pitfalls (and how to avoid them)

6. Practice Questions (AP Chemistry Style)

Question 1 (Multiple Choice)

Question 2 (Free Response)

Question 3 (Application / Real-World Style)

7. Quick Reference Cheatsheet

8. What's Next

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