Types of Chemical Reactions

AP Chemistry· AP Chemistry CED — Chemical Reactions· 14 min read

1. Core Framework for Classifying Chemical Reactions★★☆☆☆⏱ 2 min

Reaction classification organizes chemical transformations by patterns of reactant rearrangement and chemical change. This topic makes up 7-9% of your total AP Chemistry exam score, assessed in both multiple-choice and free-response sections. It is not just memorization: classification lets you predict unknown products, write accurate net ionic equations, and identify the driving force for a reaction, connecting macroscopic observations to microscopic bond changes. Exam questions typically ask you to apply classification to new contexts, such as identifying a reaction type from an unbalanced equation or predicting products for a given set of reactants.

2. Double Displacement: Precipitation and Acid-Base Neutralization★★☆☆☆⏱ 4 min

📘 Definition

Double Displacement (Metathesis) Reaction

AX + BY \rightarrow AY + BX

A reaction where two ionic reactants exchange anionic partners to form two new products, following the general form above.

This reaction only proceeds to completion if one product is removed from solution (as an insoluble solid, gas, or neutral weak electrolyte like water); if all ions remain dissolved, no net reaction occurs. The two most common double displacement reactions tested are precipitation and acid-base neutralization.

Precipitation reactions form an insoluble ionic solid (precipitate) that falls out of solution. Solubility rules let you predict which product will be insoluble. Acid-base neutralization occurs between an acid (proton donor) and base (proton acceptor), producing a salt and water. The driving force here is formation of non-ionized water, a weak electrolyte. Net ionic equations remove spectator ions (ions unchanged on both sides) to show only reacting species.

📐 Worked Example

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

1
Write the full balanced molecular equation, using solubility rules to identify the precipitate:
$Ba(NO_3)_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaNO_3(aq)$
2
Barium sulfate is insoluble, so it is marked as a solid.
3
Split all soluble strong electrolytes (aqueous ionic compounds) into their constituent ions to get the full ionic equation:
$Ba^{2+}(aq) + 2NO_3^-(aq) + 2Na^+(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s) + 2Na^+(aq) + 2NO_3^-(aq)$
4
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.
5
The remaining terms give the net ionic equation:
$Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s)$
6
This reaction 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★★★☆☆⏱ 4 min

📘 Definition

Oxidation-Reduction (Redox) Reaction

A reaction that involves transfer of one or more electrons between reactants, resulting in a change in oxidation number for reacting species.

Oxidation number (ON) is the hypothetical charge an atom would have if all bonds in the compound were fully ionic, and follows a set of standard 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 causes an increase in oxidation number, while reduction causes a decrease in oxidation number. 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.

1
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.
2
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.
3
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.
4
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★★☆☆☆⏱ 3 min

These three reaction classes are grouped by the number of reactants and products, and most are redox reactions:

Combination (Synthesis): $A + B \to A B$ , two or more reactants form one product. Most are redox.
Decomposition: $A B \to A + B$ , one reactant breaks into two or more simpler products. Many are redox.
Combustion: A reaction where a fuel (usually a hydrocarbon) reacts with oxygen gas. All combustion reactions are redox; complete combustion of hydrocarbons produces only carbon dioxide and water.

📐 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.

1
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:
$C_3H_8(g) + O_2(g) \rightarrow CO_2(g) + H_2O(g)$
2
Balance C first: 3 C on the reactant side, so add a coefficient of 3 to $C O_{2}$ :
$C_3H_8 + O_2 \rightarrow 3CO_2 + H_2O$
3
Balance H next: 8 H on the reactant side, so add a coefficient of 4 to $H_{2} O$ :
$C_3H_8 + O_2 \rightarrow 3CO_2 + 4H_2O$
4
Balance O last: the product side has $(3 \times 2) + (4 \times 1) = 10$ O atoms, so add a coefficient of 5 to $O_{2}$ . The final balanced equation is:
$C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(g)$
5
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. AP-Style Concept Check★★★☆☆⏱ 3 min

✓ Quick check

Test your understanding with these AP-style questions:

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: $Z n (s) + C u^{2 +} (a q) \to Z n^{2 +} (a q) + C u (s)$
Reveal answer
C —
Option A: Precipitation is not redox. Option B: Weak acid cannot be split into ions. Option D: This is single displacement, not double displacement. Only C is correct.

6. Common Pitfalls

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.

7. Quick Reference Cheatsheet

Reaction Category	General Form / Key Rule	AP Exam Notes
Double Displacement (Metathesis)	$A X + B Y \to A Y + B X$	Proceeds only if one product is insoluble, gas, or neutral water
Precipitation Reaction	Double displacement forming insoluble solid	Requires solubility rules to identify precipitate
Acid-Base Neutralization	Acid + Base → Salt + Water	Double displacement; driving force is neutral water formation
Combination (Synthesis)	$A + B \to A B$	Two reactants → one product; often redox
Decomposition	$A B \to A + B$	One reactant → multiple products; often redox
Single Displacement	$A + B X \to A X + B$	Element + ionic compound; always redox
Complete Hydrocarbon Combustion	$C_{x} H_{y} + (x + \frac{y}{4}) O_{2} \to x C O_{2} + \frac{y}{2} H_{2} O$	Always redox; only $C O_{2}$ and $H_{2} O$ are products

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2023 · MCQ
Classify reaction type
2022 · FRQ
Write net ionic equation
2021 · MCQ
Assign oxidation numbers

What's Next

Mastering reaction classification is a foundational skill for all remaining AP Chemistry units. You will apply these concepts when studying reaction stoichiometry to calculate product yields for precipitation and acid-base reactions, and when exploring electrochemistry, where all electrochemical reactions are redox reactions by definition. Reaction classification also helps you predict the direction of spontaneous change in thermodynamics and identify reaction mechanisms in kinetics. This subtopic builds directly on your understanding of chemical bonding and ionic compounds, and it prepares you for more advanced topics involving net ionic equation writing and electron transfer that are heavily assessed on the AP Chemistry exam.