Types of Chemical Reactions — AP Chemistry
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
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.
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 $BaSO_4$, $PbSO_4$, $CaSO_4$) and does not require memorization of obscure exceptions.
3. Oxidation-Reduction (Redox) Reactions and Oxidation Number Assignment ★★★☆☆ ⏱ 4 min
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.
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):** $A + B \rightarrow AB$, two or more reactants form one product. Most are redox.
- **Decomposition:** $AB \rightarrow A + B$, 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.
Exam tip: For incomplete combustion questions, the main carbon product is carbon monoxide (CO) instead of $CO_2$; always read the question carefully to confirm if it specifies complete or incomplete combustion.
5. AP-Style Concept Check ★★★☆☆ ⏱ 3 min
Common Pitfalls
Why: Students confuse all ionic compounds with strong electrolytes; only soluble strong electrolytes dissociate in net ionic equations.
Why: Students forget that single displacement reactions always have one elemental reactant and are always redox, while double displacement almost never has elemental reactants.
Why: Students memorize the general rule for oxygen but forget the common peroxide exception tested on the AP exam.
Why: Students confuse complete and incomplete combustion.
Why: Students rush through the canceling step and forget to remove all unchanged ions.
Why: Students see two reactants forming two products and misapply the double displacement definition.