Chemistry · Unit 4: Chemical Reactions · 14 min read · Updated 2026-05-11
Introduction to Reactions — AP Chemistry
AP Chemistry · Unit 4: Chemical Reactions · 14 min read
1. Core Definition of Chemical Reactions★☆☆☆☆⏱ 2 min
Introduction to chemical reactions is the opening topic of AP Chemistry Unit 4, covering core conventions for representing chemical change. This topic contributes approximately 2-3% of your total AP Chemistry exam score, with the full Unit 4 accounting for 7-9% of total weight. Balanced equations are almost always required as the first step for longer FRQs covering stoichiometry, titrations, or equilibrium.
2. Balancing Chemical Equations by Inspection★★☆☆☆⏱ 4 min
Balancing chemical equations adjusts stoichiometric coefficients (whole numbers in front of each species) to satisfy the law of conservation of mass: the number of atoms of every element must be equal on the reactant and product sides. The most critical rule: **never change subscripts in chemical formulas** to balance an equation, because this changes the identity of the substance.
Write the unbalanced equation with all correct chemical formulas and state symbols first
Count atoms of each element on both sides
Start with elements that appear in only one reactant and one product
Leave pure elemental species (like $O_2$, $H_2$) for last
Clear any fractions by multiplying all coefficients by the denominator
Reduce coefficients to the lowest whole-number ratio
Exam tip: Always recount oxygen last after adjusting coefficients for $CO_2$ and $H_2O$ in combustion problems; it is the most common element to have an unbalanced count.
3. Molecular, Full, and Net Ionic Equations★★★☆☆⏱ 4 min
For reactions in aqueous solution, three levels of representation are used: (1) Molecular equations show all compounds as neutral molecules, even if they dissociate in water. (2) Full ionic equations split all strong electrolytes into free ions, since these exist as separated ions in solution. (3) Net ionic equations remove spectator ions to show only the species that actually react. Net ionic equations must balance both atoms and total charge.
Exam tip: Always confirm solubility before splitting into ions; if a compound is insoluble, leave it as a neutral solid in the net ionic equation to earn full credit.
4. Classifying Common Chemical Reaction Types★★☆☆☆⏱ 3 min
AP Chemistry requires you to classify reactions by their pattern to predict products for unknown scenarios, a common exam skill. The five core reaction types you need to master are:
**Combination (Synthesis)**: Two or more reactants combine to form a single product: $A + B \rightarrow AB$. Example: $2Mg(s) + O_2(g) \rightarrow 2MgO(s)$.
**Decomposition**: A single reactant breaks down into two or more smaller products: $AB \rightarrow A + B$. Example: $2H_2O_2(aq) \rightarrow 2H_2O(l) + O_2(g)$.
**Combustion**: A fuel (usually a hydrocarbon) reacts with oxygen gas. Complete combustion of hydrocarbons produces only carbon dioxide and water.
**Double Displacement (Metathesis)**: Two ionic compounds swap cations and anions to form two new compounds: $AB + CD \rightarrow AD + CB$. Includes precipitation and acid-base neutralization.
**Single Displacement**: An elemental species displaces another element from its compound: $A + BC \rightarrow AC + B$. Example: $Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)$.
Exam tip: For any reaction involving a hydrocarbon and oxygen, assume complete combustion (only $CO_2$ and $H_2O$ as products) unless the problem explicitly states incomplete combustion.
5. AP Style Concept Check★★★☆☆⏱ 2 min
Common Pitfalls
Why: Students confuse stoichiometric coefficients (which count number of molecules) with subscripts (which count number of atoms per molecule).
Why: Students remember that ionic compounds split into ions, so they split all ionic compounds regardless of solubility.
Why: Students think state symbols are trivial, so they skip them.
Why: Students stop balancing after getting the correct atom count without clearing fractions.
Why: Students focus on atom count and forget that net ionic equations must have equal total charge on both sides.