AP Chemistry Unit 4 Notes: Classifying Reactions, Acid–Base Chemistry, and Redox Thinking

Chemical reaction

A process where atoms are rearranged as old bonds break and new bonds form, producing substances with new compositions and properties.

Driving force (aqueous reactions)

A factor that makes an aqueous reaction proceed by effectively removing products from solution—commonly formation of a precipitate, a gas, or a weak electrolyte (often water), or favorable electron transfer in redox.

Molecular equation

An equation that shows reactants and products as intact chemical formulas (compounds not split into ions).

Complete ionic equation

An equation that shows strong electrolytes in aqueous solution as separated ions; includes spectator ions.

Net ionic equation

An equation that shows only the species that undergo chemical change by canceling spectator ions from the complete ionic equation.

Spectator ions

Ions present in solution that appear unchanged on both sides of an ionic equation and cancel when writing the net ionic equation.

Strong electrolyte

A substance that dissociates/ionizes essentially completely in water (e.g., soluble ionic compounds, strong acids, strong bases), so it is written as ions in complete ionic equations.

Weak electrolyte

A substance that ionizes only partially in water (e.g., weak acids/bases) and is typically written mostly as molecules in ionic equations.

Combination (synthesis) reaction

A reaction pattern where two or more reactants form a single product (A + B → AB).

Decomposition reaction

A reaction pattern where one compound breaks into simpler substances (AB → A + B).

Combustion (complete, hydrocarbons)

A reaction with O2 where many hydrocarbons/oxygenated hydrocarbons produce CO2 and H2O (requires careful balancing; not every O2 reaction fits this).

Single-replacement (displacement) reaction

A reaction where an element replaces another element in a compound (often associated with redox when oxidation states change).

Double-replacement (metathesis) reaction

A reaction in aqueous solution where ions from two ionic compounds exchange partners (AB + CD → AD + CB) and proceeds only with a driving force (precipitate, gas, or water).

Precipitation reaction

An aqueous reaction that forms an insoluble solid (precipitate) from dissolved ions; predicted using solubility rules.

Solubility guidelines (high-yield AP patterns)

Rules used to predict ($aq$) vs ($s$): nitrates, Group 1, and $NH_4^+$ salts are usually soluble; $Cl^-$/$Br^-$/$I^-$ soluble except with $Ag^+$, $Pb^{2+}$, $Hg_2^{2+}$; $SO_4^{2-}$ soluble except with $Ba^{2+}$, $Sr^{2+}$, $Pb^{2+}$ ($Ca^{2+}$ sometimes low); $CO_3^{2-}$ and $PO_4^{3-}$ usually insoluble except Group 1/$NH_4^+$; $OH^-$ usually insoluble except Group 1 and $Ba^{2+}$/$Sr^{2+}$ ($Ca^{2+}$ slightly); $S^{2-}$ usually insoluble except Group 1, Group 2, $NH_4^+$ (often treated as soluble enough).

Gas-forming reaction

An aqueous reaction driven forward by production of a gas that escapes (common patterns: carbonate/bicarbonate + acid $\rightarrow CO_2 + H_2O$; sulfite + acid $\rightarrow SO_2 + H_2O$; $NH_4^+$ salt + strong base $\rightarrow NH_3 + H_2O$).

Neutralization reaction

An acid–base reaction where $H^+$($aq$) and $OH^-$($aq$) form $H_2O$($l$) (and typically a salt), with water formation acting as a driving force by reducing free ions.

Arrhenius acid

A substance that produces $H^+$ (more precisely $H_3O^+$) in water.

Arrhenius base

A substance that produces OH− in water.

Brønsted–Lowry acid

A proton (H+) donor.

Brønsted–Lowry base

A proton (H+) acceptor; includes bases like NH3 that do not contain OH− but generate OH− by reacting with water.

Strong acid

An acid that ionizes essentially completely in water (common AP list: $HCl$, $HBr$, $HI$, $HNO_3$, $HClO_4$, and $H_2SO_4$ for its first proton).

Oxidation state (oxidation number)

A bookkeeping charge assigned to atoms to track electron transfer; oxidation numbers are used to determine whether a reaction is redox and must sum to the overall species charge.

Redox reaction

A reaction involving electron transfer, evidenced by changes in oxidation states; includes processes like corrosion, batteries, and many single-replacement reactions.

Half-reaction method (acidic solution)

A systematic redox-balancing procedure: split into oxidation/reduction halves; balance atoms (not $O/H$), then $O$ with $H_2O$, $H$ with $H^+$, charge with $e^-$; scale to cancel $e^-$; add and cancel common species (then convert to basic conditions by adding $OH^-$ to neutralize $H^+$ if needed).