UnofficialAP ChemistryUnit 4 of 9

AP Chemistry Unit 4: Chemical Reactions

Balanced equations, stoichiometry and reaction types.

Unit overview

This unit is about representing and quantifying reactions. A balanced equation gives mole ratios, which are the bridge in every stoichiometry problem: convert the given quantity to moles, use the ratio, then convert to the requested unit.

Net ionic equations show only the species that change, leaving out spectator ions. You should recognise precipitation, acid-base and oxidation-reduction reactions, and assign oxidation numbers to identify what is oxidised and reduced. Titrations use a known solution to find an unknown concentration, and physical versus chemical changes are distinguished by whether bonds in substances are broken and formed.

Key topics

Balancing and net ionic equations: Conserve atoms and charge; remove spectator ions.
Stoichiometry: Mass to moles, mole ratio, moles to mass; limiting reactant and percent yield.
Types of reactions: Precipitation, acid-base and redox.
Titration: At the equivalence point moles of titrant react exactly with the analyte.

Worked example

How many grams of water form when 4 g of hydrogen gas reacts completely with excess oxygen ( $2H_2 + O_2 \to 2H_2O$ )? Use $H_2 = 2$ g/mol and $H_2O = 18$ g/mol.

Moles of $H_2$ : $4/2 = 2$ mol.
Ratio $H_2 : H_2O = 2 : 2$ , so 2 mol of water form.
Mass: $2 \cdot 18 = 36$ g.

Answer: $36$ g

Common mistakes

Using mass ratios instead of mole ratios from the balanced equation.
Splitting weak acids or solids into ions in a net ionic equation.
Forgetting to identify the limiting reactant when both amounts are given.

Exam tips

Write units and substance labels on every quantity in a stoichiometry chain.
For redox, track oxidation numbers rather than guessing from the equation.

Chemistry Core Formulas (6 flashcards)

See the AP Chemistry exam format and every unit on the course page.