Chemical Reactions: every key term you need (+ practice quiz)
87 flashcard terms for AP Chemistry Unit 4, written to match the course framework. Read them here, drill them as flashcards, or take the 46-question quiz. Free, no account needed.
A process where reactants are transformed into products through the breaking and forming of bonds, rearranging atoms without creating or destroying them.
Law of Conservation of Mass
Matter is neither created nor destroyed in a chemical reaction; the total mass of reactants equals the total mass of products.
Balancing Chemical Equations
Adjusting coefficients (never subscripts) so the number of each type of atom is equal on both sides of the equation, satisfying conservation of mass.
Coefficients vs. Subscripts
Coefficients (in front of a formula) scale the whole molecule's quantity; subscripts (within a formula) are fixed and define the compound itself — changing them creates a different substance.
Synthesis (Combination) Reaction
Two or more reactants combine to form one product: A + B → AB.
Decomposition Reaction
One reactant breaks down into two or more simpler products: AB → A + B.
Single Replacement Reaction
One element displaces another element in a compound: A + BC → AC + B.
Double Replacement Reaction
Two compounds exchange ions or components: AB + CD → AD + CB; often produces a precipitate, gas, or water.
Combustion Reaction
A substance (often a hydrocarbon) reacts rapidly with oxygen, releasing energy and typically producing CO₂ and H₂O.
Precipitation Reaction
A double replacement reaction where two soluble ionic solutions combine to form an insoluble solid (precipitate) that separates from solution.
Solubility Rules
Guidelines predicting which ionic compounds dissolve in water and which form precipitates (e.g., most nitrates are soluble; most carbonates are insoluble).
Net Ionic Equation
An equation showing only the species that actually participate in a reaction, omitting spectator ions that remain unchanged in solution.
Spectator Ions
Ions present in a reaction mixture that do not participate in the actual chemical change and appear unchanged on both sides of a full ionic equation.
Complete Ionic Equation
An equation showing all soluble ionic compounds as separated (dissociated) ions in solution, before removing spectator ions to get the net ionic equation.
Acid-Base (Neutralization) Reaction
A reaction between an acid and a base, typically producing water and a salt (e.g., HCl + NaOH → NaCl + H₂O).
Arrhenius Acid and Base
An Arrhenius acid increases H⁺ (H3O+) concentration in water; an Arrhenius base increases OH⁻ concentration in water.
Brønsted-Lowry Acid and Base
A Brønsted-Lowry acid is a proton (H⁺) donor; a Brønsted-Lowry base is a proton acceptor — a broader definition than Arrhenius, not requiring water.
Conjugate Acid-Base Pairs
An acid and base that differ by a single proton (H⁺); when an acid donates H⁺, it becomes its conjugate base, and vice versa.
A reaction involving the transfer of electrons between species, changing their oxidation states.
Oxidation
The loss of electrons by a species, causing its oxidation state to increase (become more positive).
Reduction
The gain of electrons by a species, causing its oxidation state to decrease (become more negative) — 'reduces' the charge.
OIL RIG Mnemonic
Oxidation Is Loss (of electrons); Reduction Is Gain (of electrons) — a memory aid for redox direction.
Oxidizing Agent
The species that is REDUCED in a redox reaction (it gains electrons), thereby causing another species to be oxidized.
Reducing Agent
The species that is OXIDIZED in a redox reaction (it loses electrons), thereby causing another species to be reduced.
Oxidation State (Number)
A bookkeeping charge assigned to an atom in a compound, tracking electron 'ownership' — used to identify which species are oxidized or reduced in a reaction.
Assigning Oxidation States
Free elements = 0; monatomic ions = their charge; O is usually −2; H is usually +1; the sum of oxidation states in a neutral compound equals 0 (or the ion's charge).
Stoichiometry
The quantitative relationships between reactants and products in a chemical reaction, based on the mole ratios given by a balanced equation's coefficients.
Mole Ratio
The ratio of coefficients between two substances in a balanced equation, used as a conversion factor between moles of one substance and moles of another.
Stoichiometric Calculations — Steps
Convert given quantity to moles → use the mole ratio from the balanced equation to find moles of the desired substance → convert to the desired unit (grams, particles, liters).
Limiting Reactant
The reactant that is completely consumed first in a reaction, stopping the reaction and determining the maximum amount of product that can form.
Excess Reactant
The reactant that remains partially unreacted because the limiting reactant runs out first.
Identifying the Limiting Reactant
Calculate the amount of product each reactant COULD produce (given enough of the other); the reactant yielding the LEAST product is limiting.
Theoretical Yield
The maximum amount of product predicted by stoichiometric calculation, based entirely on the limiting reactant and assuming the reaction goes to completion.
Actual Yield
The amount of product actually obtained in a real experiment or reaction, usually less than the theoretical yield due to side reactions, impurities, or incomplete reactions.
Percent Yield
(Actual yield ÷ Theoretical yield) × 100% — a measure of a reaction's efficiency in practice.
Why Actual Yield Is Usually Lower
Side reactions, impure reactants, reactions that don't go to completion, and product loss during collection/purification all reduce actual yield below the theoretical maximum.
When reactants are in solution, molarity (mol/L) converts volume of solution directly to moles, feeding into the same mole-ratio stoichiometry as solids or gases.
Titration
A technique using a solution of known concentration (titrant) to determine the unknown concentration of another solution, based on the stoichiometry of their reaction.
Equivalence Point
The point in a titration where moles of acid exactly equal moles of base (or the stoichiometrically equivalent amounts have reacted completely).
Indicator (Titration)
A substance that changes color at or near the equivalence point, signaling when to stop adding titrant.
Titration Calculation
Using the molarity and volume of the known solution to find moles reacted, then the balanced equation's mole ratio to find moles (and then molarity) of the unknown solution.
Gas Stoichiometry
Using the ideal gas law (PV = nRT) or molar volume at STP (22.4 L/mol) to convert between gas volume and moles for stoichiometric calculations involving gaseous reactants or products.
Molar Volume at STP
One mole of any ideal gas occupies 22.4 liters at Standard Temperature and Pressure (0°C, 1 atm) — a shortcut for gas-phase mole conversions.
Empirical Verification of Reactions
Chemists confirm proposed reactions and mechanisms through observable evidence: color change, precipitate formation, gas bubbles, temperature change, or spectroscopic data.
Evidence of a Chemical Reaction
Signs a reaction occurred include: color change, precipitate forming, gas produced (bubbles), temperature change (exo/endothermic), or odor change — though some are also physical changes.
Physical vs. Chemical Change
A physical change alters form/appearance without changing chemical identity (melting ice); a chemical change transforms substances into new ones with different properties (rusting iron).
Reaction Mechanisms (Intro)
Many overall reactions actually proceed through a series of simpler elementary steps; the sum of these steps' equations gives the overall balanced equation.
Combustion Analysis
A technique burning an organic compound completely in oxygen and measuring the CO₂ and H₂O produced to determine the compound's empirical formula.
Avogadro's Number in Stoichiometry
6.022 × 10²³ particles per mole connects moles (from stoichiometric ratios) to countable numbers of atoms, molecules, or ions when needed.
Consecutive (Sequential) Reactions
When the product of one reaction becomes the reactant of a next reaction, requiring stoichiometric calculations to be chained through multiple balanced equations.
Half-Reactions (Redox)
Splitting a redox reaction into two separate equations — one showing oxidation (electron loss) and one showing reduction (electron gain) — that must balance in both mass and charge.
Balancing Redox Half-Reactions
Electrons lost in the oxidation half-reaction must equal electrons gained in the reduction half-reaction when the two are combined into the overall balanced redox equation.
Activity Series (Metals)
A ranked list of metals by their tendency to be oxidized (lose electrons); a metal higher on the series can displace (reduce) the ion of a metal lower on the series in a single replacement reaction.
Predicting Single Replacement Outcomes
Using the activity series: a reaction only proceeds if the free element is more reactive (higher on the series) than the element it's displacing from a compound.
Remove spectator ions that appear unchanged on both sides; keep solids, gases, liquids, and weak electrolytes as molecules (e.g., HC₂H₃O₂ + OH⁻ → C₂H₃O₂⁻ + H₂O).
Solubility Rules Highlights
Group 1 and NH₄⁺ salts, nitrates, and acetates are always soluble; most halides are soluble except Ag⁺, Pb²⁺, Hg₂²⁺; most carbonates, phosphates, hydroxides, sulfides are insoluble.
Limiting Reactant Logic
Convert each reactant to moles of product; the reactant giving the least product is limiting. Percent yield = actual/theoretical × 100.
Titration Stoichiometry
At the equivalence point, moles of acid × (H⁺ per acid) = moles of base × (OH⁻ per base). Diprotic acids need twice the base per mole.
Gravimetric Analysis
Precipitate an ion, filter, dry, and weigh; use the precipitate mass and molar mass to find the original ion amount. Incomplete drying inflates results.
Oxidation Number Rules
Free elements 0; O usually −2 (peroxides −1); H +1 with nonmetals; sum equals the species charge. Changes in oxidation number identify redox.
Oxidizing vs. Reducing Agent
The oxidizing agent is reduced (gains electrons); the reducing agent is oxidized. In Zn + Cu²⁺, Cu²⁺ is the oxidizing agent.
Half-Reaction Method (Acidic)
Balance atoms other than O and H, add H₂O for O, H⁺ for H, electrons for charge; multiply to equalize electrons; add and cancel.
Half-Reaction Method (Basic)
Balance as in acid, then add OH⁻ to both sides equal to the H⁺, combining H⁺ + OH⁻ into water and canceling.
Redox Titration
Permanganate (MnO₄⁻, purple → colorless Mn²⁺) is self-indicating; 1 mol MnO₄⁻ accepts 5 mol e⁻ in acid, so mole ratios follow the balanced half-reactions.
Brønsted-Lowry Acids and Bases
Acids donate H⁺, bases accept H⁺; conjugate pairs differ by one proton. Water is amphoteric.
Classifying Reactions
Precipitation (ion exchange forms a solid), acid-base (proton transfer), redox (electron transfer). Some reactions fit more than one class.
Physical vs. Chemical Change Evidence
Chemical change forms new substances: gas evolution, precipitate, color change, energy change. Dissolving NaCl is physical; dissolving Zn in acid is chemical.
Combustion Reactions
Hydrocarbon + O₂ → CO₂ + H₂O; balance C, then H, then O last. Incomplete combustion produces CO or soot.
Molecular-Level Representations
Particle diagrams must show correct stoichiometry and the limiting reactant left over; spectator ions remain dissolved in beaker drawings.
Synthesis and Decomposition Patterns
Metal carbonates decompose to metal oxide + CO₂ on heating; metal + nonmetal form ionic salts; nonmetal oxides + water form acids.
Spectator Ion Identification
Ions that appear unchanged and fully dissolved on both sides of a complete ionic equation are spectators and are removed to give the net ionic equation.
Writing Net Ionic Equations
Split only strong electrolytes into ions; keep solids, gases, weak acids, and water intact. The remaining species show the actual chemical change.
Combine cations and anions crosswise, then apply solubility rules. If either new pairing is insoluble, a precipitate forms and drives the reaction forward.
Strong vs. Weak Electrolyte Representation
Strong acids like HNO₃ appear as separated ions, while acetic acid is written as intact CH₃COOH because only a small fraction ionizes.
Percent Yield Analysis
Percent yield = (actual ÷ theoretical) × 100. Values below 100% often reflect side reactions, incomplete reactions, or product lost during transfer and filtration.
Excess Reactant Remaining
After identifying the limiting reactant, subtract the moles consumed from the moles supplied of the other reactant to find what is left over.
Back Titration Strategy
Add a known excess of one reagent, let it react completely, then titrate the leftover. Moles reacted equal moles added minus moles titrated.
Standardization of a Titrant
Titrate a primary standard of exactly known mass to fix the titrant's true molarity, since solids like NaOH absorb water and cannot be weighed accurately.
Equivalence Point vs. Endpoint
The equivalence point is where stoichiometric amounts have reacted; the endpoint is where the indicator changes. Good indicator choice keeps the difference negligible.
Gravimetric Analysis Calculation
Convert precipitate mass to moles, use the formula to find moles of the target ion, then divide by the original sample mass for percent composition.
Balancing Redox in Basic Solution
Balance as if acidic, then add enough OH⁻ to both sides to neutralize every H⁺, combining pairs into water and canceling duplicates.
Disproportionation
A single species is both oxidized and reduced, as when Cl₂ in base forms Cl⁻ and ClO⁻; the same element appears in two different oxidation states among the products.
Oxidation States in Peroxides
Oxygen is normally −2 but is −1 in peroxides such as H₂O₂ and −1/2 in superoxides, so the usual rule must be checked against the formula.
Assigning Oxidation States in Polyatomic Ions
The oxidation states within an ion must sum to the ion's charge. In Cr₂O₇²⁻, seven oxygens at −2 total −14, so each chromium must be +6.
Titration Curve Shape Logic
pH changes slowly in the buffer region, jumps sharply near equivalence, and levels off in excess titrant; the steep region's midpoint locates the equivalence volume.
Stoichiometry with Solutions
moles = molarity × volume in liters. Combining that with the balanced mole ratio links volumes of two solutions directly.
Limiting Reactant from Particle Diagrams
Count each reactant's particles, divide by its coefficient, and the smaller quotient identifies the limiting reactant regardless of which looks more numerous.