AP Chemistry Vocabulary

Absolute zero

The lowest possible temperature (0 K or −273.15°C) at which all molecular motion theoretically ceases.

Example: "         is the starting point of the Kelvin temperature scale."

Absorption spectrum

A spectrum showing dark lines or bands at wavelengths where light has been absorbed by a substance, corresponding to electron transitions to higher energy levels.

Example: "The          of hydrogen shows dark lines at specific wavelengths where photons are absorbed by electrons."

Accuracy

The closeness of a measured value to the true or accepted value of a quantity.

Example: "         describes how close a measurement is to the actual value being measured."

Acid

A substance that donates protons (H⁺), accepts electron pairs, or produces H₃O⁺ in aqueous solution.

Example: "Hydrochloric acid is a strong          that completely ionizes in water."

Acid dissociation constant (Ka)

The equilibrium constant for the ionization of a weak acid in water, indicating the extent to which the acid donates protons.

Similar definitions: Ka

Example: "The          of acetic acid is 1.8 × 10⁻⁵, indicating it is a weak acid."

Acid-base equilibrium

The equilibrium established when a weak acid or weak base partially ionizes in water, described by Ka or Kb.

Example: "         calculations are used to find the pH of weak acid and weak base solutions."

Acid-base indicator

A weak acid or base that changes color depending on the pH of the solution.

Similar definitions: Indicator

Example: "Litmus is an          that turns red in acidic solutions and blue in basic solutions."

Acid-base titration

A titration in which an acid is neutralized by a base (or vice versa) to determine the concentration of the unknown solution.

Example: "An          of HCl with NaOH reaches its equivalence point when moles of acid equal moles of base."

Activated complex

A temporary, high-energy, unstable arrangement of atoms that forms at the peak of the energy profile during a chemical reaction.

Similar definitions: Transition state

Example: "The          exists at the top of the energy barrier and can either form products or revert to reactants."

Activation energy

The minimum amount of energy required for reactant molecules to undergo a chemical reaction and form products.

Similar definitions: Ea

Example: "A catalyst lowers the          of a reaction, allowing it to proceed faster."

Activity series

A list of metals arranged by their tendency to be oxidized (lose electrons), used to predict whether a single displacement reaction will occur.

Example: "According to the         , zinc can displace copper from a CuSO₄ solution because zinc is more reactive."

Actual yield

The amount of product actually obtained from a chemical reaction, which is typically less than the theoretical yield.

Example: "The          of a reaction may be lower than expected due to incomplete reactions or product loss during collection."

Adhesion

The attractive force between molecules of different substances.

Example: "         between water and glass causes a meniscus to curve upward in a glass tube."

Alkali metal

An element in Group 1 of the periodic table (excluding hydrogen) that is highly reactive, has one valence electron, and forms +1 cations.

Example: "Sodium is an          that reacts vigorously with water to produce hydrogen gas."

Alkaline earth metal

An element in Group 2 of the periodic table that has two valence electrons, is reactive, and forms +2 cations.

Example: "Calcium is an          commonly found in bones and limestone."

Allotrope

Different structural forms of the same element in the same physical state.

Example: "Diamond and graphite are          forms of carbon with very different physical properties."

Amorphous solid

A solid that lacks long-range order in the arrangement of its atoms or molecules.

Similar definitions: Noncrystalline solid

Example: "Glass is an          because its atoms are arranged randomly rather than in a repeating lattice."

Amphiprotic

A substance capable of both donating and accepting a proton (H⁺) in acid-base reactions.

Example: "The hydrogen carbonate ion (HCO₃⁻) is          because it can act as either an acid or a base."

Amphoteric

A substance that can act as either an acid or a base depending on the reaction conditions.

Example: "Water is          because it can donate or accept protons in different reactions."

Anhydrous

Describes a substance from which all water has been removed.

Example: "         copper sulfate (CuSO₄) is a white powder that turns blue when water is added."

Anion

A negatively charged ion formed when an atom or molecule gains one or more electrons.

Example: "Chloride (Cl⁻) is an          formed when chlorine gains one electron."

Anode

The electrode at which oxidation occurs in an electrochemical cell.

Example: "In a galvanic cell, the          is the negative terminal where the metal is oxidized."

Antibonding orbital

A molecular orbital that is higher in energy than the atomic orbitals from which it was formed, with a node between the nuclei that destabilizes the bond.

Example: "Electrons in an          weaken the bond between atoms and are designated with an asterisk (σ* or π*)."

Aqueous solution

A solution in which water is the solvent.

Example: "When sodium chloride dissolves in water, it forms an          containing Na⁺ and Cl⁻ ions."

Arrhenius acid

A substance that increases the concentration of H⁺ ions when dissolved in water.

Example: "HNO₃ is an          because it produces H⁺ ions in aqueous solution."

Arrhenius base

A substance that increases the concentration of OH⁻ ions when dissolved in water.

Example: "KOH is an          because it produces OH⁻ ions in aqueous solution."

Arrhenius equation

A mathematical expression that relates the rate constant of a reaction to temperature and activation energy: k = Ae^(-Ea/RT).

Example: "The          shows that increasing temperature exponentially increases the rate constant."

Arrhenius theory

An acid-base theory defining acids as substances that produce H⁺ ions and bases as substances that produce OH⁻ ions in aqueous solution.

Example: "The          is limited to aqueous solutions and cannot explain the basicity of ammonia."

Atmosphere (atm)

A unit of pressure equal to 101,325 pascals, approximately the average air pressure at sea level.

Example: "Standard pressure is defined as 1         , which equals 760 mmHg."

Atom

The smallest unit of an element that retains the chemical properties of that element, consisting of protons, neutrons, and electrons.

Example: "An          of carbon has 6 protons, 6 electrons, and typically 6 neutrons."

Atomic mass

The weighted average mass of all naturally occurring isotopes of an element, expressed in atomic mass units (amu).

Example: "The          of chlorine is approximately 35.45 amu due to the natural abundance of its isotopes."

Atomic mass unit

A unit of mass equal to one-twelfth the mass of a carbon-12 atom, approximately 1.66 × 10⁻²⁴ grams.

Similar definitions: amu, Dalton

Example: "One          (amu) is defined relative to the mass of a carbon-12 atom."

Atomic number

The number of protons in the nucleus of an atom, which defines the element and its position in the periodic table.

Example: "The          of oxygen is 8, meaning every oxygen atom has 8 protons."

Atomic radius

Half the distance between the nuclei of two identical atoms bonded together.

Example: "The          generally decreases across a period due to increasing effective nuclear charge."

Atomization energy

The enthalpy change required to convert one mole of a substance into gaseous atoms.

Example: "The          of molecular oxygen is the energy needed to break O₂ into two separate O atoms."

Aufbau diagram

A diagram showing the order in which atomic orbitals are filled with electrons, from lowest to highest energy.

Similar definitions: Orbital filling diagram

Example: "An          shows that the 4s orbital fills before the 3d orbitals."

Aufbau principle

The rule stating that electrons fill atomic orbitals in order of increasing energy, from lowest to highest.

Example: "According to the         , the 2s orbital fills before the 2p orbitals."

Autoionization

The spontaneous reaction of a pure liquid to form small concentrations of cations and anions.

Example: "The          of water produces equal concentrations of H₃O⁺ and OH⁻ ions."

Average rate

The change in concentration of a reactant or product divided by the change in time over a specified interval.

Example: "The          of a reaction can be calculated from concentration data at two different time points."

Avogadro's law

A gas law stating that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules (V ∝ n).

Example: "         explains why the molar volume of any ideal gas at STP is 22.4 L."

Avogadro's number

The number of particles (atoms, molecules, or ions) in one mole of a substance, equal to 6.022 × 10²³.

Example: "         tells us there are 6.022 × 10²³ atoms in exactly 12 grams of carbon-12."

Balanced equation

A chemical equation in which the number of atoms of each element is equal on both sides, satisfying the law of conservation of mass.

Example: "A          for combustion of methane is CH₄ + 2O₂ → CO₂ + 2H₂O."

Base

A substance that accepts protons (H⁺), donates electron pairs, or produces OH⁻ in aqueous solution.

Example: "Sodium hydroxide is a strong          that fully dissociates in water."

Base dissociation constant (Kb)

The equilibrium constant for the reaction of a weak base with water, indicating the extent of proton acceptance.

Similar definitions: Kb

Example: "The          of ammonia is 1.8 × 10⁻⁵, reflecting its strength as a weak base."

Battery

A device consisting of one or more electrochemical cells that converts chemical energy into electrical energy.

Example: "A          produces electricity through spontaneous redox reactions occurring at its electrodes."

Beer-Lambert law

A law relating the absorbance of light by a solution to its concentration, path length, and molar absorptivity: A = εbc.

Example: "The          is used in spectrophotometry to determine the concentration of a colored solution."

Bent shape

A molecular geometry in which the central atom has two bonding pairs and one or two lone pairs, resulting in a V-shaped arrangement.

Example: "Water has a          with a bond angle of approximately 104.5° due to two lone pairs on oxygen."

Bimolecular reaction

An elementary reaction involving the collision of two reactant molecules.

Example: "A          has a rate law that is second order overall, with rate = k[A][B] or rate = k[A]²."

Bohr model

A model of the atom in which electrons orbit the nucleus in fixed circular paths at specific energy levels.

Example: "The          successfully explained the line spectrum of hydrogen but failed for multi-electron atoms."

Boiling point

The temperature at which a liquid's vapor pressure equals the external atmospheric pressure and the liquid changes to a gas.

Example: "The          of water at standard pressure is 100°C or 373 K."

Boiling point elevation

A colligative property in which the boiling point of a solvent increases when a nonvolatile solute is dissolved in it.

Example: "         explains why adding salt to water raises the temperature needed for boiling."

Bomb calorimeter

A device used to measure the heat of combustion of a substance at constant volume.

Example: "A          is used to determine the caloric content of food by burning it in excess oxygen."

Bond angle

The angle formed between three atoms across at least two bonds, determined by the repulsion of electron pairs around the central atom.

Example: "The          in water is approximately 104.5° due to the presence of two lone pairs on oxygen."

Bond energy

The average energy required to break one mole of a particular type of bond in gaseous molecules.

Similar definitions: Bond dissociation energy

Example: "The          of a C–H bond is approximately 413 kJ/mol."

Bond length

The equilibrium distance between the nuclei of two bonded atoms in a molecule.

Example: "A triple bond has a shorter          than a single bond between the same two atoms."

Bond order

The number of chemical bonds between a pair of atoms, which can be 1, 2, or 3 for single, double, or triple bonds respectively.

Example: "The          of nitrogen gas (N₂) is 3 because the two nitrogen atoms share a triple bond."

Bond polarity

The unequal distribution of electron density in a covalent bond due to a difference in electronegativity between the bonded atoms.

Example: "         increases as the electronegativity difference between two bonded atoms increases."

Bonding orbital

A molecular orbital that is lower in energy than the atomic orbitals from which it was formed, with electron density concentrated between the nuclei.

Example: "Electrons in a          stabilize the bond and hold the atoms together."

Born-Haber cycle

A thermodynamic cycle used to calculate the lattice energy of an ionic compound from experimentally measurable quantities.

Example: "The          uses Hess's law to relate lattice energy to ionization energy, electron affinity, and enthalpy of formation."

Boyle's law

A gas law stating that the volume of a gas is inversely proportional to its pressure at constant temperature.

Example: "         is expressed as P₁V₁ = P₂V₂ when temperature and amount of gas are held constant."

Brønsted-Lowry acid

A species that donates a proton (H⁺) in a chemical reaction.

Similar definitions: Proton donor

Example: "HCl acts as a          when it donates a proton to water."

Brønsted-Lowry base

A species that accepts a proton (H⁺) in a chemical reaction.

Similar definitions: Proton acceptor

Example: "Ammonia acts as a          when it accepts a proton from water."

Brønsted-Lowry theory

An acid-base theory defining acids as proton (H⁺) donors and bases as proton acceptors.

Example: "The          is broader than the Arrhenius theory because it applies to non-aqueous solutions as well."

Buffer capacity

The amount of acid or base a buffer solution can absorb before its pH changes significantly.

Example: "The          is greatest when the concentrations of weak acid and conjugate base are both high and approximately equal."

Buffer region

The portion of a titration curve where the solution acts as a buffer, with pH changing gradually as titrant is added.

Example: "The          of a weak acid titration curve is centered around the pKa of the acid."

Buffer solution

A solution that resists changes in pH upon the addition of small amounts of acid or base, typically containing a weak acid and its conjugate base.

Similar definitions: Buffer

Example: "A          made of acetic acid and sodium acetate maintains a nearly constant pH around 4.75."

Calorimeter

A device used to measure the amount of heat absorbed or released during a chemical or physical process.

Example: "A coffee-cup          is commonly used to measure enthalpy changes of reactions in solution."

Calorimetry

The experimental technique of measuring heat changes in chemical or physical processes.

Example: "         can be used to determine the specific heat capacity of a metal by measuring temperature changes."

Capillary action

The ability of a liquid to flow in narrow spaces against gravity due to adhesive and cohesive forces.

Example: "         allows water to rise in thin glass tubes because water molecules adhere to the glass surface."

Catalyst

A substance that increases the rate of a chemical reaction by lowering the activation energy without being consumed.

Similar definitions: Catalysis

Example: "Enzymes are biological          molecules that speed up biochemical reactions."

Catalyst poison

A substance that blocks or deactivates a catalyst, preventing it from increasing the reaction rate.

Example: "Lead is a          for platinum catalytic converters, which is why leaded gasoline was banned."

Cathode

The electrode at which reduction occurs in an electrochemical cell.

Example: "In a galvanic cell, the          is the positive terminal where metal ions gain electrons."

Cation

A positively charged ion formed when an atom or molecule loses one or more electrons.

Example: "Na⁺ is a          formed when sodium loses one electron."

Cell potential

The voltage difference between two half-cells in an electrochemical cell, indicating the driving force for the redox reaction.

Similar definitions: Electromotive force, EMF

Example: "The          of a galvanic cell must be positive for the reaction to be spontaneous."

Charles's law

A gas law stating that the volume of a gas is directly proportional to its absolute temperature at constant pressure.

Example: "         is expressed as V₁/T₁ = V₂/T₂ when pressure and amount of gas are held constant."

Chemical kinetics

The branch of chemistry that studies the rates and mechanisms of chemical reactions.

Example: "         helps us understand why some reactions are fast and others are slow."

Clausius-Clapeyron equation

An equation that relates the vapor pressure of a liquid to temperature, using the enthalpy of vaporization.

Example: "The          can be used to determine the vapor pressure of water at different temperatures."

Closed system

A system that can exchange energy but not matter with its surroundings.

Example: "A sealed calorimeter is an example of a          because heat can transfer through its walls but no mass enters or leaves."

Coefficient

A number placed before a formula in a chemical equation to indicate the relative number of moles of that substance.

Example: "In 2H₂O, the          of 2 indicates two moles of water."

Cohesion

The attractive force between molecules of the same substance.

Example: "         in water is primarily due to hydrogen bonding and is responsible for forming water droplets."

Colligative properties

Physical properties of solutions that depend on the number of solute particles rather than their identity.

Example: "         include boiling point elevation, freezing point depression, and osmotic pressure."

Collision theory

A theory stating that chemical reactions occur when reactant particles collide with sufficient energy (activation energy) and proper orientation.

Example: "         explains why increasing temperature and concentration both increase reaction rates."

Combined gas law

A gas law that combines Boyle's, Charles's, and Gay-Lussac's laws into a single expression: P₁V₁/T₁ = P₂V₂/T₂.

Example: "The          is used when pressure, volume, and temperature all change simultaneously."

Combustion analysis

An analytical method used to determine the empirical formula of a compound by burning it and measuring the products.

Example: "         of a hydrocarbon produces CO₂ and H₂O, from which the original formula can be determined."

Combustion reaction

A rapid exothermic reaction between a substance and oxygen, producing heat and light.

Example: "The          of methane produces carbon dioxide and water: CH₄ + 2O₂ → CO₂ + 2H₂O."

Common ion effect

The decrease in solubility of an ionic compound when a solution already contains one of the ions in that compound.

Example: "The          causes AgCl to be less soluble in a NaCl solution than in pure water."

Compound

A substance composed of two or more elements chemically combined in fixed proportions.

Example: "Water is a          made of hydrogen and oxygen in a 2:1 molar ratio."

Concentration

The amount of solute present in a given quantity of solvent or solution.

Example: "The          of a solution can be expressed in units such as molarity, molality, or percent by mass."

Concentration cell

An electrochemical cell in which both half-cells contain the same species but at different concentrations, generating a potential from the concentration difference.

Example: "A          operates until the concentrations in both half-cells become equal."

Condensation

The phase change from gas to liquid, releasing heat to the surroundings.

Example: "         of water vapor on a cold surface forms dew drops."

Conjugate acid

The species formed when a base accepts a proton (H⁺).

Example: "NH₄⁺ is the          of the base NH₃."

Conjugate acid-base pair

Two species that differ by a single proton (H⁺), where one is the acid and the other is its conjugate base.

Example: "NH₃ and NH₄⁺ form a          because they differ by one proton."

Conjugate base

The species that remains after an acid donates a proton (H⁺).

Example: "CH₃COO⁻ is the          of acetic acid (CH₃COOH)."

Coordination number

The number of atoms or ions immediately surrounding a central atom or ion in a crystal or complex.

Example: "In a face-centered cubic unit cell, each atom has a          of 12."

Core electrons

The inner electrons of an atom that are not in the outermost (valence) shell and are not typically involved in bonding.

Example: "In sodium, the          include the 1s², 2s², and 2p⁶ electrons."

Coulomb's law

A law stating that the force between two charged particles is proportional to the product of their charges and inversely proportional to the square of the distance between them.

Example: "         explains why the lattice energy of MgO is much greater than that of NaCl."

Coulomb's law (for ionic bonds)

The electrostatic force between two ions is proportional to the product of their charges and inversely proportional to the square of the distance between them.

Example: "         explains why lattice energy increases with ionic charge and decreases with ionic radius."

Coulombic attraction

The electrostatic force of attraction between oppositely charged particles, which increases with charge magnitude and decreases with distance.

Example: "         between the nucleus and valence electrons determines ionization energy and atomic radius."

Covalent bond

A chemical bond formed when two atoms share one or more pairs of electrons.

Example: "The H–H bond in hydrogen gas is a          where each atom contributes one electron to the shared pair."

Critical point

The temperature and pressure above which a substance exists as a supercritical fluid with properties of both liquid and gas.

Example: "Above the         , there is no distinct boundary between liquid and gas phases."