Mole: 6.022 x 10^23 particles; the chemist's counting unit Avogadro's number: 6.022 x 10^23 per mole Molar mass: Mass in grams of one mole of a substance Empirical formula: The simplest whole-number ratio of atoms Molecular formula: The actual number of each atom in a molecule Percent composition: Mass percent of each element in a compound Limiting reactant: The reactant that runs out first and caps the product Theoretical yield: The maximum product predicted by stoichiometry Percent yield: (actual yield / theoretical yield) x 100 Molarity: Moles of solute per liter of solution Dilution equation: M1V1 = M2V2 Isotope: Atoms of one element with different numbers of neutrons Average atomic mass: Weighted average of an element's isotopes Mass spectrometry: Measures isotope masses and abundances Photoelectron spectroscopy: Measures energies needed to remove electrons from atoms Electron configuration: Arrangement of electrons in orbitals Aufbau principle: Fill lowest-energy orbitals first Pauli exclusion principle: No two electrons in an atom share all four quantum numbers Hund's rule: Fill degenerate orbitals singly before pairing Valence electrons: Outer-shell electrons involved in bonding Effective nuclear charge: Net pull felt by outer electrons Atomic radius trend: Decreases across a period, increases down a group Ionization energy trend: Generally increases across a period, decreases down a group Electronegativity: An atom's pull on shared electrons in a bond Coulomb's law: Force depends on charges and decreases with distance squared Ionic bond: Attraction between oppositely charged ions Covalent bond: A shared pair of electrons Metallic bond: Positive ions in a sea of delocalized electrons Lattice energy: Energy released when gaseous ions form an ionic solid Bond order: Number of bonding pairs between two atoms Bond length trend: Higher bond order means shorter, stronger bonds Lewis structure: A diagram of valence electrons and bonds Formal charge: Valence electrons minus nonbonding electrons minus half the bonding electrons Resonance: Multiple valid Lewis structures for one species VSEPR theory: Electron groups arrange to minimize repulsion Linear geometry: Two electron groups; 180 degrees Trigonal planar: Three electron groups; 120 degrees Tetrahedral: Four electron groups; 109.5 degrees Hybridization sp3: Four electron groups, tetrahedral Sigma bond: Head-on overlap of orbitals Pi bond: Side-by-side overlap of p orbitals Polar bond: A bond with unequal sharing London dispersion forces: Weak attractions from temporary dipoles; present in all molecules Dipole-dipole force: Attraction between polar molecules Hydrogen bonding: Strong dipole attraction involving H bonded to N, O or F Ideal gas law: PV = nRT Combined gas law: P1V1/T1 = P2V2/T2 Dalton's law: Total pressure is the sum of partial pressures Kinetic molecular theory: Gas particles move randomly with energy proportional to temperature Graham's law: Lighter gases effuse faster Real gas deviation: Gases act least ideally at high pressure and low temperature Solute: The substance dissolved Solvent: The substance doing the dissolving Electrolyte: A substance that forms ions in water and conducts Net ionic equation: Shows only the species that change, omitting spectators Spectator ion: An ion unchanged by a reaction Precipitate: An insoluble solid formed in solution Beer-Lambert law: Absorbance is proportional to concentration: A = abc Oxidation: Loss of electrons Reduction: Gain of electrons Oxidizing agent: The species that is reduced Reducing agent: The species that is oxidized Rate law: Equation relating reaction rate to concentrations Reaction order: The exponent of a reactant in the rate law Rate constant: The proportionality constant k in the rate law First-order half-life: ln 2 / k; independent of concentration Collision theory: Reactions need collisions with enough energy and proper orientation Activation energy: Minimum energy for a successful collision Catalyst: Speeds a reaction by giving a lower-energy path; not consumed Rate-determining step: The slowest step in a mechanism Reaction intermediate: A species made then used up within a mechanism Arrhenius equation: Relates rate constant to temperature and activation energy Endothermic: Absorbs heat; positive delta H Exothermic: Releases heat; negative delta H Specific heat: Energy needed to raise 1 g of a substance by 1 degree C Calorimetry: Measuring heat flow, q = mc delta T Enthalpy of formation: Heat change when 1 mol forms from elements in standard states Hess's law: Total enthalpy change is the sum of steps Bond enthalpy: Energy needed to break a bond Entropy: A measure of disorder or energy dispersal Gibbs free energy: Delta G = delta H - T delta S; negative means spontaneous Standard state: Reference conditions for thermodynamic data, 1 bar Equilibrium: Forward and reverse rates are equal Equilibrium constant K: Ratio of products to reactants at equilibrium Reaction quotient Q: Same ratio at any moment; compare with K to predict direction Le Chatelier's principle: A system at equilibrium shifts to counteract a stress Ksp: Solubility product constant for a slightly soluble salt Common ion effect: A shared ion lowers solubility Arrhenius acid: Produces H+ in water Bronsted-Lowry acid: A proton donor Lewis acid: An electron pair acceptor Conjugate acid-base pair: Two species differing by one proton pH: Negative log of hydrogen ion concentration Kw: Ion product of water, 1.0 x 10^-14 at 25 degrees C Strong acid: Dissociates completely in water Buffer: A solution that resists pH change; weak acid plus its conjugate base Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]) Titration equivalence point: Moles of acid equal moles of base Indicator: A dye that changes color near the endpoint Electrochemical cell: Converts chemical energy to electrical energy or the reverse Anode: Electrode where oxidation occurs Cathode: Electrode where reduction occurs Standard cell potential: E-cell = E-cathode minus E-anode Salt bridge: Allows ion flow to keep solutions neutral Electrolysis: Using electricity to drive a nonspontaneous reaction Faraday's constant: About 96,485 coulombs per mole of electrons Free flashcards from Peabody at peabody.kids/flashcards/tp-ap-chemistry