OSAT Chemistry flashcards

34 cards. Read the term and the definition here. The deck at the top of this page is optional.

Mass number

The sum of protons and neutrons in an atom's nucleus; written as a superscript before the symbol. It differs among isotopes of the same element, which share proton count but vary in neutrons.

Isotopes

Atoms of the same element with different neutron counts, hence different mass numbers. They have identical chemical properties but different masses and may differ in nuclear stability.

Aufbau principle

Electrons fill the lowest-energy orbitals first: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. This ordering explains the structure of the periodic table and ground-state configurations.

Hund's rule

Within a subshell, electrons occupy separate orbitals with parallel spins before pairing. This minimizes electron-electron repulsion and maximizes total spin, giving the lowest-energy configuration.

Atomic radius trend

Radius decreases across a period as nuclear charge grows with electrons added to the same shell, and increases down a group as additional shells are added.

Ionization energy

The energy needed to remove an electron from a gaseous atom. It increases across a period and decreases down a group; each successive removal requires more energy.

Electronegativity

The tendency of an atom to attract shared electrons in a bond. It rises across a period and falls down a group; fluorine is the most electronegative element.

Mole concept

One mole contains 6.022e23 representative particles. Molar mass in grams per mole equals the formula mass in atomic mass units, linking mass to particle count.

Limiting reactant

The reactant consumed first and that determines the maximum product. Identify it by dividing each reactant's moles by its coefficient and comparing the quotients.

Percent yield

Actual yield divided by theoretical yield, multiplied by 100. Theoretical yield comes from stoichiometry; the actual yield is measured experimentally and never exceeds theory.

Ideal gas law

PV = nRT relates pressure, volume, moles, and absolute temperature. R equals 0.08206 L atm per mole kelvin, so temperature must be in kelvin.

Molar volume at STP

One mole of any ideal gas occupies 22.4 liters at 0 degrees Celsius and 1 atmosphere. This follows from the ideal gas law and allows mole-volume conversions.

Dilution equation

M1V1 = M2V2 conserves moles of solute when solvent is added. Solve for the unknown concentration or volume; units must match on both sides.

Colligative properties

Properties depending on solute particle count, not identity: boiling point elevation, freezing point depression, vapor pressure lowering, and osmotic pressure. The van't Hoff factor accounts for dissociation.

pH definition

pH equals the negative base-10 logarithm of hydronium ion concentration. At 25 degrees Celsius, pH plus pOH equals 14, and neutral solutions have pH 7.

Le Chatelier's principle

A system at equilibrium shifts to partially offset a change in concentration, pressure, or temperature. Adding a reactant shifts toward products; raising temperature favors the endothermic direction.

Gibbs free energy

Delta G equals delta H minus T delta S. A negative value means the reaction is spontaneous at that temperature; positive means nonspontaneous; zero means the system is at equilibrium.

Exothermic reaction

A reaction that releases heat to the surroundings, giving a negative enthalpy change. Products have lower enthalpy than reactants, and raising temperature typically shifts equilibrium toward reactants.

Oxidation and reduction

Oxidation is loss of electrons and an increase in oxidation number; reduction is gain of electrons and a decrease. They always occur together in a redox reaction.

Empirical vs molecular formula

The empirical formula gives the simplest whole-number ratio of atoms; the molecular formula gives the actual atom counts and is a whole-number multiple of the empirical formula.

Intermolecular forces

Attractions between molecules ranked by strength: ion-dipole, hydrogen bonding, dipole-dipole, London dispersion. Stronger forces raise boiling point and surface tension.

Sigma and pi bonds

A sigma bond is a head-on orbital overlap allowing rotation; a pi bond is a side-by-side overlap above and below the axis. Single bonds are sigma; double and triple bonds add pi bonds.

VSEPR geometry

Electron domains around a central atom repel and spread apart. Two domains give linear, three trigonal planar, four tetrahedral; lone pairs compress bond angles below ideal values.

Significant figures

All nonzero digits count; interior zeros count; leading zeros do not; trailing zeros count only with a decimal point. Multiplication keeps the fewest sig figs.

Accuracy versus precision

Accuracy is closeness to the accepted value; precision is closeness of repeated measurements to each other. A systematic error harms accuracy; random error limits precision.

Meniscus reading

Read the bottom of the curved liquid surface at eye level for water solutions. Volumetric glassware is calibrated to this point, so misreading shifts every measurement systematically.

Standardization in titration

Determining an approximate titrant concentration using a primary standard of known purity, such as potassium hydrogen phthalate for sodium hydroxide. It corrects for absorbed water and impurities.

Beer-Lambert law

Absorbance equals molar absorptivity times path length times concentration. A linear calibration of absorbance against concentration lets you determine an unknown concentration.

Flame test

Metal cations heated in a flame emit characteristic visible light from excited electrons returning to lower energy levels. Colors identify elements such as sodium, potassium, and copper.

Filtration versus distillation

Filtration separates an insoluble solid from a liquid by passing it through a porous barrier. Distillation separates miscible liquids by differences in boiling point through vaporization and condensation.

Chromatography

Components separate because they partition differently between a mobile phase and a stationary phase. Greater affinity for the stationary phase slows migration along the medium.

Diluting concentrated acid

Add acid slowly to water with stirring, never water to acid. The highly exothermic hydration can boil the mixture and spatter if water is poured onto concentrated acid.

Chemical waste disposal

Halogenated and non-halogenated organic solvents are collected separately, and heavy-metal solutions are never poured down the drain. Containers must be labeled with contents and hazard.

Controlled experiment

An investigation that changes one independent variable while holding all other conditions constant, using a control group for comparison. It supports causal inference better than observation alone.