Catalonia PAU Chemistry flashcards

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

Hund's rule

Electrons fill degenerate orbitals singly with parallel spins before pairing, minimizing electron–electron repulsion; this explains the paramagnetism of atoms with partly filled p or d subshells.

Ionization energy trend

Increases left to right across a period as effective nuclear charge rises and radius shrinks, and decreases down a group as the outer electron is farther from the nucleus and better shielded.

Isotope average atomic mass

The weighted mean of isotope masses using natural abundances: mass = Σ(fraction × isotope mass). Chlorine's 75.8% chlorine-35 and 24.2% chlorine-37 give about 35.45 u.

VSEPR lone pair effect

Lone pairs occupy more space than bonding pairs, so they compress bond angles below the ideal value: methane 109.5°, ammonia 107°, water 104.5° in the tetrahedral electron-domain family.

Hydrogen bond

A strong dipole–dipole attraction between H bonded to N, O or F and a lone pair on a nearby electronegative atom; it raises boiling points and explains water's unusually high enthalpy of vaporization.

Formal charge rule

Formal charge equals valence electrons minus nonbonding electrons minus half the bonding electrons. The preferred Lewis structure minimizes formal charges and places negative charge on the more electronegative atom.

Hybridization from steric number

Steric number (sigma bonds plus lone pairs) sets hybridization: 2 is sp and linear, 3 is sp² and trigonal planar, 4 is sp³ and tetrahedral; unhybridized p orbitals form pi bonds.

Ideal gas law

PV = nRT relates pressure, volume, moles and absolute temperature; R is 0.0821 L·atm·mol⁻¹·K⁻¹ or 8.314 J·mol⁻¹·K⁻¹, and one mole occupies 22.4 L at STP.

Raoult's law

The vapor pressure of a solution equals the solvent mole fraction times the pure solvent vapor pressure: P = x_solvent·P°. Adding a nonvolatile solute lowers vapor pressure and raises boiling point.

Colligative properties

Boiling-point elevation, freezing-point depression, vapor-pressure lowering and osmotic pressure depend on the number of dissolved particles, not identity; use the van't Hoff factor i for electrolytes.

Triple point

The unique temperature and pressure at which solid, liquid and gas coexist in equilibrium; on a phase diagram it is where the three boundary curves meet, below the critical point.

Reaction quotient Q

The equilibrium-constant expression evaluated at current concentrations. If Q < K the reaction proceeds forward, if Q > K it reverses, and if Q = K the system is at equilibrium.

Le Chatelier's principle

A system at equilibrium shifts to partially offset a change in concentration, pressure or temperature. Adding heat favors the endothermic direction; compression favors the side with fewer gas moles.

Solubility product Ksp

The equilibrium constant for dissolving a sparingly soluble salt, omitting the solid; for M_mX_x, Ksp = [M]^m[X]^x. A precipitate forms when the ion product exceeds Ksp.

Common-ion effect

Adding an ion already present in a solubility or acid–base equilibrium suppresses ionization or dissolution, lowering solubility or percent dissociation by shifting the equilibrium toward the undissociated side.

Henderson–Hasselbalch equation

pH = pKa + log([conjugate base]/[weak acid]); it gives buffer pH and shows that a buffer is most effective when the acid and base concentrations are comparable, near pH = pKa.

Salt hydrolysis

The reaction of a salt's ions with water. Salts of weak acids give basic solutions, salts of weak bases give acidic solutions, and salts of strong acid plus strong base stay neutral.

Weak acid approximation

For a weak acid with small Ka and concentration C₀, [H₃O⁺] ≈ √(Ka·C₀), valid when dissociation is under about 5%; otherwise solve the quadratic expression.

Buffer capacity

The amount of strong acid or base a buffer absorbs with little pH change; it rises with the absolute concentrations of the conjugate pair and is greatest near pH = pKa.

Nernst equation

E = E° − (RT/nF)ln Q, or E° − (0.0592/n)log Q at 25 °C, correcting cell potential for nonstandard concentrations; E becomes zero at equilibrium.

Standard cell potential

E°cell = E°cathode − E°anode using standard reduction potentials. A positive value means a spontaneous galvanic reaction, related to free energy by ΔG° = −nFE°cell.

Faraday's law of electrolysis

The moles of substance produced or consumed equal the charge passed (Q = I·t) divided by n·F, where n is the electrons per formula unit and F is 96485 C/mol.

Anode and cathode roles

Oxidation always occurs at the anode and reduction at the cathode. In a galvanic cell the anode is negative; in an electrolytic cell the external supply makes the anode positive.

First-order half-life

For a first-order reaction t½ = ln 2/k ≈ 0.693/k, independent of initial concentration; each half-life reduces the remaining reactant to 50 percent of its current amount.

Arrhenius equation

k = A·e^(−Ea/RT) shows rate constant rising with temperature and falling with activation energy; plotting ln k against 1/T gives a straight line of slope −Ea/R.

Rate-determining step

The slowest elementary step in a mechanism limits the overall rate; the experimental rate law reflects the species involved up to and including that step, after substituting intermediates.

Hess's law

Enthalpy change is independent of pathway, so reactions can be added, reversed (sign changed) or multiplied (enthalpy scaled) to obtain an unknown ΔH from known values.

Gibbs energy criterion

ΔG = ΔH − TΔS determines spontaneity at constant temperature and pressure: negative is spontaneous, zero is equilibrium, positive is nonspontaneous; ΔG° = −RT ln K.

Standard enthalpy of formation

The enthalpy change when one mole of a compound forms from its elements in standard states; elements have ΔH°f = 0, and ΔH°rxn = Σ products − Σ reactants.

Markovnikov's rule

In electrophilic addition of HX to an alkene, hydrogen adds to the carbon with more hydrogens, giving the more stable carbocation intermediate and the more substituted halogen product.

SN1 versus SN2

SN1 is two-step, favored by tertiary halides and polar protic solvents, with racemization; SN2 is one-step backside attack, favored by primary halides, giving inversion of configuration.

Addition versus condensation polymers

Addition polymers form when alkene monomers join without losing atoms (polyethylene); condensation polymers form with loss of a small molecule such as water (polyester, polyamide).