Castilla y León PAU Chemistry flashcards

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

Bohr energy formula

For hydrogen-like atoms, E_n = −13.6 Z²/n² eV. The energy of an emitted or absorbed photon equals the absolute difference between the two level energies, hν = hc/λ.

Pauli exclusion principle

No two electrons in the same atom can have identical sets of all four quantum numbers. An orbital therefore holds at most two electrons, and they must have opposite spin values, +½ and −½.

First ionization energy trend

Energy needed to remove the most loosely bound electron from a gaseous atom. It increases across a period as nuclear charge grows, and decreases down a group as the outer electron is farther out and better shielded.

Anomalous electron configurations

Chromium is [Ar]3d⁵4s¹ and copper is [Ar]3d¹⁰4s¹. Half-filled and fully filled d subshells give extra exchange stability, so one 4s electron moves into the 3d subshell.

VSEPR steric number rule

Steric number equals bonded atoms plus lone pairs on the central atom. It fixes the electron geometry: 2 linear, 3 trigonal planar, 4 tetrahedral, 5 trigonal bipyramidal, 6 octahedral. Lone pairs compress bond angles.

Hydrogen bonding

A strong dipole–dipole attraction between H bonded to N, O, or F and a lone pair on a nearby N, O, or F. It explains the unusually high boiling points of water, ammonia, and hydrogen fluoride.

Molecular polarity decision rule

A molecule is polar only if it has polar bonds and an asymmetric shape, so bond dipoles do not cancel. CO₂ and CCl₄ are nonpolar; H₂O, NH₃, and HCl are polar.

Hybridization and geometry

Steric number 2 gives sp and 180°; 3 gives sp² and 120°; 4 gives sp³ and about 109.5°. Lone pairs occupy hybrid orbitals, so water is sp³ and bent, not sp².

Hess's law

The enthalpy change of a reaction is independent of the route taken. Add the enthalpies of the chosen steps, reversing a step to change the sign of its ΔH and multiplying a step to scale its ΔH.

Standard enthalpy of formation

The enthalpy change when one mole of a compound forms from its elements in their standard states. Elements in their standard states have ΔH_f° = 0, so ΔH_r° = Σ products − Σ reactants.

Gibbs free energy criterion

ΔG = ΔH − TΔS. A process is spontaneous when ΔG is negative, at equilibrium when ΔG = 0, and nonspontaneous when positive. When ΔH and ΔS have opposite signs, temperature decides.

Entropy change signs

Entropy rises when gas moles increase, when a solid melts or a liquid boils, and when temperature rises. ΔS is positive for such changes and negative when order increases, as in gas condensation.

First-order half-life

For a first-order reaction, t½ = ln2/k ≈ 0.693/k. The half-life is independent of initial concentration, unlike zero-order and second-order reactions, whose half-lives depend on the starting concentration.

Arrhenius equation

k = A·e^(−Ea/RT). A larger activation energy makes the rate constant more sensitive to temperature. The linear form ln k = ln A − Ea/RT gives Ea from the slope of ln k against 1/T.

Le Chatelier's principle

When a system at equilibrium is disturbed by a change in concentration, pressure, or temperature, it shifts in the direction that partly counteracts the change. Catalysts alter neither K nor the equilibrium position.

Reaction quotient versus K

Q has the same form as K but uses current concentrations. If Q < K the reaction proceeds forward, if Q > K it proceeds backward, and if Q = K the system is at equilibrium.

Henderson–Hasselbalch equation

pH = pKa + log([conjugate base]/[weak acid]). It gives the pH of a buffer and shows that pH equals pKa when the acid and base concentrations are equal, as at half-neutralization.

Weak acid pH approximation

For a weak monoprotic acid of concentration C₀, [H₃O⁺] ≈ √(Ka·C₀) when Ka is small. Then pH = −log[H₃O⁺]. The approximation fails when Ka is not negligible versus C₀.

Salt hydrolysis classification

A salt of a strong acid and weak base gives an acidic solution, a salt of a weak acid and strong base gives a basic solution, and a strong acid–strong base salt is neutral.

Buffer capacity condition

A buffer resists pH change when it contains comparable amounts of a weak acid and its conjugate base. It works best near pH = pKa, with a base-to-acid ratio between 0.1 and 10.

Solubility product Ksp

For a sparingly soluble salt MₐX_b dissolving to aM + bX, Ksp = [M]^a[X]^b at saturation. The concentration of the undissolved solid does not appear in the expression.

Common-ion effect

Adding an ion already present in a solubility equilibrium shifts the dissolution equilibrium toward the solid, decreasing molar solubility. The effect follows Le Chatelier's principle and is used in gravimetric analysis.

Precipitation prediction

Calculate the ion product Qsp from the mixed concentrations after dilution. If Qsp exceeds Ksp a precipitate forms, if it equals Ksp the solution is saturated, and if it is below Ksp the solution remains unsaturated.

Selective precipitation

When several ions could precipitate with one reagent, the salt whose Qsp reaches Ksp at the lowest reagent concentration precipitates first. This allows fractional separation of ions from a mixture.

Oxidation state rules

Free elements are 0; monatomic ions equal their charge; oxygen is usually −2 and hydrogen usually +1; the sum of oxidation states equals the overall charge of the species.

Balancing redox half-reactions

Split into oxidation and reduction halves, balance atoms, add H₂O for oxygen and H⁺ for hydrogen in acid, add electrons to balance charge, equalize electrons, then add and cancel.

Standard cell potential

E°cell = E°cathode − E°anode, using reduction potentials for both electrodes. A positive E°cell indicates a spontaneous galvanic reaction, and it relates to free energy by ΔG° = −nFE°cell.

Nernst equation

E = E° − (RT/nF)ln Q, which at 25 °C becomes E = E° − (0.0592/n)log Q. It corrects the cell potential for nonstandard concentrations or gas pressures.

Markovnikov's rule

When HX adds to an unsymmetrical alkene, the hydrogen adds to the carbon bearing more hydrogens, and the halogen goes to the more substituted carbon, through the more stable carbocation intermediate.

Alcohol oxidation products

Primary alcohols oxidize to aldehydes and then to carboxylic acids, secondary alcohols to ketones, and tertiary alcohols resist oxidation because the carbinol carbon has no hydrogen to remove.

Esterification condensation

A carboxylic acid reacts with an alcohol in the presence of an acid catalyst to form an ester plus water. The reaction is reversible, and the ester is named alkyl alkanoate.

Addition versus condensation polymers

Addition polymers form when alkene monomers join without losing atoms, as in polyethene and PVC. Condensation polymers form with loss of a small molecule such as water, as in PET and nylon.