UEE Chemistry flashcards
UEE Chemistry · Build recall with a focused review session.
32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Effective nuclear charge
- The net positive charge experienced by an outer electron after accounting for shielding by inner electrons; it increases across a period and governs radius and ionisation energy trends.
- Hund's rule
- Within a set of degenerate orbitals, electrons occupy separate orbitals with parallel spins before any pairing occurs, minimising electron–electron repulsion and lowering energy.
- First ionisation energy
- The energy required to remove one mole of electrons from one mole of gaseous atoms, forming gaseous 1+ ions; it rises across a period and falls down a group.
- VSEPR theory
- Electron domains around a central atom repel and adopt arrangements that maximise separation; lone pairs occupy more space than bonding pairs, compressing bond angles.
- Bond order
- Half the difference between bonding and antibonding electrons in molecular orbital theory; higher bond order means a shorter, stronger bond, as in N2 (order 3).
- Hydrogen bonding
- A strong intermolecular 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 high specific heat.
- Limiting reagent
- The reactant that is completely consumed first, determined by dividing each reactant's moles by its stoichiometric coefficient; it fixes the maximum product yield.
- Molar volume at STP
- The volume occupied by one mole of an ideal gas at 0 °C and 1 atm, equal to 22.4 L; at 25 °C and 1 atm it is approximately 24.5 L.
- Empirical formula
- The simplest whole-number ratio of atoms of each element in a compound, found by dividing percentage composition by atomic masses and then by the smallest ratio.
- Ideal gas equation
- PV = nRT relates pressure, volume, amount and absolute temperature; R equals 8.314 J mol⁻¹ K⁻¹ and temperature must be expressed in kelvin.
- Dalton's law of partial pressures
- The total pressure of a non-reacting gas mixture equals the sum of the partial pressures of its components, each proportional to its mole fraction.
- Critical temperature
- The highest temperature above which a substance cannot be liquefied by pressure alone, regardless of how much pressure is applied.
- Hess's law
- The enthalpy change of a reaction is independent of the route taken, so step enthalpies can be summed to find an overall ΔH that is hard to measure directly.
- Standard enthalpy of formation
- The enthalpy change when one mole of a compound forms from its elements in their standard states at 298 K and 1 bar; elements have zero value.
- Gibbs free energy
- ΔG = ΔH − TΔS determines spontaneity: negative ΔG means a spontaneous process, zero means equilibrium, and positive means non-spontaneous.
- 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 counteracts the disturbance.
- Buffer solution
- A mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid, that resists pH change on adding small amounts of acid or alkali.
- Solubility product
- The equilibrium constant for dissolution of a sparingly soluble salt, equal to the product of ion concentrations each raised to its stoichiometric power in a saturated solution.
- Rate-determining step
- The slowest elementary step in a multi-step mechanism; it controls the overall reaction rate and appears in the experimentally observed rate law.
- Arrhenius equation
- k = A e^(−Ea/RT) shows that the rate constant increases exponentially with temperature and decreases with activation energy.
- Half-life (first order)
- The time for the concentration of a first-order reactant to fall to half its initial value; equal to ln2/k and independent of starting concentration.
- Standard electrode potential
- The potential of a half-cell measured against the standard hydrogen electrode at 1 M, 1 bar and 298 K; more positive values indicate greater reducing-agent strength of the conjugate.
- Nernst equation
- E = E° − (RT/nF)ln Q, often written with 0.0592/n at 25 °C, correcting cell potential for non-standard concentrations.
- Faraday's laws of electrolysis
- The mass deposited at an electrode is proportional to charge passed and to molar mass divided by electrons transferred: m = MIt/(nF).
- Inert pair effect
- The tendency of heavy p-block elements such as Tl, Pb and Bi to favour an oxidation state two below the group maximum, because the s electrons are poorly shielded.
- Ligand substitution
- Replacement of one ligand in a complex by another, often accompanied by a colour change, as when blue aqua-copper ions become deep blue with ammonia.
- Markovnikov's rule
- In electrophilic addition of HX to an alkene, the hydrogen adds to the carbon bearing more hydrogens, giving the more stable carbocation intermediate.
- SN1 versus SN2
- SN1 is a two-step mechanism via a carbocation, favoured by tertiary halides and giving racemisation; SN2 is one concerted step with inversion, favoured by primary halides.
- Tollens' reagent
- Ammoniacal silver nitrate, which oxidises aldehydes to carboxylates and deposits a silver mirror; ketones give no reaction.
- Rf value
- In chromatography, the distance travelled by the solute divided by the distance travelled by the solvent front; it is characteristic of a compound under fixed conditions.
- Recrystallisation
- A purification method in which a crude solid is dissolved in the minimum hot solvent, cooled slowly to form pure crystals, then filtered and dried.
- Flame test colours
- Characteristic emission colours identify metal ions: sodium yellow, potassium lilac, calcium brick-red, copper blue-green, lithium crimson.