BCS Written Chemistry flashcards

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

Hund's rule

For degenerate orbitals, electrons occupy separate orbitals with parallel spins before any pairing occurs, giving maximum total spin and explaining the paramagnetism of atoms with partly filled subshells.

Effective nuclear charge

The net positive charge experienced by an outer electron after inner electrons shield it; estimated by Slater's rules as Z minus the screening constant S.

Half-filled stability

A subshell that is exactly half filled, such as 2p³ in nitrogen or 3d⁵ in manganese, has extra exchange-energy stability, so its ionization energy is anomalously high.

De Broglie relation

Matter has an associated wavelength equal to Planck's constant divided by momentum, lambda equals h over mv, which explains electron diffraction and quantized Bohr orbits.

VSEPR theory

Electron pairs around a central atom repel and adopt the arrangement that maximizes their separation; lone pairs repel more strongly than bonding pairs, so they compress bond angles.

Hybridization from steric number

Steric number counts sigma bonds plus lone pairs on the central atom: two gives sp linear, three gives sp2 trigonal planar, four gives sp3 tetrahedral, five or six gives expanded octahedral sets.

Bond order from MO theory

Half the difference between bonding and antibonding electron counts; it predicts bond strength and length, and shows that dioxygen has bond order two and two unpaired electrons.

Fajans' rules

Covalent character in an ionic bond rises when the cation is small and highly charged and the anion is large and polarizable, as in lithium iodide compared with lithium fluoride.

Gibbs energy criterion

Delta G equals Delta H minus T Delta S. A negative value means the process is spontaneous at constant temperature and pressure; zero means equilibrium.

Arrhenius equation

Rate constant k equals A times exp of minus Ea over RT. Plotting ln k against 1/T gives a straight line of slope minus Ea/R and intercept ln A.

Nernst equation

Cell potential E equals standard potential minus RT over nF times ln Q; at 25 degrees Celsius the factor becomes 0.0592/n for base-ten logarithm.

Henderson–Hasselbalch equation

pH equals pKa plus log of the ratio of conjugate base to weak acid concentration; it sets buffer pH and shows maximum buffer capacity when pH equals pKa.

SN2 mechanism

A one-step bimolecular substitution in which the nucleophile attacks the carbon opposite the leaving group, giving inversion of configuration and a rate depending on both reactants.

Markovnikov's rule

In electrophilic addition to an alkene, hydrogen attaches to the carbon bearing more hydrogens, so the more stable carbocation forms; peroxides reverse this for hydrogen bromide only.

Electrophilic aromatic substitution

An aromatic ring attacks an electrophile, forming a resonance-stabilized arenium ion, then loses a proton to restore aromaticity; substituents direct ortho, para, or meta accordingly.

Grignard reagent

An organomagnesium halide, RMgX, made from alkyl halide and magnesium in dry ether; it is a strong nucleophile that adds to carbonyls and carbon dioxide.

Cannizzaro reaction

Aldehydes lacking alpha hydrogens, such as benzaldehyde, undergo base-promoted disproportionation to give equimolar alcohol and carboxylate through a hydride-transfer intermediate.

Crystal field splitting

Ligand electrons repel metal d orbitals unequally, splitting them into a lower and a higher set; the gap size determines color, magnetic moment, and high-spin versus low-spin behavior.

Spectrochemical series

An empirical ordering of ligands by the size of the d-orbital splitting they produce, from weak iodide and chloride to strong cyanide and carbon monoxide.

Chelate effect

Polydentate ligands form more stable complexes than equivalent monodentate ligands because fewer free particles are produced, so entropy of complex formation increases.

Spin-only magnetic moment

Mu equals the square root of n times n plus two, in Bohr magnetons, where n is the number of unpaired d electrons; used to deduce spin state from measurement.

Beer–Lambert law

Absorbance equals molar absorptivity times path length times concentration. It underpins colorimetry but fails at high concentration or with polychromatic light.

Primary standard

A highly pure, stable, non-hygroscopic reagent of known composition, such as potassium hydrogen phthalate or sodium carbonate, used to standardize a titrant solution accurately.

Iodometric titration

An indirect method in which an oxidizing analyte liberates iodine from iodide, and the iodine is titrated with standard sodium thiosulfate using freshly prepared starch indicator.

Retention factor Rf

In thin-layer chromatography, the distance travelled by the solute divided by the distance travelled by the solvent front, used to identify compounds under fixed conditions.

Haber process conditions

Nitrogen and hydrogen combine over an iron catalyst at roughly 450 degrees Celsius and 200 atmospheres. The reaction is exothermic and fewer moles of gas form, favoring high pressure.

Contact process

Sulfur is burned to sulfur dioxide, oxidized to sulfur trioxide over vanadium pentoxide, and absorbed in concentrated sulfuric acid rather than water to avoid acid mist.

Biological oxygen demand

The oxygen consumed by microorganisms decomposing organic matter in a water sample over a set period, commonly five days; it indicates organic pollution load.

Eutrophication

Excess nitrate and phosphate in water trigger algal blooms; their decay consumes dissolved oxygen, causing fish kills and degraded water quality.

Arsenic contamination

Shallow tubewells in many parts of Bangladesh exceed the drinking-water limit for arsenic, so testing, deep wells, and treatment are used to reduce exposure.

Urea fertilizer

The most widely used nitrogen fertilizer in Bangladesh, containing about 46 percent nitrogen; in soil it hydrolyses to ammonium and then nitrifies, acidifying the soil.

TSP and DAP

Triple superphosphate and diammonium phosphate are phosphorus fertilizers used in Bangladesh; DAP also supplies nitrogen, and both are applied with urea and potash.