ACS flashcards

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

Limit of detection (LOD)

The lowest analyte concentration reliably distinguished from blank noise, conventionally defined as the blank signal plus three times the standard deviation of the blank (S/N = 3).

Standard addition method

A calibration strategy in which known analyte increments are added to sample aliquots, allowing quantification when matrix effects prevent use of an external calibration curve.

Chromatographic resolution

R = 2(t₂ − t₁)/(w₁ + w₂), measuring separation of two peaks. A value of at least 1.5 corresponds to baseline separation for quantitative work.

Beer-Lambert law

Absorbance equals molar absorptivity times path length times concentration (A = εbc). Deviations occur at high absorbance or high concentration due to stray light and interactions.

Internal standard

A known amount of a compound added to all samples and standards that corrects for injection variability, detector drift, and sample preparation losses during quantification.

SN2 mechanism

A concerted one-step nucleophilic substitution with second-order kinetics, in which the nucleophile attacks the substrate from the side opposite the leaving group, inverting stereochemistry.

Hückel aromaticity rule

A cyclic, planar, fully conjugated system with 4n + 2 pi electrons is aromatic. Systems with 4n pi electrons are antiaromatic if planar and conjugated.

Markovnikov's rule

In electrophilic addition of HX to an alkene, the hydrogen adds to the carbon bearing more hydrogens, generating the more stable carbocation intermediate.

Grignard reagent

An organomagnesium compound (RMgX) formed from alkyl halide and magnesium, acting as a strong nucleophile and base; it requires anhydrous, aprotic conditions.

CIP R/S assignment

Stereocenter configuration determined by ranking substituents by atomic number; with the lowest priority pointing away, clockwise is R and counterclockwise is S.

First law of thermodynamics

Energy is conserved: the change in internal energy equals heat added to the system plus work done on it (ΔU = q + w).

Gibbs free energy criterion

ΔG = ΔH − TΔS determines spontaneity at constant temperature and pressure. Negative ΔG means spontaneous; zero means equilibrium.

Arrhenius equation

The rate constant depends exponentially on temperature: k = A exp(−Ea/RT). Plotting ln k versus 1/T gives a slope of −Ea/R.

Clausius-Clapeyron equation

Relates vapor pressure to temperature through the enthalpy of vaporization: ln(P₂/P₁) = −(ΔHvap/R)(1/T₂ − 1/T₁).

First-order half-life

For first-order kinetics the half-life equals ln2 divided by the rate constant and is independent of initial concentration, unlike second-order reactions.

Nernst equation

Cell potential depends on reaction quotient: E = E° − (RT/nF) ln Q, which at 298 K becomes E° − (0.0592/n) log Q.

Spectrochemical series

A ranking of ligands by crystal field strength, from weak-field iodide and chloride to strong-field cyanide and carbon monoxide, determining high- versus low-spin configurations.

18-electron rule

Transition-metal complexes are especially stable when the metal d electrons plus ligand-donated electrons total 18, filling the nine bonding molecular orbitals.

Lanthanide contraction

The steady decrease in ionic radius across the lanthanide series caused by poor shielding by 4f electrons, increasing effective nuclear charge.

HSAB principle

Hard acids prefer hard bases and soft acids prefer soft bases. For example, Al³⁺ binds oxygen donors, while Hg²⁺ and Ag⁺ bind sulfur and iodide.

Jahn-Teller distortion

Geometric distortion that removes degeneracy in non-linear molecules, pronounced in octahedral high-spin d⁴ and d⁹ complexes, typically elongating one axis.

Michaelis-Menten equation

v = Vmax[S]/(Km + [S]) describes enzyme kinetics; Km is the substrate concentration giving half-maximal velocity and reflects substrate affinity.

Competitive inhibition

An inhibitor competes with substrate at the active site, raising apparent Km while leaving Vmax unchanged; effects are reversed by increasing substrate concentration.

Enzyme transition-state stabilization

Enzymes accelerate reactions by binding and stabilizing the transition state, lowering activation energy without altering the overall free energy change of the reaction.

DNA base pairing

Adenine pairs with thymine by two hydrogen bonds, and guanine pairs with cytosine by three; higher GC content raises duplex melting temperature.

Protein secondary structure

Local folding patterns, chiefly alpha helices and beta sheets, stabilized by hydrogen bonds between backbone amide groups rather than side chains.

Number-average molecular weight

Mn equals the sum of NiMi divided by the sum of Ni, weighting each chain equally; it is sensitive to low-molecular-weight species and measured by osmometry.

Polydispersity index

PDI equals Mw divided by Mn. It equals one for a perfectly monodisperse polymer and increases as the molecular weight distribution broadens.

Glass transition temperature

The temperature below which an amorphous polymer becomes rigid and glassy. It rises with chain stiffness and crosslinking and falls with added plasticizer.

Step-growth polymerization

Polymerization in which monomers react to form dimers, trimers, and longer chains, requiring very high conversion to achieve high molecular weight.

Thermoset versus thermoplastic

Thermosets are irreversibly crosslinked networks that cannot be remelted, while thermoplastics soften reversibly on heating and can be reprocessed.

Ziegler-Natta catalyst

A transition-metal catalyst system that polymerizes olefins with controlled stereochemistry, producing stereoregular, crystalline polymers such as isotactic polypropylene.

Carothers equation

Relates number-average degree of polymerization to extent of reaction p for step-growth systems: Xn = 1/(1 − p), showing high p is needed for high molecular weight.