IIT JAM CY key terms

The thermodynamic quantity H = U + PV, equal to internal energy plus pressure times volume. At constant pressure the heat exchanged equals the enthalpy change, so it is the natural bookkeeping quantity for calorimetry and thermochemistry.

A state function measuring how widely energy and particles are dispersed among the available microstates. The second law states that the total entropy of an isolated system never decreases, so entropy increase marks spontaneous change.

The quantity G = H - TS. At constant temperature and pressure a process is spontaneous only if G decreases; equilibrium corresponds to the minimum of G, and the standard change is linked to the equilibrium constant.

The partial vapour pressure of each component in an ideal solution equals its mole fraction times the vapour pressure of the pure liquid. Positive or negative deviations signal non-ideal mixing and differing intermolecular interactions.

Properties of dilute solutions that depend on the number of dissolved particles rather than their identity: vapour-pressure lowering, boiling-point elevation, freezing-point depression and osmotic pressure. They are used to determine molar masses.

The relation k = A exp(-Ea/RT) showing that a rate constant rises exponentially with temperature. A is the pre-exponential factor and Ea the activation energy; plotting ln k against 1/T gives a straight line.

The minimum energy that colliding molecules must possess for a reaction to occur. It equals the height of the energy barrier between reactants and products and controls how strongly the rate responds to temperature.

The sum of the exponents of the concentration terms in an experimentally determined rate law. It is not necessarily related to the stoichiometric coefficients and can be fractional, zero or even negative.

The relation E = E° - (RT/nF) ln Q giving the cell potential at any composition. It links measured voltage to the reaction quotient and hence to equilibrium constants and unknown ion concentrations.

The statement that position and momentum of a particle cannot both be known precisely at the same time; the product of their uncertainties is at least h/4π. It explains why electrons occupy orbitals rather than well-defined trajectories.

The wavelength λ = h/p associated with any particle of momentum p. Its experimental confirmation for electrons established wave-particle duality and underpins the quantization of energy levels in atoms.

The set of labels n, l, m and s that specifies the state of an electron in an atom, fixing its shell, subshell, orbital orientation and spin. No two electrons in an atom may share all four values.

The energy separation Δ between d orbitals that arises when ligands approach a metal ion and their fields break the degeneracy. Its size decides colour, magnetism and whether a complex is high-spin or low-spin.

An empirical ordering of ligands by the size of the crystal-field splitting they produce, roughly I- < Br- < Cl- < F- < OH- < H2O < NH3 < en < CN- < CO. Strong-field ligands favour low-spin complexes.

The geometrical distortion of a non-linear complex that removes degeneracy when the d-electron configuration is unequally filled in the high-energy orbitals. It is strongest for d9 octahedral and d4 high-spin octahedral systems, usually elongating one axis.

The value μ = √(n(n+2)) Bohr magnetons calculated from n unpaired electrons, ignoring orbital contribution. It is used to estimate unpaired electron counts in transition-metal complexes and to compare with measured moments.

The extra thermodynamic stability of complexes with polydentate ligands that form rings, compared with analogous monodentate complexes. It arises largely from the favourable entropy change of releasing several small ligands.

The empirical rule that hard acids prefer hard bases and soft acids prefer soft bases, where hardness means low polarizability and high charge density. It predicts the direction of complexation, precipitation and substitution reactions.

The steady decrease in atomic and ionic radius across the lanthanides caused by poor shielding by 4f electrons. It makes the elements after the lanthanides smaller than expected and causes close similarity in their chemistry.

The tendency of heavier p-block elements to form compounds in which the outer s2 pair stays unshared, giving an oxidation state two units below the group maximum. Relativistic stabilization and poor shielding are the usual explanations.

A bonding model in which atomic orbitals combine to give delocalized molecular orbitals spread over the whole molecule. Filling these orbitals with electrons explains bond order, the paramagnetism of O2 and the stability of species such as H2+.

An electron-rich species that donates a pair of electrons to an electron-poor centre to form a bond. Nucleophilicity depends on charge, polarizability and solvent, and is not the same as basicity, though the two often run parallel.

An electron-poor species that accepts a pair of electrons from a nucleophile. Common examples are carbocations, carbonyl carbons and halonium ions, and electrophiles are central to addition and substitution mechanisms.

A one-step bimolecular substitution in which nucleophile attack and leaving-group departure occur together through a five-coordinate transition state. It gives inversion of configuration at carbon and is fastest at unhindered methyl or primary centres.

A two-step unimolecular substitution: the leaving group departs first to give a carbocation, which the nucleophile then attacks. The rate depends only on the substrate, racemization is common, and tertiary or benzylic substrates react fastest.

When a protic reagent adds to an unsymmetrical alkene, the hydrogen attaches to the carbon bearing more hydrogens, giving the more stable carbocation intermediate. Peroxide-initiated radical addition reverses this regiochemistry.

The special stability of cyclic, planar, fully conjugated systems containing 4n + 2 π electrons in a continuous overlapping ring. Such compounds resist addition, undergo substitution, and show characteristic ring-current effects in NMR.

The permanent polarization transmitted through sigma bonds because of an electronegativity difference, weakening with distance from the substituent. It stabilizes or destabilizes ions and shifts acidities, basicities and reaction rates.

Delocalization of sigma-bond electrons, usually from an adjacent C-H or C-C bond, into an empty or partly empty p orbital. It stabilizes carbocations, alkenes and radicals and is often counted by the number of alpha hydrogens.

The absolute configuration assigned to a stereocentre by ranking its four substituents with the Cahn-Ingold-Prelog rules. With the lowest-priority group pointing away, clockwise order is R and anticlockwise order is S.

The fragment that departs with the bonding electrons in a substitution or elimination reaction. Good leaving groups are weak bases such as halide ions or tosylate, and their ability strongly affects SN1 and SN2 rates.

The stage of a titration at which the amount of titrant added exactly matches the amount of analyte according to the reaction stoichiometry. It is detected by an indicator or instrumental break and is not necessarily the same as the endpoint.

A solution of a weak acid with its conjugate base, or a weak base with its conjugate acid, that resists pH change when small amounts of acid or base are added. Maximum buffering occurs near the pKa.

The statement that absorbance of a dilute solution is proportional to path length and to the concentration of the absorbing species, A = εcl. Deviations occur at high concentration or when the species dissociates or aggregates.

The equilibrium constant Ksp for dissolution of a sparingly soluble salt, written as the product of ion concentrations each raised to its stoichiometric power. It predicts whether a precipitate forms when solutions are mixed.

A titration in which a chelating ligand such as EDTA binds metal ions quantitatively in a fixed, usually 1:1, ratio. Metal-ion indicators show the endpoint, and the pH must be controlled to obtain selectivity.

The ratio of the equilibrium concentrations of a solute in two immiscible phases, usually organic over aqueous. It is the basis of solvent extraction and is simple only when the solute exists in one form in each phase.

In chromatography, the ratio of the distance travelled by a solute to the distance travelled by the solvent front, or the equivalent ratio of adjusted retention times. It characterizes distribution between stationary and mobile phases.

The agreement among repeated measurements of the same quantity, reflecting random scatter rather than closeness to the true value. It is described by the standard deviation or relative standard deviation of the replicate results.

A reproducible deviation from the true value that affects results in one direction, caused by faulty calibration, impure reagents or a biased method. It cannot be reduced by repeating measurements, only by correcting the source.