Tamil Nadu HSE First Year Chemistry flashcards

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

Mole

The amount of substance containing 6.022×10^23 particles. One mole of any gas occupies 22.4 L at STP, and its mass in grams equals its molar mass.

Limiting reagent

The reactant that is completely consumed first and so determines the maximum product formed. Find it by dividing moles of each reactant by its stoichiometric coefficient.

Empirical formula

The simplest whole-number ratio of atoms in a compound. Obtain it by dividing percentage composition by atomic masses and then by the smallest quotient.

Heisenberg uncertainty principle

Position and momentum of a microscopic particle cannot both be known exactly at the same time. The product of their uncertainties is at least h/4π.

de Broglie relation

A particle of mass m moving with velocity v has wavelength λ = h/mv. It explains why electrons show diffraction and why Bohr's orbits must contain whole wavelengths.

Aufbau principle

Electrons fill orbitals in order of increasing (n+l) value; when values tie, the orbital with lower n fills first, giving 4s before 3d.

Hund's rule

Degenerate orbitals are each singly occupied with parallel spins before any pairing occurs. This minimises electron repulsion and maximises total spin.

Radial nodes

Spherical surfaces where the probability of finding the electron is zero. Their number equals n - l - 1, while angular nodes equal l.

Ionisation enthalpy

Energy required to remove an electron from an isolated gaseous atom. It rises across a period and falls down a group, with exceptions at half-filled configurations.

Electron gain enthalpy

Enthalpy change when an electron is added to a gaseous atom. Chlorine has the most negative value because fluorine's small size causes electron repulsion.

Diagonal relationship

Similarity in properties between certain elements of period 2 and the next group of period 3, such as Li-Mg and Be-Al, arising from comparable charge/radius ratios.

VSEPR theory

Electron pairs around a central atom arrange to minimise repulsion. Lone pairs repel more strongly than bond pairs, reducing bond angles, as in water and ammonia.

Hybridisation

Mixing of atomic orbitals to give equivalent orbitals. Steric number 2, 3, 4, 5 and 6 correspond to sp, sp2, sp3, sp3d and sp3d2 respectively.

Bond order

Half the difference between bonding and antibonding electrons in molecular orbital theory. Higher bond order means shorter, stronger bonds; O2 has order 2 and is paramagnetic.

Hydrogen bond

An electrostatic attraction between a hydrogen attached to N, O or F and a lone pair on a nearby electronegative atom, typically 10-40 kJ per mole.

Graham's law

The rate of diffusion of a gas is inversely proportional to the square root of its molar mass, so lighter gases such as hydrogen diffuse faster than heavier ones.

van der Waals equation

A corrected ideal gas equation, (P + an^2/V^2)(V - nb) = nRT, where a accounts for intermolecular attraction and b for the finite volume of molecules.

Critical temperature

The highest temperature above which a gas cannot be liquefied however much pressure is applied. Gases with stronger intermolecular forces have higher critical temperatures.

First law of thermodynamics

Energy is conserved: ΔU = q + w, where w is work done on the system. At constant volume no expansion work occurs, so q_v equals ΔU.

Hess's law

The enthalpy change of a reaction is independent of the route taken. Thermochemical equations can therefore be added, reversed or multiplied to find an unknown enthalpy change.

Gibbs energy

Defined as G = H - TS. A process is spontaneous at constant temperature and pressure when ΔG is negative; at equilibrium ΔG equals zero.

Le Chatelier principle

When a system at equilibrium is disturbed by a change in concentration, pressure or temperature, it shifts in the direction that opposes the change.

Reaction quotient Q

The mass-action expression using instantaneous rather than equilibrium concentrations. If Q is less than K the reaction proceeds forward; if greater, it proceeds backward.

Buffer solution

A mixture of a weak acid with its salt, or a weak base with its salt, that resists pH change on adding small amounts of acid or base.

Solubility product

The product of ionic concentrations in a saturated solution of a sparingly soluble salt, each raised to its stoichiometric power. Precipitation occurs when the ionic product exceeds it.

Common ion effect

The suppression of ionisation of a weak electrolyte by adding a strong electrolyte that supplies an identical ion, thereby shifting the equilibrium backward.

Raoult's law

For a solution of non-volatile solute, the vapour pressure of each volatile component equals its mole fraction multiplied by the vapour pressure of the pure component.

van't Hoff factor

The ratio of observed colligative property to that calculated for a non-electrolyte. It is greater than one for dissociating solutes and less than one for associating solutes.

Azeotrope

A liquid mixture that boils at constant composition and temperature. Positive deviations give minimum-boiling azeotropes; negative deviations give maximum-boiling azeotropes.

Order of reaction

The sum of the powers of concentration terms in the experimentally determined rate law. It may be zero, fractional or negative and is unrelated to stoichiometry.

Half-life

The time for the concentration of a reactant to fall to half its initial value. For a first-order reaction it equals 0.693/k and is independent of concentration.

Markovnikov's rule

In addition of HX to an unsymmetrical alkene, hydrogen attaches to the carbon bearing more hydrogens, giving the more stable carbocation intermediate. Peroxides reverse this only for HBr.

Huckel's rule

A cyclic, planar and fully conjugated ring is aromatic if it contains 4n+2 π electrons. Benzene with six π electrons satisfies this condition.

Inductive effect

Permanent displacement of σ-bond electrons along a chain due to an electronegativity difference. Electron-withdrawing groups increase acidity of carboxylic acids and stabilise carbanions.

Chirality

A molecule with a carbon bearing four different groups is non-superimposable on its mirror image and rotates plane-polarised light; an equal mixture of both forms is optically inactive.