Assam ASSEB HS Chemistry key terms

Smallest repeating portion of a crystal lattice whose translation in three dimensions generates the whole crystal; described by edge lengths, angles and particle positions.

Number of nearest-neighbour particles touching a given particle in a crystal lattice; it determines packing arrangement and influences density and packing efficiency.

Percentage of the total volume of a unit cell occupied by its constituent particles; simple cubic is about 52%, body-centred cubic about 68%, and face-centred cubic about 74%.

Ratio of the number of moles of one component to the total number of moles of all components in a mixture; it is dimensionless and used in Raoult's law calculations.

For an ideal solution, the partial vapour pressure of each volatile component equals its mole fraction in the liquid multiplied by the vapour pressure of the pure component.

Solution property that depends on the number of solute particles rather than their identity; examples are relative lowering of vapour pressure, boiling-point elevation, freezing-point depression and osmotic pressure.

Ratio of the observed colligative effect to that expected for a non-electrolyte of the same molality; it reflects the number of particles produced by dissociation or association.

Conductivity of an electrolyte solution divided by its molar concentration; it measures conduction by one mole of electrolyte and increases on dilution, especially for weak electrolytes.

At infinite dilution, the molar conductivity of an electrolyte equals the sum of the limiting molar conductivities of its cation and anion, each multiplied by its stoichiometric coefficient.

Equation relating electrode or cell potential to standard potential, temperature and reaction quotient; at 298 K it is E = E° − (0.059 V/n) log Q.

Electrode potential measured under standard conditions, with unit activity for solutes, 1 bar for gases and pure solids or liquids, relative to the standard hydrogen electrode.

Device containing a concentrated inert electrolyte in a gel that connects two half-cells, completes the circuit and minimises liquid junction potential without allowing bulk mixing.

Electrochemical corrosion of iron in the presence of oxygen and moisture; iron dissolves at anodic sites while oxygen is reduced at cathodic sites, forming hydrated iron(III) oxide.

Equation that expresses reaction rate as a function of reactant concentrations, with exponents determined experimentally; it need not correspond to stoichiometric coefficients in the balanced equation.

Sum of the powers of concentration terms in the experimentally determined rate law; it may be zero, fractional or negative and is not necessarily equal to molecularity.

Time required for the concentration of a reactant to fall to half its initial value; for a first-order reaction it is independent of initial concentration and equals ln2/k.

Equation k = A e^(−Ea/RT) relating the rate constant to temperature, activation energy and pre-exponential factor; its logarithmic form gives a straight-line plot.

Minimum additional energy that reactant molecules must acquire for collisions to lead to products; a higher activation energy makes the rate more sensitive to temperature.

Weak, reversible adsorption held by van der Waals forces; it has low enthalpy, is not specific, may form multilayers, and generally decreases as temperature rises.

Strong adsorption involving chemical bond formation between adsorbate and adsorbent surface; it has high enthalpy, is specific, is usually monolayer, and may be irreversible.

Scattering of a light beam by colloidal particles so that its path becomes visible; used to distinguish colloids from true solutions and depends on particle size.

Colloidal dispersion of one liquid in another immiscible liquid, stabilised by an emulsifier; common types are oil-in-water and water-in-oil.

Method of concentrating sulphide ores by agitating powdered ore with water, oil and froth; oil-wet ore particles rise with the froth while water-wet gangue settles.

Tendency of heavier p-block elements to form compounds in an oxidation state two less than the group maximum because the ns² pair is reluctant to be lost or shared.

Gradual decrease in atomic and ionic radii across the lanthanoid series caused by poor shielding of 4f electrons; it makes later lanthanoids chemically similar and increases density.

d-block element whose atom or common ion has an incomplete d subshell; such elements typically show variable oxidation states, coloured compounds, catalysis and complex formation.

Ion or molecule that donates one or more electron pairs to a central metal ion, forming coordinate bonds in a complex; ligands are classified by charge and denticity.

Complex in which a polydentate ligand binds the same metal ion through two or more donor atoms, forming a ring; chelates are often more stable than analogous monodentate complexes.

Separation of degenerate d orbitals of a metal ion into different energy levels by the electrostatic field of surrounding ligands; it explains colour and magnetic behaviour of complexes.

Unimolecular nucleophilic substitution whose rate depends only on substrate concentration; it proceeds through a carbocation and is favoured by tertiary substrates and polar protic solvents.

Bimolecular nucleophilic substitution whose rate depends on substrate and nucleophile concentrations; it occurs by backside attack through a transition state and is favoured by primary substrates.

Organomagnesium halide of formula RMgX, prepared from an alkyl or aryl halide and magnesium in dry ether; its carbon–magnesium bond makes the organic group nucleophilic and strongly basic.

Reaction of an alkoxide with a primary alkyl halide by SN2 to form an ether; primary halides give good yields, while tertiary halides mainly undergo elimination.

Base-catalysed reaction in which an aldehyde or ketone with α-hydrogen forms a β-hydroxy carbonyl, which may lose water to give an α,β-unsaturated carbonyl; forms carbon–carbon bonds.

Disproportionation of an aldehyde with no α-hydrogen in strong base, giving equimolar amounts of a carboxylic acid salt and an alcohol; one aldehyde is oxidised and another reduced.

Benzenesulphonyl chloride, used to distinguish primary, secondary and tertiary amines; primary amines give an alkali-soluble sulphonamide, secondary an insoluble one, and tertiary no stable product.

Amide bond formed between the carboxyl group of one amino acid and the amino group of another with loss of water; repeated peptide bonds create the primary structure of proteins.

Polymer formed when unsaturated monomers join repeatedly without elimination of any small molecule; examples include polyethylene, polyvinyl chloride and polystyrene.

Polymer formed by stepwise reaction of bifunctional monomers with elimination of a small molecule such as water; examples are nylon-6,6 and poly(ethylene terephthalate).

Highly pure, stable and non-hygroscopic substance of accurately known formula mass, used to prepare a standard solution or to standardise another solution in volumetric analysis.