RBSE SS Chemistry key terms

The smallest portion of a crystal lattice that, when repeated in three dimensions, generates the entire crystal. It is described by its edge lengths and the angles between those edges.

The percentage of the total volume of a unit cell that is actually occupied by the constituent particles. It depends on the lattice type and is highest for close-packed face-centred cubic arrangements.

A point defect in ionic crystals in which an equal number of cations and anions are missing from their lattice sites, so electrical neutrality is preserved but the density of the crystal falls.

An anion vacancy in an ionic crystal that has trapped one or more electrons. Such centres absorb visible light and give crystals such as sodium chloride a yellow colour.

Adsorption in which the adsorbed species forms chemical bonds with the surface of the adsorbent. It is usually irreversible, highly specific, and accompanied by a large enthalpy of adsorption.

The scattering of a beam of light by colloidal particles, which makes the path of the beam visible. It is used to distinguish a colloidal solution from a true solution.

The number of moles of solute dissolved in one kilogram of solvent. Unlike molarity it does not change with temperature, because it is based on mass rather than volume.

For a solution of volatile liquids, the partial vapour pressure of each component is proportional to its mole fraction in the liquid, the constant being its vapour pressure when pure.

Properties of dilute solutions that depend only on the number of solute particles present and not on their identity: relative lowering of vapour pressure, boiling point elevation, freezing point depression and osmotic pressure.

The ratio of the observed colligative effect of a solution to the value expected if the solute neither associated nor dissociated. It is greater than one for electrolytes that dissociate.

At constant temperature the solubility of a gas in a liquid is proportional to the partial pressure of that gas above the liquid; the constant depends on the gas, the solvent and the temperature.

The potential of a half-cell measured against the standard hydrogen electrode under standard conditions of one bar pressure, unit concentration of ions and, usually, 298 K.

An equation relating the potential of a cell or half-cell to its standard potential and to the reaction quotient, allowing prediction of how concentration and temperature change the cell voltage.

The conductance of a solution containing one mole of electrolyte placed between electrodes one centimetre apart. It increases on dilution for strong and weak electrolytes alike.

At infinite dilution the molar conductivity of an electrolyte equals the sum of the limiting molar conductivities of its individual ions, each multiplied by the number of such ions in the formula unit.

The sum of the powers of the concentration terms in the experimentally determined rate law. It may be zero, fractional or negative and need not equal the molecularity of the reaction.

The minimum extra energy that reacting molecules must acquire, above their average energy, for a collision to convert reactants into products. It controls the temperature dependence of the rate.

The time needed for the concentration of a reactant to fall to half of its initial value. For a first-order reaction it is independent of the starting concentration.

An expression of the rate constant as k = A exp(−Ea/RT), where A is the frequency factor and Ea the activation energy, showing how rate constants respond to changes in temperature.

The tendency of the two electrons in the outermost s orbital of heavier p-block elements to stay out of bonding, so these elements show an oxidation state two units below the group maximum.

Compounds formed between two different halogens, such as chlorine monofluoride. The central atom is the larger, less electronegative halogen and is usually in an odd oxidation state.

The steady decrease in atomic and ionic radii across the lanthanoid series caused by poor shielding by 4f electrons, which makes later members similar in size to the elements above them.

The magnetic moment calculated from unpaired electrons alone using μ = √(n(n+2)) Bohr magnetons, where n is the number of unpaired electrons; it helps assign oxidation states and geometries.

An ion or molecule that donates one or more electron pairs to a central metal atom or ion in a coordination compound, binding through a donor atom such as nitrogen, oxygen or carbon.

The separation in energy of the d orbitals of a metal ion when ligands approach, caused by unequal electrostatic repulsion. Its size decides the colour and magnetic behaviour of the complex.

A complex in which a polydentate ligand is attached to the same metal ion through two or more donor atoms, forming a ring. Chelates are usually more stable than comparable unidentate complexes.

A two-step substitution in which the leaving group departs first to give a planar carbocation that is then attacked by the nucleophile. The rate depends only on the substrate concentration.

An equimolar mixture of two enantiomers. It is optically inactive because the two rotations cancel, and the composition is often written with the prefix (±).

The preparation of an ether by treating an alkyl halide with a sodium or potassium alkoxide. Primary halides give good yields, whereas tertiary halides favour elimination instead.

A test that distinguishes alcohols by how quickly they give a cloudy layer with Lucas reagent, a solution of anhydrous zinc chloride in concentrated hydrochloric acid; tertiary alcohols react fastest.

A base-catalysed reaction of two aldehyde or ketone molecules, one acting as nucleophile, giving a β-hydroxy carbonyl compound that may then lose water to form an α,β-unsaturated product.

A disproportionation in which an aldehyde having no α-hydrogen, such as methanal, reacts with concentrated alkali to give one molecule of a carboxylate salt and one of an alcohol.

The conversion of a primary amide into a primary amine containing one carbon fewer, using bromine and aqueous or alcoholic alkali; the amine formed is pure and the method is preparatively useful.

The reaction of a primary amine with chloroform and alcoholic potassium hydroxide to give an isocyanide, whose foul smell serves as a test for primary amines.

The two cyclic stereoisomers of a sugar that differ only in the configuration at the new chiral centre created when the ring closes, known as the anomeric carbon.

A species carrying both a positive and a negative charge on different atoms, as amino acids do in neutral solution. It has no net charge but behaves like a salt.

A chain reaction in which unsaturated monomers join together without losing any small molecule, giving a polymer whose repeat unit contains the same atoms as the monomer.

The process of heating natural rubber with sulfur, which forms cross-links between polymer chains and makes the rubber harder, stronger and less sticky while improving its elasticity.