Maharashtra HSC Chemistry key terms

The smallest repeating portion of a crystal lattice which, when repeated in three dimensions, generates the whole crystal. It is described by edge lengths and the angles between them.

The number of nearest neighbour particles surrounding a given particle in a crystal lattice. It is 12 for hexagonal close packing, 8 for body-centred cubic and 6 for simple cubic structures.

The percentage of total volume of a unit cell actually occupied by constituent particles. Close packing gives about 74 percent, body-centred cubic about 68 percent and simple cubic about 52 percent.

A point defect in ionic crystals in which an equal number of cations and anions are missing from their lattice sites, leaving vacant spaces. It lowers density and is favoured when ions are of similar size.

A point defect in which an ion leaves its normal lattice site and occupies an interstitial position. It does not change the density of the crystal and is favoured when the cation is much smaller than the anion.

An anion vacancy in an ionic crystal that has trapped one or more electrons. These trapped electrons absorb visible light, which gives crystals such as heated sodium chloride a characteristic colour.

A property of a dilute solution that depends only on the number of solute particles present, not on their identity. Relative lowering of vapour pressure, boiling point elevation, freezing point depression and osmotic pressure belong here.

The number of moles of solute dissolved in one kilogram of solvent. Because it uses mass rather than volume, molality does not change with temperature and is preferred for colligative property calculations.

The number of moles of solute present in one litre of solution. It is temperature dependent because the volume of a solution changes with temperature, so molality is often used instead.

The ratio of the observed colligative effect to the value expected for an ideal non-dissociating solute. It is greater than one for solutes that dissociate and less than one for solutes that associate.

The excess pressure that must be applied to a solution to prevent pure solvent from crossing a semipermeable membrane into it. It equals the product of molarity, gas constant and absolute temperature for dilute solutions.

A solution that resists a significant change in pH when a small amount of strong acid or base is added. It usually contains a weak acid with its conjugate base, or a weak base with its conjugate acid.

An equation relating the pH of a buffer to the acid dissociation constant and the ratio of conjugate base to weak acid concentrations. It is written as pH equals pKa plus log of that ratio.

A measure of how much strong acid or base a buffer can absorb before its pH changes appreciably. It is greatest when the concentrations of the conjugate acid and base components are large and nearly equal.

The equilibrium constant for the dissolution of a sparingly soluble salt, written as the product of the molar concentrations of its ions each raised to the power of its stoichiometric coefficient in the balanced equation.

The decrease in ionisation of a weak electrolyte caused by adding a strong electrolyte that supplies one of the same ions. It shifts the ionisation equilibrium towards the undissociated species.

The product of the actual concentrations of the ions of a sparingly soluble salt in solution at a given moment, each raised to its stoichiometric coefficient. It is compared with the solubility product to predict precipitation.

A property whose value depends only on the current state of the system and not on the path taken to reach that state. Internal energy, enthalpy, entropy and Gibbs energy are examples.

A thermodynamic quantity equal to internal energy plus the product of pressure and volume. The change in enthalpy at constant pressure equals the heat exchanged with the surroundings.

The statement that the enthalpy change of a reaction is the same whether it occurs in one step or in several steps, provided the initial and final states are identical. It follows from enthalpy being a state function.

A thermodynamic function equal to enthalpy minus the product of absolute temperature and entropy. A negative change indicates a spontaneous process at constant temperature and pressure.

A measure of the disorder or randomness of a system, related to the number of ways energy can be distributed among its particles. It increases in spontaneous processes when the system becomes more disordered.

The potential developed by a half cell containing the electrode in contact with a solution of unit activity of its ions, measured against the standard hydrogen electrode at 298 kelvin and one bar.

An equation giving the electrode potential or cell potential at non-standard concentrations. It subtracts from the standard potential a term containing the reaction quotient, temperature and number of electrons transferred.

A list of standard reduction potentials of half reactions arranged in order of increasing potential. It allows prediction of which species will be oxidised or reduced and which metal can displace another from solution.

The sum of the powers to which the concentration terms are raised in the experimentally determined rate law. It may be zero, fractional or negative and must be found by experiment, not from the balanced equation.

The number of reacting species that collide simultaneously in a single elementary step. It is always a small whole number, unlike order of reaction which can be fractional.

The proportionality constant in a rate law, equal to the rate when all reactant concentrations are unity. Its value depends on temperature and is related to activation energy by the Arrhenius equation.

The minimum extra energy that reactant molecules must possess for their collisions to lead to a reaction. A larger value makes the reaction slower and more temperature sensitive.

The slowest elementary step in a multi-step reaction, which controls the overall rate. The overall rate law is derived from the species involved in this step.

The time required for half of the reactant concentration to be consumed in a reaction. For a first-order reaction it is independent of initial concentration and equals 0.693 divided by the rate constant.

The steady decrease in atomic and ionic radii across the lanthanoid series caused by poor shielding of the nuclear charge by 4f electrons. It explains the similarity in size of zirconium and hafnium.

An element whose atom or common ion has an incompletely filled d subshell. Such elements show variable oxidation states, form coloured ions and act as catalysts.

An ion or molecule that donates one or more electron pairs to a central metal ion to form a coordinate bond in a complex. Water, ammonia, cyanide and chloride are common examples.

The number of donor atoms directly bonded to the central metal ion in a coordination entity. It is commonly 4 or 6 and is decided by the metal, its oxidation state and the ligands.

A metal complex in which a single ligand occupies two or more coordination sites on the same metal ion, forming a ring. Such complexes are usually more stable than similar complexes with monodentate ligands.

The existence of two forms of a substance that are non-superimposable mirror images of each other and rotate plane-polarised light in opposite directions. Such molecules are called enantiomers.

A reaction in which a nucleophile replaces a leaving group on a carbon atom. In halogen derivatives it can proceed by the SN1 route through a carbocation or the SN2 route through a single concerted step.

The geometric property of a molecule that makes it non-superimposable on its mirror image, usually because a carbon atom carries four different groups. Chiral molecules rotate plane-polarised light.

A reaction in which phenol is treated with chloroform and aqueous sodium hydroxide to introduce a formyl group mainly at the ortho position. The product is salicylaldehyde, formed through a dichlorocarbene intermediate.

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

A reaction in which an aldehyde or ketone with alpha hydrogen atoms reacts in dilute base to form a beta-hydroxy carbonyl compound, which may lose water to give an alpha,beta-unsaturated carbonyl compound.

A disproportionation shown by aldehydes that lack alpha hydrogen atoms when treated with concentrated alkali. One molecule is oxidised to a carboxylate salt while another is reduced to an alcohol.

The conversion of a primary amide into a primary amine having one carbon atom fewer, by heating with bromine and alkali. The carbonyl carbon is lost as carbonate.

The ability of an amine to donate its lone pair to a proton. In aqueous solution it is influenced by the electron-releasing alkyl groups, solvation and the availability of the lone pair on nitrogen.

A molecule that contains both a positive and a negative charge on different atoms within the same structure. Amino acids exist mainly as zwitterions in neutral aqueous solution.

The pH at which an amino acid or protein carries no net electrical charge because the positive and negative charges balance. At this pH it does not move in an electric field.

The amide linkage formed between the carboxyl group of one amino acid and the amino group of another, with loss of water. It joins amino acids into peptides and proteins.

A chain reaction in which unsaturated monomers join together without losing any small molecule. Polyethylene, polyvinyl chloride and polystyrene are made this way.

A stepwise reaction in which monomers with two or more reactive groups join with the elimination of a small molecule such as water. Nylon 6,6 and terylene are formed in this way.

A polymer formed from two or more different monomer species. Buna-S, made from butadiene and styrene, is a common example used in the rubber industry.

The percentage of the total mass of reactants that ends up in the desired product. A high atom economy means little waste is produced and is a key measure of greenness.

The design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances. It aims at safer solvents, higher atom economy and reduced energy demand.

A particle whose size lies roughly between one and one hundred nanometres in at least one dimension. Such particles have a very large surface area to volume ratio and show properties different from the bulk material.