European Baccalaureate Chemistry flashcards

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

Isotopes

Atoms of the same element with identical proton number but different neutron numbers, so they differ in mass number while sharing chemical properties; their relative abundances determine the weighted mean atomic mass.

First ionisation energy

The energy needed to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions. It increases across a period and decreases down a group.

Hund's rule

When filling degenerate orbitals of equal energy, electrons occupy separate orbitals with parallel spins before any pairing occurs, minimising electron–electron repulsion and giving the lowest-energy configuration.

VSEPR theory

Electron domains around a central atom repel each other and arrange to maximise separation. Lone pairs repel more strongly than bonding pairs, so they compress bond angles below ideal values.

Hydrogen bonding

A strong intermolecular attraction between a hydrogen atom bonded to N, O or F and a lone pair on a nearby electronegative atom, explaining water's high boiling point and ice's lower density.

Molecular dipole moment

A measure of molecular polarity arising from polar bonds and molecular geometry. Symmetrical arrangements such as linear CO2 or tetrahedral CH4 cancel individual bond dipoles, giving a net zero moment.

Limiting reagent

The reactant that is completely consumed first, determined by dividing each reactant's amount by its stoichiometric coefficient. It fixes the maximum theoretical yield of product.

Ideal gas law

pV = nRT, relating pressure, volume, amount and absolute temperature with R = 8.314 J mol⁻¹ K⁻¹. It assumes negligible molecular volume and no intermolecular forces.

Atom economy

The molar mass of the desired product divided by the total molar mass of all reactants, expressed as a percentage. It measures how efficiently atoms are incorporated into the target product.

Hess's law

The enthalpy change of a reaction is independent of the route taken, because enthalpy is a state function. It allows unknown enthalpies to be calculated from formation or combustion data.

Gibbs free energy

ΔG = ΔH − TΔS determines spontaneity at constant temperature and pressure: negative values indicate a spontaneous process, zero means equilibrium, positive means non-spontaneous under those conditions.

Entropy

A measure of the dispersal of energy among available microstates. It increases with temperature, on melting or boiling, and when a reaction produces more moles of gas.

Rate-determining step

The slowest step in a multi-step reaction mechanism, which controls the overall rate. Its participants appear in the experimentally determined rate equation.

Dynamic equilibrium

A state in which forward and reverse reaction rates are equal, so concentrations remain constant while both reactions continue. The equilibrium constant depends only on temperature.

Le Chatelier's 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, partially counteracting the disturbance.

Activation energy

The minimum collision energy needed for a reaction to occur. It equals the energy difference between the transition state and the reactants and is lowered by a catalyst.

Buffer solution

A mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid, that resists pH change on adding small amounts of acid or alkali, working best near pH = pKa.

Equivalence point

The point in a titration where the acid and base have reacted in exact stoichiometric proportions. Its pH depends on the strength of the acid and base involved.

Weak acid dissociation constant

Ka = [H⁺][A⁻]/[HA] for a weak acid HA, with pKa = −log Ka. A smaller Ka indicates a weaker acid and a higher pH for a given concentration.

Standard electrode potential

The potential of a half-cell measured under standard conditions against the standard hydrogen electrode, set at 0.00 V. More positive values indicate a greater tendency to be reduced.

Oxidising agent

A species that accepts electrons from another substance, causing its oxidation while itself being reduced. Its oxidation number decreases during the reaction.

Faraday constant

The charge carried by one mole of electrons, approximately 96 485 C mol⁻¹. It converts charge passed during electrolysis into moles of electrons using n = Q/F.

Markovnikov's rule

In electrophilic addition of HX to an alkene, the hydrogen adds to the carbon with more hydrogens, generating the more stable carbocation intermediate and giving the major product.

SN1 mechanism

A two-step nucleophilic substitution in which the leaving group departs first, forming a carbocation intermediate. It is favoured by tertiary halogenoalkanes and gives racemised products.

Chiral centre

A carbon atom bonded to four different groups, producing non-superimposable mirror-image molecules. Enantiomers rotate plane-polarised light in opposite directions.

Esterification

The reversible, acid-catalysed reaction of a carboxylic acid with an alcohol to form an ester and water. Removing water or using excess alcohol increases the equilibrium yield.

Infrared spectroscopy

A technique identifying functional groups from absorption of infrared radiation at characteristic wavenumbers, such as a sharp C=O peak near 1700 cm⁻¹ or a broad O–H peak near 3300 cm⁻¹.

Chemical shift

The position of an NMR signal relative to a reference, measured in ppm. It reflects the electronic environment of the nucleus and helps identify the functional group present.

Retention time

The time a component takes to pass through a chromatographic column. Comparing it with standards identifies substances, while peak area enables quantitative analysis.

Beer–Lambert law

A = εcl, stating that absorbance is proportional to concentration and path length for a given species and wavelength. It underpins quantitative colorimetric and atomic absorption analysis.

Haber–Bosch process

The industrial synthesis of ammonia from nitrogen and hydrogen over an iron catalyst at about 450 °C and 200 atm. The exothermic reaction favours high pressure and moderate temperature.

Catalytic converter

A device using platinum, palladium and rhodium on a support to oxidise carbon monoxide and hydrocarbons and reduce nitrogen oxides in vehicle exhaust to carbon dioxide, water and nitrogen.