Cameroon GCE A-Level Chemistry flashcards

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

Effective nuclear charge

The net positive charge experienced by an outer electron after subtracting shielding by inner electrons. It increases across a period, raising ionisation energy and electronegativity.

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.

Lattice enthalpy

The enthalpy change when one mole of an ionic solid forms from its gaseous ions under standard conditions. It is exothermic and becomes more negative with higher charges and smaller ionic radii.

VSEPR theory

A model predicting molecular shape from the repulsion between electron pairs around a central atom. Lone pairs repel more strongly than bonding pairs, reducing bond angles.

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. It raises boiling points above expectation.

Enthalpy of vaporisation

The enthalpy change when one mole of a liquid is converted to vapour at its boiling point. It exceeds the enthalpy of fusion because all intermolecular forces must be overcome.

Avogadro's law

Equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules. It underlies molar volume calculations in gas stoichiometry.

Graham's law of diffusion

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

Hess's law

The total enthalpy change of a reaction is independent of the route taken. It allows unknown enthalpy changes to be calculated from standard enthalpies of formation or combustion.

Standard enthalpy of formation

The enthalpy change when one mole of a compound forms from its elements in their standard states at 298 K and 1 atm. It is zero for an element in its standard state.

Le Chatelier's principle

When a system at equilibrium is disturbed by a change in concentration, pressure or temperature, the system shifts to oppose that change. Catalysts do not shift the position.

Equilibrium constant Kc

The ratio of product concentrations to reactant concentrations, each raised to its stoichiometric coefficient, at equilibrium. It depends only on temperature.

Rate-determining step

The slowest step in a reaction mechanism, which controls the overall rate. It determines the orders in the experimentally derived rate equation.

Activation energy

The minimum energy colliding particles must possess for a reaction to occur. Catalysts lower it by providing an alternative pathway with lower energy.

Oxidation number

A number assigned to an atom showing its electron gain or loss relative to the element. Oxidation increases the value; reduction decreases it.

Buffer solution

A solution that resists pH change when small amounts of acid or base are added. It contains a weak acid and its conjugate base in comparable concentrations.

Henderson–Hasselbalch equation

pH = pKa + log([salt]/[acid]) for a weak acid buffer. It shows that pH equals pKa when the acid and salt concentrations are equal.

Salt hydrolysis

The reaction of ions from a dissolved salt with water, producing an acidic or alkaline solution. Salts of weak acids with strong bases give alkaline solutions.

Thermal stability of carbonates

The ease of decomposition of Group 2 carbonates to oxide and carbon dioxide. Stability increases down the group as the cation becomes larger and less polarising.

Displacement of halogens

A more reactive halogen displaces a less reactive halide from solution. Chlorine displaces bromide and iodide, bromine displaces iodide, iodine displaces none.

Ligand

An ion or molecule that donates one or more lone pairs of electrons to a central metal ion, forming a coordinate bond in a complex.

Coordination number

The number of ligand donor atoms bonded to the central metal ion in a complex. It is commonly 4 (tetrahedral) or 6 (octahedral).

Ligand substitution

The replacement of one ligand in a complex by another, often causing a colour change. Adding excess ammonia to a hexaaqua complex is a common example.

Markovnikov's rule

When HBr adds to an unsymmetrical alkene, the hydrogen adds to the carbon with more hydrogens, forming the more stable carbocation intermediate.

Tollens' reagent

A solution of silver nitrate in aqueous ammonia used to distinguish aldehydes from ketones. Aldehydes form a silver mirror; ketones do not react.

Esterification

The reaction of a carboxylic acid with an alcohol in the presence of an acid catalyst to form an ester and water. It is a reversible condensation reaction.

SN1 mechanism

A two-step nucleophilic substitution in which the leaving group departs first to form a carbocation. Tertiary haloalkanes favour it and the product is racemic.

Electrophilic addition

A reaction in which an electrophile adds to an alkene, converting the double bond to a single bond. The alkene's pi electrons attack the electrophile.

Electrophilic substitution

A reaction in which an electrophile replaces a hydrogen atom on an aromatic ring. Benzene's delocalised ring is preserved, so addition is not favoured.

Concordant titres

Titration volumes that agree within 0.10 cm³ of each other. Only concordant values are averaged; the rough titre is discarded.

Flame test colours

Characteristic flame colours identify metal ions: lithium red, sodium yellow, potassium lilac, calcium brick-red and copper blue-green. A clean wire must be used.

Percentage error

The uncertainty of a measurement expressed as a percentage of the measured value. It is calculated as (error/measured value) × 100 and guides precision choices.