Pancyprian 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 screening by inner electrons, approximated as Z minus the shielding constant; it increases across a period and governs atomic radius and ionisation energy.

Hund's rule

Within a set of degenerate orbitals, electrons occupy separate orbitals with parallel spins before any pairing occurs. This minimises electron repulsion and explains the paramagnetism of atoms with unpaired electrons.

Lattice enthalpy

The enthalpy change when one mole of an ionic solid forms from its gaseous ions. It becomes more exothermic as ionic charges rise and ionic radii fall, as in MgO versus NaCl.

Hydrogen bonding

A strong dipole–dipole attraction between a hydrogen atom covalently bonded to nitrogen, oxygen or fluorine and a lone pair on a nearby electronegative atom. It raises boiling points and structures ice and proteins.

Limiting reagent

The reactant that is completely consumed first, determined by dividing each reactant's moles by its stoichiometric coefficient and choosing the smallest quotient. It fixes the maximum theoretical yield.

Empirical formula

The simplest whole-number ratio of atoms of each element in a compound, obtained by dividing percentage composition by atomic mass and reducing to the smallest ratio. The molecular formula is a whole-number multiple of it.

Dalton's law of partial pressures

In a mixture of non-reacting gases, the total pressure equals the sum of the partial pressures each gas would exert alone. Each partial pressure equals its mole fraction times the total pressure.

Colligative properties

Properties depending on the number, not identity, of dissolved particles: vapour pressure lowering, boiling point elevation, freezing point depression and osmotic pressure. Electrolytes give larger effects through the van't Hoff factor.

Hess's law

The enthalpy change of a reaction is independent of the route taken, depending only on initial and final states. It allows unknown enthalpies to be calculated from known formation or combustion enthalpies.

Gibbs energy change

Defined as ΔG = ΔH − TΔS, it determines spontaneity at constant temperature and pressure: negative means spontaneous, zero means equilibrium, positive means non-spontaneous in the forward direction.

Rate-determining step

The slowest elementary step in a reaction mechanism, which limits the overall rate. The experimental rate law reflects the species involved up to and including this step.

Arrhenius equation

k = A·e^(−E_a/RT), relating the rate constant to activation energy and temperature. A plot of ln k against 1/T gives a straight line of slope −E_a/R.

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. Catalysts alter neither the position nor the equilibrium constant.

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. pH is given by Henderson–Hasselbalch.

Solubility product K_sp

The equilibrium constant for dissolution of a sparingly soluble salt, equal to the product of ion concentrations raised to their stoichiometric powers. Precipitation occurs when the ion product exceeds K_sp.

Salt hydrolysis

The reaction of a salt's ions with water, altering pH. Salts of weak acids or weak bases hydrolyse, giving alkaline or acidic solutions respectively, while strong acid–strong base salts remain neutral.

Standard electrode potential

The potential of a half-cell measured against the standard hydrogen electrode at 298 K, 1 bar and 1 mol dm⁻³. Cell emf equals cathode potential minus anode potential.

Faraday's laws of electrolysis

The mass deposited or liberated at an electrode is proportional to charge passed and to molar mass divided by electrons transferred. Charge Q = It, and moles of electrons equal Q/96 485.

Disproportionation

A redox reaction in which the same element is simultaneously oxidised and reduced, as when chlorine reacts with cold dilute alkali to give chloride and chlorate(I) ions.

Transition metal complex ion

A central metal ion surrounded by ligands donating lone pairs. Variable oxidation states, coloured d–d transitions, catalysis and ligand substitution characterise these species in solution.

Halogen displacement reactions

A halogen higher in group 17 displaces a halide ion of a lower halogen from solution, because oxidising power decreases down the group. Chlorine displaces bromide but iodine cannot displace chloride.

Markovnikov's rule

When hydrogen halide adds to an unsymmetrical alkene, the hydrogen attaches to the carbon bearing more hydrogens, giving the more stable carbocation intermediate and the major branched product.

SN1 and SN2 mechanisms

Nucleophilic substitution of haloalkanes proceeds either in one step with inversion (SN2, favoured by primary halides) or via a carbocation with racemisation (SN1, favoured by tertiary halides).

Fischer esterification

A reversible acid-catalysed reaction between a carboxylic acid and an alcohol producing an ester and water. Removing water or distilling the ester shifts equilibrium toward the product.

Concentration titration calculation

Moles of titrant equal concentration times volume in dm³, converted to moles of analyte by the balanced equation, then divided by analyte volume to give its concentration.

Weighing by difference

Mass of substance transferred is found by subtracting the final container mass from the initial mass. This avoids transfer losses and is more accurate than weighing the receiving vessel directly.

Percentage uncertainty

The measurement uncertainty divided by the measured value, expressed as a percentage. The largest percentage uncertainty among the measurements dominates the overall experimental error.

Heating to constant mass

Repeated heating, cooling and weighing until successive masses agree, confirming complete drying or decomposition. It removes residual water or volatile products that would inflate the final mass.

Flame test procedure

A clean nichrome or platinum wire is dipped in the sample and held in a non-luminous flame; the characteristic colour identifies the metal cation, such as yellow for sodium or lilac for potassium.

Concordant titration results

Titre volumes agreeing within about 0.10 cm³ of each other, which are averaged to give the mean titre. Scattered values indicate technique error and should be repeated rather than averaged.

Oxidation number

A bookkeeping charge assigned to an atom assuming ionic bonding. Free elements are zero, oxygen is usually −2, and the sum over a species equals its overall charge.

Aromatic electrophilic substitution

Benzene reacts by replacing a hydrogen with an electrophile rather than by addition, because substitution preserves the delocalised pi system and its stabilisation energy.