NZ Scholarship Chemistry flashcards

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

Avogadro constant

Number of particles in one mole, 6.022 × 10²³ mol⁻¹. Multiply amount in moles by this value to convert to a particle count.

Molar mass calculation

M is the sum of relative atomic masses in g mol⁻¹. Amount n = m/M links mass in grams to moles for stoichiometric calculations.

Limiting reagent test

Divide each reactant's amount by its stoichiometric coefficient; the smallest quotient identifies the limiting reagent, which determines maximum product yield.

Ideal gas equation

pV = nRT with R = 8.314 J K⁻¹ mol⁻¹ and temperature in kelvin. It relates pressure, volume, amount and temperature for a gas.

Equilibrium constant expression

K_c is the product of product concentrations raised to coefficients divided by reactant concentrations raised to coefficients. Pure solids and liquids are omitted because their concentration is constant.

Reaction quotient versus K

Q has the same form as K but uses current concentrations. Q < K means net forward reaction; Q > K means net reverse; Q = K means equilibrium.

Le Chatelier temperature rule

Raising temperature shifts equilibrium in the endothermic direction and changes the value of K. Concentration or pressure changes shift position but leave K unchanged.

pH and Kw

pH = −log₁₀[H₃O⁺]. At 298 K, K_w = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴, so pH + pOH = 14 for aqueous solutions.

Weak acid Ka

K_a = [H₃O⁺][A⁻]/[HA]. A larger K_a means a stronger weak acid; the approximation [H₃O⁺] ≈ √(K_a·c) holds only for slight dissociation.

Buffer pH equation

pH = pK_a + log([A⁻]/[HA]) for a weak acid and its conjugate base. A buffer works best when the ratio is near one, so pH ≈ pK_a.

Calorimetry relation

q = mcΔT gives heat transferred, with m mass, c specific heat capacity and ΔT temperature change. For aqueous solutions c is about 4.18 J g⁻¹ K⁻¹.

Hess's law

The enthalpy change of a reaction is independent of route. Sum step enthalpies, or use ΣΔH°_f(products) − ΣΔH°_f(reactants) to obtain ΔH°_rxn.

Gibbs free energy criterion

ΔG° = ΔH° − TΔS°. A negative ΔG° indicates a thermodynamically spontaneous reaction at that temperature; spontaneity depends on both enthalpy and entropy terms.

Cell potential calculation

E°_cell = E°_cathode − E°_anode using standard reduction potentials. A positive E°_cell corresponds to a spontaneous galvanic reaction with ΔG° = −nFE°_cell.

Nernst equation

E = E° − (RT/nF)ln Q adjusts cell potential for non-standard concentrations. At 298 K the term simplifies to (0.0592/n)log Q in volts.

VSEPR shape rule

Electron domains, including lone pairs, set geometry. Repulsion order is lone pair–lone pair > lone pair–bond pair > bond pair–bond pair, so water is bent and ammonia pyramidal.

Hydrogen bonding requirement

Occurs when hydrogen is covalently bonded to nitrogen, oxygen or fluorine and interacts with a lone pair on another such atom. It raises boiling points markedly, as in water.

Hybridisation and steric number

Steric number 2 gives sp linear 180°, 3 gives sp² trigonal planar 120°, 4 gives sp³ tetrahedral 109.5°. Count bonded atoms plus lone pairs.

Chirality test

A carbon bonded to four different groups is a stereocentre. The molecule is chiral if it lacks a plane of symmetry, giving non-superimposable mirror-image enantiomers.

E/Z geometric isomerism

Occurs about a C=C double bond when each carbon carries two different groups. Priority follows higher atomic number on each carbon; Z means high-priority groups on the same side.

SN1 versus SN2

Primary haloalkanes react by SN2, a one-step bimolecular process with inversion. Tertiary haloalkanes react by SN1 via a carbocation, giving mixtures and racemisation.

Esterification conditions

Carboxylic acid plus alcohol with concentrated acid catalyst gives an ester and water. The reaction is reversible, so removing water or using excess alcohol improves yield.

IR spectroscopy use

Absorption bands identify functional groups: broad O–H near 3300 cm⁻¹, sharp C=O near 1700 cm⁻¹, C–O near 1000–1300 cm⁻¹. Fingerprint region supports identification.

Proton NMR splitting

A signal splits into n+1 peaks where n is the number of non-equivalent hydrogens on adjacent carbons. Integration gives the relative hydrogen ratio.

Mass spectrometry molecular ion

The molecular ion peak gives molecular mass; fragment peaks reveal loss of groups. For chlorine or bromine, isotope patterns (3:1, 1:1) confirm halogen presence.

Titration equivalence relation

At equivalence, c₁V₁ divided by the acid coefficient equals c₂V₂ divided by the base coefficient. Use this to find unknown concentration from burette volumes.

Percentage yield

Percentage yield = actual amount divided by theoretical amount × 100. Theoretical amount comes from the limiting reagent's stoichiometry.

Solubility product Ksp

For a sparingly soluble salt, K_sp is the product of ion concentrations raised to coefficients. A precipitate forms when the ion product exceeds K_sp.

Common-ion effect

Adding an ion already present in a solubility or acid equilibrium shifts the position to reduce that ion's concentration, lowering solubility or suppressing ionisation.

Conductivity requirement

Electrical conduction requires mobile charge carriers: delocalised electrons in metals and graphite, or mobile ions in molten salts and electrolyte solutions. Solid ionic lattices do not conduct.

Thermodynamic versus kinetic control

A negative ΔG means a reaction is favourable but says nothing about rate. A high activation energy can make a favourable reaction slow; a catalyst lowers E_a only.

Random versus systematic error

Random error scatters repeated readings and is reduced by averaging; systematic error offsets all readings in one direction and is not removed by repetition, only by calibration or method change.