NSEC flashcards

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

Mole concept

One mole equals 6.022×10²³ particles (Avogadro constant) and, for a gas at STP, occupies 22.4 L; molar mass in g·mol⁻¹ links mass to moles by n = m/M.

Molarity versus molality

Molarity is moles of solute per litre of solution and changes with temperature; molality is moles per kilogram of solvent and is temperature-independent, so molality is used in colligative-property formulas.

Raoult's law

The partial vapour pressure of each volatile component equals its mole fraction in the solution times its pure vapour pressure; positive deviation gives a minimum-boiling azeotrope and negative deviation a maximum-boiling azeotrope.

van't Hoff factor

The ratio of observed colligative effect to the value expected for an undissociated solute; i > 1 for dissociation (NaCl gives about 2), i < 1 for association, and i = 1 for non-electrolytes.

Nernst equation

E = E° − (0.0591/n)·log Q at 298 K, where n is electrons transferred and Q the reaction quotient; it predicts how cell potential falls as products accumulate or reactant concentration drops.

Faraday's laws of electrolysis

Mass deposited is proportional to charge passed, and for a given charge proportional to equivalent mass; 1 faraday (96,500 C) deposits one gram-equivalent, so m = (E·I·t)/96,500.

First-order half-life

For a first-order reaction t½ = 0.693/k, independent of initial concentration; in contrast, zero-order half-life t½ = a/2k depends on the starting concentration a.

Arrhenius equation

k = A·e^(−Ea/RT); a plot of ln k against 1/T is a straight line of slope −Ea/R, so activation energy can be found from rate constants at two temperatures.

pH of weak acid

For a weak monobasic acid of concentration c and dissociation constant Ka, [H⁺] = √(Ka·c) and pH = ½(pKa − log c), valid when dissociation is small compared with c.

Buffer pH

Henderson–Hasselbalch equation: pH = pKa + log([conjugate base]/[acid]); a buffer resists pH change best when the ratio is near 1, that is, pH close to pKa.

Le Chatelier's principle

When a system at equilibrium is disturbed by concentration, pressure or temperature change, it shifts in the direction that partly opposes the change; catalysts do not shift equilibrium, only speed its approach.

Kp and Kc relation

Kp = Kc(RT)^Δn, where Δn is moles of gaseous products minus gaseous reactants; R = 0.0831 L·bar·K⁻¹·mol⁻¹ gives Kp in bar units when concentrations are in mol·L⁻¹.

Ionisation enthalpy trend

Ionisation enthalpy increases across a period and decreases down a group; exceptions occur at Be→B and N→O because of filled or half-filled subshell stability and increased shielding.

Electron gain enthalpy

Energy released when a gaseous atom accepts an electron; chlorine's is more negative than fluorine's because fluorine's small size causes electron–electron repulsion in the compact 2p subshell.

VSEPR geometry rules

Electron pairs around a central atom repel and arrange to minimise repulsion; lone pairs occupy more space than bond pairs, compressing bond angles, for example 104.5° in water versus 109.5° in methane.

Hybridisation from steric number

Steric number = sigma bonds + lone pairs on the central atom: 2 gives sp (linear), 3 gives sp² (trigonal planar), 4 gives sp³ (tetrahedral), 5 gives sp³d and 6 gives sp³d².

Markovnikov's rule

When HX adds to an unsymmetrical alkene, hydrogen attaches to the carbon bearing more hydrogens, giving the more stable carbocation; peroxide-initiated HBr addition reverses this regiochemistry.

SN1 versus SN2

SN1 is two-step, first order, favoured by tertiary halides and polar protic solvents with racemisation; SN2 is one-step, second order, favoured by primary halides and polar aprotic solvents with inversion.

Saytzeff rule

In dehydrohalogenation of alkyl halides or dehydration of alcohols, the major alkene is the more substituted, more stable one, formed by removal of hydrogen from the carbon with fewer hydrogens.

Cannizzaro reaction

Aldehydes lacking an alpha-hydrogen, such as formaldehyde and benzaldehyde, undergo disproportionation with concentrated alkali to give one molecule of alcohol and one of carboxylate salt.

Aldol condensation

An aldehyde or ketone with alpha-hydrogens forms a beta-hydroxy carbonyl (aldol) in dilute base; heating causes dehydration to an alpha,beta-unsaturated carbonyl compound.

Tollens' test

Ammoniacal silver nitrate oxidises aldehydes to carboxylate, depositing a silver mirror; ketones and most other carbonyls give no reaction, so the test distinguishes aldehydes.

Iodoform test

Iodine with alkali gives a yellow precipitate of CHI3 for compounds containing CH3CO– or CH3CH(OH)– groups, such as acetaldehyde, ethanol, acetone and acetophenone.

Haloform reaction

Methyl ketones are fully halogenated at the methyl group in basic halogen, then the trihalomethyl group leaves as haloform (CHX3), leaving a carboxylate salt.

Crystal field splitting

In an octahedral field d orbitals split into lower t2g and higher eg sets by Δo; strong-field ligands give low-spin complexes, weak-field ligands high-spin complexes.

Coordination number and EAN

Coordination number counts donor atoms bonded to the central metal; the effective atomic number rule states metal electrons plus donated pairs often reach the next noble-gas count.

Optical isomerism in complexes

Octahedral complexes with three bidentate ligands, such as [Co(en)3]³⁺, exist as non-superimposable mirror-image Δ and Λ enantiomers that rotate plane-polarised light oppositely.

Radioactive decay law

Activity A = λN and N = N0·e^(−λt); half-life t½ = 0.693/λ, and after n half-lives the remaining fraction is (1/2)^n. Carbon-14 dating uses t½ ≈ 5730 years.

Order versus molecularity

Order is the experimentally determined sum of concentration exponents in the rate law and may be zero or fractional; molecularity is the number of species colliding in an elementary step and is a whole number.

Collision theory

Reaction rate depends on collision frequency and the fraction of collisions with energy at least Ea and correct orientation; the steric factor P accounts for orientation in the rate expression.

Lanthanoid contraction

The steady decrease in atomic and ionic radii across the lanthanoids due to poor shielding of 4f electrons; it causes Zr and Hf to have nearly identical radii and very similar chemistry.

Molar conductivity

Λm = κ/c, where κ is conductivity in S·cm⁻¹ and c concentration in mol·cm⁻³; Λm increases on dilution, and for strong electrolytes Λm = Λ°m − A√c (Debye–Hückel–Onsager).