WBCHSE HS Chemistry flashcards

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

Bohr radius of hydrogen

The radius of the n = 1 orbit of hydrogen, 0.529 Å, calculated from r = 0.529 n²/Z Å; it sets the scale for atomic size.

Heisenberg uncertainty principle

Position and momentum of a microscopic particle cannot both be known exactly; Δx·Δp ≥ h/4π. It rules out fixed Bohr orbits and explains why electrons are described by orbitals.

Bond order from MO theory

Half the difference between bonding and antibonding electrons. O₂ has bond order 2 and two unpaired π* electrons, which correctly explains its paramagnetism.

van der Waals constants a and b

In (P + an²/V²)(V − nb) = nRT, a measures intermolecular attraction and b the excluded volume of the molecules; larger a means easier liquefaction.

Gibbs energy criterion

ΔG = ΔH − TΔS decides spontaneity: negative for spontaneous, zero at equilibrium, positive for non-spontaneous. It links enthalpy, entropy and temperature in one equation.

Relationship of Kp and Kc

K_p = K_c(RT)^Δn, where Δn is gaseous product moles minus reactant moles. For PCl₅ dissociation Δn = +1, so K_p is larger than K_c.

Le Chatelier pressure rule

Increasing pressure shifts an equilibrium toward the side with fewer gaseous molecules. It has no effect when gaseous moles are equal on both sides, as in H₂ + I₂ ⇌ 2HI.

Henderson-Hasselbalch equation

pH = pK_a + log([salt]/[acid]) for an acidic buffer. A buffer works best when the ratio is near 1, that is when pH is close to pK_a.

Solubility product expression

For AₓB_y, K_sp = xˣyʸs^(x+y), where s is molar solubility. AgCl gives s², while Ag₂CrO₄ gives 4s³, so the two cannot be compared by s alone.

First-order half-life

For a first-order reaction t₁/₂ = 0.693/k, independent of starting concentration. After n half-lives the fraction remaining is (½)ⁿ.

Arrhenius equation

k = A e^(−Ea/RT); plotting log k against 1/T gives a straight line of slope −Ea/2.303R. A catalyst raises k by lowering Ea, not by raising A.

Hardy-Schulze rule

Coagulation of a lyophobic sol is caused by ions of opposite charge; the higher the ion charge, the smaller the amount needed. Al³⁺ coagulates negative sols far better than Na⁺.

van't Hoff factor

i = observed colligative effect divided by the calculated value for the same molality. NaCl gives i near 2 through dissociation, while benzoic acid in benzene gives i below 1 through dimerisation.

Raoult's law

The partial vapour pressure of a solvent equals its mole fraction times the vapour pressure of the pure solvent. Positive deviations occur when solute-solvent attractions are weaker than solvent-solvent attractions.

Nernst equation

E = E° − (0.0591/n) log Q at 298 K. It shows how cell potential changes with concentration and gives E°_cell = (0.0591/n) log K at equilibrium.

Kohlrausch's law

The limiting molar conductivity of an electrolyte is the sum of the limiting ionic conductivities of its ions. It allows Λ°_m of weak electrolytes such as acetic acid to be found indirectly.

Faraday's laws of electrolysis

Mass deposited is proportional to charge passed and to the equivalent mass: m = (It/F)(M/z). One faraday, 96500 C, deposits one mole of silver but only half a mole of copper.

Inert-pair effect

Heavier p-block elements show an oxidation state two less than the group value more readily, because the ns² pair becomes reluctant to participate. Pb(II) and Bi(III) are more stable than Pb(IV) and Bi(V).

Anomalous behaviour of fluorine

F₂ has a low bond dissociation energy and fluorine forms only one bond, with no d-orbital participation. As a result HF is a weak acid and F₂ is the strongest halogen oxidiser.

Lanthanoid contraction

The steady decrease in ionic radius from La to Lu caused by poor shielding by 4f electrons. It makes Zr and Hf nearly identical in size and chemistry.

Magnetic moment formula

μ = √(n(n+2)) Bohr magnetons, where n is the number of unpaired electrons. Fe³⁺ with d⁵ high spin has five unpaired electrons and μ near 5.92 BM.

Crystal field splitting

Δ_o is the energy gap between t₂g and e_g orbitals in an octahedral complex. Strong-field ligands such as CN⁻ give a large gap and low-spin configurations.

Chelate effect

Polydentate ligands such as EDTA form more stable complexes than the same number of monodentate ligands, because ring formation gives a favourable entropy change on binding.

SN1 versus SN2 mechanism

SN2 is a single bimolecular step with inversion and favours primary halides; SN1 proceeds through a carbocation, is first order, and favours tertiary halides with racemisation.

Lucas test

Anhydrous ZnCl₂ in concentrated HCl gives immediate turbidity with tertiary alcohols, about five minutes with secondary, and requires heating for primary alcohols.

Cannizzaro reaction

An aldehyde with no α-hydrogen, such as benzaldehyde, disproportionates in concentrated alkali to give the corresponding alcohol and carboxylate salt. It is not an aldol reaction.

Tollens' test

Ammoniacal silver nitrate is reduced by aldehydes to a silver mirror on the test tube. Ketones give no reaction, so the test distinguishes the two carbonyl classes.

Hinsberg test

Benzenesulphonyl chloride in alkali gives a soluble product with primary amines, an insoluble sulphonamide with secondary amines, and no reaction with tertiary amines.

Condensation polymerisation

Monomers join with loss of a small molecule such as water or HCl. Nylon-6,6 from adipic acid and hexamethylenediamine and PET from a diol and diacid are examples.

Vulcanisation

Heating natural rubber with sulphur forms cross-links between cis-polyisoprene chains. The network makes rubber harder, stronger and less sensitive to temperature.

Brown ring test

Adding iron(II) sulphate to a nitrate solution and then concentrated sulphuric acid slowly down the side gives a brown ring of a nitrosyl iron complex at the junction.

Group separation in salt analysis

Cations are separated by precipitation with dilute HCl, H₂S in acid, NH₄OH with NH₄Cl, H₂S in base, and ammonium carbonate. Each group reagent leaves the remaining cations in solution.