HKDSE Chemistry cheat sheet

Mole relations: n = m/M = N/L (L = 6.022×10^23 mol^-1); at room conditions molar volume ≈ 24.0 dm^3 mol^-1 (molar volume of gas at s.t.p. ≈ 22.4 dm^3 mol^-1).

Concentration: c = n/V (mol dm^-3); dilution gives c1V1 = c2V2; percentage by mass = (mass solute / mass solution) × 100%.

Avogadro's law: equal volumes of gases at the same T and P contain equal numbers of moles; volume ratio = mole ratio in gaseous reactions.

Ideal gas equation PV = nRT with R = 8.314 J K^-1 mol^-1; convert T to kelvin and P to Pa (1 atm = 1.013×10^5 Pa).

Empirical formula from combustion/mass data: divide each element's mass by its relative atomic mass, then divide by the smallest ratio; molecular formula = empirical formula × whole-number factor from M.

Percentage yield = (actual moles of product / theoretical moles) × 100%; limiting reactant is the reagent giving the smaller calculated amount of product.

Bonding decision rule: metal + non-metal → ionic lattice; non-metal + non-metal → covalent molecular or giant covalent; metal + metal → metallic.

Oxidation numbers: sum equals overall charge; O is usually −2, H usually +1, group 1 +1, group 2 +2; element in free state = 0.

Oxidation = increase in oxidation number (loss of electrons); reduction = decrease (gain of electrons); balance redox half-equations by atoms then charge, then equalise electrons.

Periodic trends across a period: atomic radius decreases, first ionisation energy increases, electronegativity increases; down a group these reverse.

First ionisation energy: X(g) → X^+(g) + e^-; successive ionisation energies jump sharply when a new shell is broken into.

Rate of reaction depends on concentration, surface area, temperature and catalyst; rate = change in amount of reactant/product per unit time.

Collision theory: reaction requires collisions with energy ≥ activation energy and correct orientation; catalyst lowers activation energy without being consumed.

Le Chatelier's principle: a system at equilibrium shifts to oppose a change in concentration, pressure or temperature; a catalyst changes rate but not equilibrium position.

Equilibrium constant Kc: for aA + bB ⇌ cC + dD, Kc = [C]^c[D]^d / ([A]^a[B]^b); only temperature changes Kc.

Acids and bases: strong acid ionises completely; pH = −log[H^+]; Kw = [H^+][OH^-] = 1.0×10^-14 at 25 °C; pH + pOH = 14.

Titration calculation: moles acid/base from c × V, then apply the stoichiometric ratio; equivalence point is not always pH 7 (weak acid–strong base > 7).

Tests: CO2 turns limewater milky; O2 relights a glowing splint; H2 gives a 'pop' with a burning splint; NH3 turns damp red litmus blue; Cl2 bleaches damp litmus.

Organic families: alkanes (single bonds, substitution), alkenes (C=C, addition, decolourise Br2 in CCl4), alcohols (oxidation to aldehyde then acid), carboxylic acids (esterification).