MATSEC IM Chemistry cheat sheet

Chemistry (IM syllabus content)

• Mole concept: n = m/M; molar volume of an ideal gas at STP (0 °C, 1 atm) is 22.4 dm³ mol⁻¹; at RTP (25 °C, 1 atm) it is 24.0 dm³ mol⁻¹.

• Empirical formula from percentage composition: divide each % by atomic mass, then divide by the smallest ratio; multiply to obtain whole numbers.

• Limiting reagent: calculate moles of each reactant, divide by the stoichiometric coefficient; the smallest quotient identifies the limiting reagent.

• Acid–base titration: at the equivalence point, moles of H⁺ = moles of OH⁻; for a diprotic acid, n(acid) = n(base)/2.

• Oxidation numbers: sum of oxidation numbers in a neutral compound is zero; in a polyatomic ion, it equals the ion charge; F is always −1, O is usually −2, H is +1 with nonmetals.

• Redox: oxidation is loss of electrons (increase in oxidation number); reduction is gain of electrons (decrease in oxidation number).

• Bonding: ionic compounds have high melting points and conduct when molten or aqueous; covalent molecular substances have low melting points and usually do not conduct.

• Periodic trends: atomic radius decreases across a period and increases down a group; first ionisation energy generally increases across a period and decreases down a group.

• Energetics: ΔH = Σ bond energies broken − Σ bond energies formed; a negative ΔH indicates an exothermic reaction.