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High-Performance Liquid Chromatography Analytical Chemistry & Spectroscopy min A technique in which a liquid mobile phase is pumped at high pressure through a column of very fine stationary phase. High-Resolution Mass Spectrometry Analytical Chemistry & Spectroscopy Da Mass spectrometry capable of measuring mass to four or more decimal places, allowing molecular formulae to be determined. HOMO and LUMO Chemical Bonding & Molecular Structure eV The highest occupied and lowest unoccupied molecular orbitals, which together govern a molecule's reactivity and spectroscopy. ΔE = E_LUMO − E_HOMO Homogeneous Catalysis Chemical Kinetics & Catalysis Dimensionless Catalysis in which the catalyst is in the same phase as the reactants. Homogeneous Equilibrium Chemical Equilibrium Dimensionless An equilibrium in which all the reacting species are present in the same phase. Homogeneous Mixture Foundations of Chemistry Dimensionless A mixture whose composition and properties are uniform throughout, with no visible boundary between the components. Homologous Series Organic Chemistry Dimensionless A family of compounds sharing the same functional group and general formula, with successive members differing by CH₂. differ by −CH₂− Hormone Biochemistry & Polymer Chemistry mol/L A chemical messenger released by one tissue that regulates the activity of cells elsewhere in the body. Hund's Rule of Maximum Multiplicity Atomic Structure & Periodicity Dimensionless The rule that electrons occupy degenerate orbitals singly with parallel spins before any orbital is doubly occupied. maximise total spin S Hybridisation Chemical Bonding & Molecular Structure Dimensionless The mixing of atomic orbitals on the same atom to form an equal number of new, equivalent orbitals suited to bonding. Hydrate Foundations of Chemistry Dimensionless A crystalline compound in which a definite number of water molecules is incorporated into the lattice. MX·nH₂O Hydrate Isomerism Inorganic & Coordination Chemistry Dimensionless Isomerism arising from water molecules occupying either the coordination sphere or lattice positions outside it. Hydration Energy Solutions & Colligative Properties kJ/mol The energy released when one mole of gaseous ions is surrounded by water molecules to form a dilute aqueous solution. ΔH_hyd ∝ z / r Hydration Reaction Stoichiometry & Chemical Reactions Dimensionless The addition of water across a multiple bond or the incorporation of water into a crystal lattice. C=C + H₂O → C(OH)–CH Hydrocarbon Organic Chemistry Dimensionless A compound composed exclusively of carbon and hydrogen. CₓH_y Hydrogen Bond Chemical Bonding & Molecular Structure kJ/mol A strong dipole interaction between a hydrogen atom bonded to N, O or F and a lone pair on another such atom. X–H···Y (X, Y = N, O, F) Hydrogen–Oxygen Fuel Cell Electrochemistry & Redox V A fuel cell in which hydrogen is oxidised at the anode and oxygen reduced at the cathode, producing only water. 2H₂ + O₂ → 2H₂O, E° = 1.23 V Hydrolysis Stoichiometry & Chemical Reactions Dimensionless A reaction in which a bond is cleaved by water, with the H and OH fragments adding to the two products. AB + H₂O → AOH + BH Hydrolysis Constant Acids, Bases & Salts mol/L The equilibrium constant for the hydrolysis of a salt ion in water. Kh = Kw / Ka (anion) Hyperconjugation Organic Chemistry kJ/mol The stabilising delocalisation of electrons from a sigma C–H bond into an adjacent empty or partly filled p orbital. Hypertonic Solution Solutions & Colligative Properties atm A solution with a higher osmotic pressure than the one it is compared with, drawing water out of the more dilute side. π_solution > π_cell Hypotonic Solution Solutions & Colligative Properties atm A solution with a lower osmotic pressure than the one it is compared with, so water flows into the more concentrated side. π_solution < π_cell Hückel's Rule Organic Chemistry Dimensionless The rule that a planar cyclic conjugated system is aromatic if it contains 4n + 2 pi electrons. 4n + 2 π electrons ICE Table Chemical Equilibrium mol/L A systematic tabulation of Initial, Change and Equilibrium amounts used to solve equilibrium problems.