Chemistry terms starting with H

Half-Life of a Reaction Chemical Kinetics & Catalysis s The time required for the concentration of a reactant to fall to half its initial value. t½ = 0.693 / k (first order)

Half-Reaction Electrochemistry & Redox Dimensionless An equation showing either the oxidation or the reduction part of a redox process, including the electrons transferred. Ox + n e⁻ ⇌ Red

Half-Reaction Method Stoichiometry & Chemical Reactions Dimensionless A technique for balancing redox equations by splitting them into separate oxidation and reduction half-equations. balance atoms, then O with H₂O, H with H⁺, charge with e⁻

Haloalkane Organic Chemistry Dimensionless An alkane in which one or more hydrogen atoms have been replaced by halogen atoms. R–X (X = F, Cl, Br, I)

Halogens Atomic Structure & Periodicity Dimensionless The group 17 elements — fluorine through astatine — which are one electron short of a full outer shell. ns² np⁵

Heat Thermochemistry & Thermodynamics J Energy transferred between a system and its surroundings solely because of a temperature difference. q = mcΔT

Heat Capacity at Constant Pressure Thermochemistry & Thermodynamics J/(mol·K) The heat needed to raise a system's temperature by one kelvin while pressure is held constant. C_p = C_V + R (ideal gas)

Heat Capacity at Constant Volume Thermochemistry & Thermodynamics J/(mol·K) The heat needed to raise a system's temperature by one kelvin while its volume is held fixed. C_V = (∂U/∂T)_V

Heisenberg Uncertainty Principle Atomic Structure & Periodicity J·s The principle that the position and momentum of a particle cannot both be known to arbitrary precision at the same time. Δx · Δp ≥ h / 4π

Helmholtz Free Energy Thermochemistry & Thermodynamics kJ/mol The thermodynamic potential that determines spontaneity at constant temperature and volume. A = U − TS

Henderson–Hasselbalch Equation Acids, Bases & Salts Dimensionless The equation giving the pH of a buffer from the pKa of its acid and the ratio of conjugate base to acid. pH = pKa + log([A⁻]/[HA])

Henry's Law Solutions & Colligative Properties Pa The solubility of a gas in a liquid is directly proportional to the partial pressure of that gas above the liquid. p = K_H × x

Hess's Law Thermochemistry & Thermodynamics kJ/mol The total enthalpy change of a reaction is the same regardless of the route taken, provided the initial and final states are identical. ΔH_total = ΣΔH_steps

Heterogeneous Catalysis Chemical Kinetics & Catalysis Dimensionless Catalysis in which the catalyst is in a different phase from the reactants, usually a solid with gaseous or liquid reagents.

Heterogeneous Equilibrium Chemical Equilibrium Dimensionless An equilibrium involving species present in more than one phase.

Heterogeneous Mixture Foundations of Chemistry Dimensionless A mixture in which the components remain physically distinct, so composition varies from one region of the sample to another.

Hexagonal Close Packing States of Matter & Gas Laws Dimensionless A close-packed arrangement in which layers of spheres repeat in an ABAB sequence. 6 atoms per cell, 74% packing

High Spin Complex Inorganic & Coordination Chemistry BM A complex in which electrons occupy the higher d orbitals singly before pairing, maximising unpaired spins. Δ < P

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.

Terms starting with H