Haber Process
The industrial synthesis of ammonia from nitrogen and hydrogen gases: N₂ + 3H₂ ⇌ 2NH₃. Conditions: iron catalyst, 400-500°C, 150-300 atm pressure. The process is a compromise betwe…
Chemical Glossary — Letter H
87 chemicals found starting with "H"
Hafnium
Hafnium is a lustrous, silvery transition metal, atomic number 72. Always found with zirconium (very similar chemistry). High neutron absorption cross-section makes it ideal for nu…
Half Cell
A half-cell is one of the two components of an electrochemical cell, consisting of an electrode in contact with an electrolyte solution, in which a half-reaction of either oxidatio…
Half Life
Half-life (t₁/₂) is the time required for half of a given quantity of a substance to undergo a specific process, most commonly radioactive decay or a first-order chemical reaction.…
Half-Cell
One half of an electrochemical cell, consisting of an electrode in contact with an electrolyte solution. The half-cell reaction involves either oxidation (at the anode) or reductio…
Half-Life
The time required for half of the atoms in a radioactive sample to decay. Different radioactive isotopes have characteristic half-lives ranging from fractions of a second to billio…
Half-Reaction
A half-reaction is one of the two components of an oxidation-reduction (redox) reaction, representing either the oxidation half (loss of electrons) or the reduction half (gain of e…
Halide
A binary compound formed between a halogen (F, Cl, Br, I, or At) and another element. Metal halides are typically ionic (e.g., NaCl, CaBr₂). Hydrogen halides (HF, HCl, HBr, HI) dis…
Halogen
Any of the Group 17 elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). Halogens are highly reactive nonmetals that form -1 ions. They exist as diat…
Halogenation
A reaction in which halogen atoms are introduced into a molecule. Halogenation of alkanes (radical mechanism) requires UV light or heat. Halogenation of alkenes (electrophilic addi…
Halogens
Halogens are the elements of Group 17 (VIIA) of the periodic table: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). They are highly reactive nonmetals cha…
Hard Acid
In HSAB (Hard Soft Acid Base) theory, a Lewis acid that is small, has high positive charge, and is not easily polarized. Hard acids form stronger bonds with hard bases. Examples in…
Hard Base
In HSAB theory, a Lewis base that is small, has high electronegativity, is difficult to polarize, and is not easily oxidized. Hard bases form stronger bonds with hard acids. Exampl…
Hard Water
Water containing dissolved calcium and magnesium ions, which prevent soap from lathering and cause scale deposits in pipes and kettles. Temporary hardness (caused by Ca(HCO₃)₂) can…
Hassium
Hassium is a synthetic radioactive transactinide element named after Hesse, Germany. Expected to behave similarly to osmium. First synthesised in 1984 at the GSI Helmholtz Centre.
Heat
Heat (Q) is a form of energy transferred between objects due to a temperature difference. Heat always flows spontaneously from a region of higher temperature to a region of lower t…
Heat
Energy transferred between objects due to a temperature difference. Heat flows from higher to lower temperature regions. It is measured in joules (J) or calories (cal). Heat is a f…
Heat Capacity
Heat capacity is the amount of heat energy required to raise the temperature of a substance by one degree (Celsius or Kelvin). Specific heat capacity is the heat capacity per unit …
Heat Capacity
The amount of heat energy required to raise the temperature of a substance by 1°C (or 1 K). Specific heat capacity is per unit mass (J/g·°C). Molar heat capacity is per mole. Water…
Heat of Condensation
The heat of condensation (enthalpy of condensation) is the amount of heat released when a substance changes from gas to liquid at constant temperature and pressure. It is equal in …
Heat of Crystallization
The heat of crystallization (enthalpy of crystallization) is the heat released when a substance transitions from a liquid (or solution) to a crystalline solid. It is equal in magni…
Heat of Formation
The enthalpy change when one mole of a compound is formed from its constituent elements in their standard states at 298 K and 1 atm. Standard heats of formation for elements in the…
Heat Of Fusion
The heat of fusion (enthalpy of fusion, ΔH_fus) is the amount of heat energy required to convert one mole of a solid substance into a liquid at constant pressure and at its melting…
Heat Of Solution
The heat of solution (enthalpy of solution, ΔH_soln) is the total heat absorbed or released when a solute dissolves completely in a solvent to form a solution at constant pressure.…
Heat of Vaporization
The heat of vaporization (enthalpy of vaporization, latent heat of vaporization) is the amount of energy required to convert a unit mass of a liquid into vapor at constant temperat…
Heavy Water
Heavy water is water in which both hydrogen atoms are replaced by deuterium (²H or D), the stable isotope of hydrogen with one proton and one neutron. Its chemical formula is D₂O. …
Heisenberg Uncertainty Principle
The Heisenberg uncertainty principle states that it is fundamentally impossible to simultaneously determine both the exact position and exact momentum of a subatomic particle with …
Heisenberg's Uncertainty Principle
The fundamental quantum mechanical principle stating that it is impossible to simultaneously determine both the exact position and exact momentum of a particle with unlimited preci…
Helium
Helium is the second lightest and second most abundant element in the universe. It is a colourless, odourless, non-toxic, inert noble gas with the lowest boiling point of any eleme…
Henderson-Hasselbalch Equation
An equation relating the pH of a buffer solution to the pKa of the weak acid and the ratio of concentrations of the conjugate base to the weak acid: pH = pKa + log([A⁻]/[HA]). Used…
Henry's Law
Henry's law states that at constant temperature, the amount of a gas dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid. It is expr…
Henry's Law
A gas law stating that at constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas above the liquid: C = kP. Used to e…
Hess's Law
The law stating that the total enthalpy change for a reaction is the same regardless of the pathway taken, as long as initial and final states are the same. This allows calculation…
Hess's Law Of Heat Summation
Hess's law states that the total enthalpy change for a chemical reaction is independent of the pathway taken and equals the sum of the enthalpy changes for any series of steps that…
Heterocyclic Amine
A heterocyclic amine is a cyclic organic compound in which the ring contains at least one nitrogen atom (and possibly other heteroatoms like oxygen or sulfur) as part of the ring s…
Heterogeneous Catalyst
A heterogeneous catalyst is a catalyst that exists in a different phase from the reactants, most commonly a solid catalyst acting on liquid or gas-phase reactants. The reaction occ…
Heterogeneous Catalyst
A catalyst that exists in a different phase from the reactants. Typically a solid catalyst with gaseous or liquid reactants. The reaction occurs on the catalyst surface. Examples i…
Heterogeneous Equilibria
Heterogeneous equilibria are chemical equilibria in which the reactants and products are present in more than one phase (e.g., solid, liquid, and gas). The equilibrium expression f…
Heterogeneous Mixture
A heterogeneous mixture is a mixture in which the composition is not uniform throughout, and the individual components can be physically distinguished. Examples include sand and wa…
Heterogeneous Mixture
A mixture in which the composition is not uniform throughout and different components are visually distinguishable. Examples include sand and water, salad dressing, and granite. Th…
Heterolytic Cleavage
The breaking of a covalent bond in which both electrons go to one atom, producing a cation and an anion (or a carbocation and a carbanion in organic chemistry). This type of bond b…
Heteronuclear
Heteronuclear refers to molecules or species composed of two or more different types of atoms. The term is used in contrast to homonuclear (same atoms throughout). Heteronuclear di…
High Spin Complex
A high-spin complex is a coordination compound in which the ligand field splitting energy (Δ) is smaller than the electron pairing energy, causing d-electrons to occupy higher-ener…
High-Spin Complex
A coordination complex in which the crystal field splitting energy is smaller than the pairing energy, causing electrons to occupy all d-orbitals singly before pairing occurs. High…
Holmium
Holmium is a soft, silvery rare earth metal, atomic number 67. Has the highest magnetic moment of any naturally occurring element (10.6 Bohr magnetons). Used in strong magnets and …
Homogeneous Catalyst
A homogeneous catalyst is a catalyst that exists in the same phase as the reactants, most commonly in solution. Because catalyst and reactants are intimately mixed, homogeneous cat…
Homogeneous Catalyst
A catalyst that is in the same phase as the reactants, typically in solution. Homogeneous catalysts offer better contact with reactants but are harder to separate. Examples include…
Homogeneous Equilibria
Homogeneous equilibria are chemical equilibria in which all reactants and products are present in the same phase (e.g., all gases or all in solution). The equilibrium constant expr…
Homogeneous Mixture
A homogeneous mixture (solution) is a mixture with a uniform composition throughout; at the molecular level, the components are evenly distributed. Common examples include salt wat…
Homogeneous Mixture
A mixture with a uniform composition throughout, where the components are indistinguishable at the molecular level. Solutions are homogeneous mixtures. Examples include seawater, a…
Homologous Series
A homologous series is a family of organic compounds that share the same general molecular formula and functional group, with consecutive members differing by a –CH₂– unit. Members…
Homologous Series
A series of organic compounds with the same functional group and general formula, differing from each other by a -CH₂- unit. Each member has similar chemical properties but gradual…
Homolytic Cleavage
The breaking of a covalent bond in which each atom takes one of the shared electrons, producing two neutral radicals. This type of bond cleavage occurs in radical reactions, includ…
Homonuclear
Homonuclear refers to molecules or species composed of only one type of atom. Homonuclear diatomic molecules consist of two identical atoms bonded together (e.g., H₂, O₂, N₂, F₂, C…
Hund's Rule
Hund's rule states that when filling degenerate orbitals (orbitals of equal energy), electrons occupy each orbital singly with parallel spins before any pairing occurs. This arrang…
Hund's Rule
Hund's Rule (Hund's Rule of Maximum Multiplicity) states that when electrons occupy orbitals of equal energy (degenerate orbitals), each orbital is occupied by one electron before …
Hybridisation
Hybridisation (or hybridization) is the concept of mixing atomic orbitals to form new hybrid orbitals with different shapes and energies suitable for bonding. sp hybridisation prod…
Hybridisation
The concept in valence bond theory describing the mixing of atomic orbitals to form new hybrid orbitals of equivalent energy for bonding. Types include sp (linear), sp² (trigonal p…
Hybridization
The mathematical mixing of atomic orbital wave functions to produce new hybrid orbitals of equal energy and shape. sp, sp², sp³, sp³d, and sp³d² hybridizations correspond to linear…
Hydrate
A hydrate is a compound that contains water molecules either loosely bound within its crystal structure (crystalline hydrate or water of crystallization) or chemically bonded to th…
Hydrate Isomers
Hydrate isomers are a type of coordination isomerism in coordination compounds where the water molecules are distributed differently — either as ligands coordinated to the metal ce…
Hydration
Hydration is the process by which water molecules surround and interact with dissolved ions or polar molecules through electrostatic attraction and hydrogen bonding. The hydration …
Hydration
The process by which water molecules surround and interact with dissolved ions or molecules (solvation in water). The release of energy during hydration (hydration enthalpy) helps …
Hydration Energy
Hydration energy (enthalpy of hydration) is the energy released when gaseous ions are surrounded by water molecules to form aqueous ions, reflecting the strength of ion-dipole inte…
Hydration Enthalpy
The enthalpy change when one mole of gaseous ions are surrounded by water molecules to form aqueous ions. It is always negative (exothermic) because ion-dipole interactions form. S…
Hydride
A hydride is a binary compound of hydrogen with another element, typically a metal. Ionic hydrides (e.g., NaH, CaH₂) contain the hydride ion (H⁻) and are strong reducing agents and…
Hydride
A compound of hydrogen with another element. Metal hydrides (e.g., NaH, CaH₂) are ionic and contain H⁻ ions. Covalent hydrides (e.g., H₂O, NH₃, CH₄) are molecular compounds. Hydrog…
Hydride Ion
The hydrogen anion H⁻, consisting of a proton and two electrons. Hydride ions are powerful reducing agents and strong bases. Metal hydrides like NaH and LiAlH₄ are important reduci…
Hydrocarbon
An organic compound containing only carbon and hydrogen atoms. Hydrocarbons are classified as aliphatic (alkanes, alkenes, alkynes) or aromatic (benzene and derivatives). They are …
Hydrocarbons
Hydrocarbons are organic compounds composed exclusively of carbon and hydrogen atoms. They are classified into aliphatic hydrocarbons (alkanes, alkenes, and alkynes), alicyclic com…
Hydrogen
Hydrogen is the lightest and most abundant element in the universe, making up about 75% of all normal matter by mass. It is a colourless, odourless, highly flammable diatomic gas a…
Hydrogen Bond
A hydrogen bond is a weak electrostatic attraction between a hydrogen atom covalently bonded to a highly electronegative atom (N, O, or F) and a lone pair on another electronegativ…
Hydrogen Bond
A relatively strong intermolecular or intramolecular force between a hydrogen atom bonded to a highly electronegative atom (N, O, or F) and another electronegative atom. Hydrogen b…
Hydrogen Half-Cell
The standard hydrogen electrode (SHE), consisting of a platinum electrode in contact with 1 M H⁺ solution through which hydrogen gas at 1 atm is bubbled. It is assigned a standard …
Hydrogen-Oxygen Fuel Cell
A hydrogen-oxygen fuel cell is an electrochemical device that converts chemical energy from hydrogen and oxygen directly into electrical energy, with water as the only product. Unl…
Hydrogenation
Hydrogenation is a chemical reaction in which hydrogen gas (H₂) is added across an unsaturated bond (C=C, C≡C, C=O, C≡N) in the presence of a catalyst. It is used extensively to co…
Hydrolysis
Hydrolysis is a chemical reaction in which a molecule is cleaved by the addition of water. In the hydrolysis of esters, for example, water breaks the ester bond to give a carboxyli…
Hydrolysis
Hydrolysis is a chemical reaction in which a molecule is cleaved into two parts by the addition of a water molecule, with the hydrogen atom of water attaching to one fragment and t…
Hydrolysis
A chemical reaction in which a compound is broken down by reaction with water. In acid-base chemistry, hydrolysis of salts produces acidic or basic solutions. Hydrolysis of esters,…
Hydrolysis Constant
The equilibrium constant for the hydrolysis reaction of a salt in water. Related to Ka and Kb of the conjugate acid-base pair through Kw. Hydrolysis constants determine whether sal…
Hydrometer
A hydrometer is an instrument used to measure the specific gravity (relative density) of liquids compared to water. It consists of a sealed glass tube with a weighted bulb at the b…
Hydronium Ion
The H₃O⁺ ion formed when a proton (H⁺) is transferred to a water molecule. In aqueous solution, free protons do not exist; they are always associated with water as hydronium ions. …
Hydrophilic
Describes molecules or groups that are attracted to water and readily dissolve in or absorb water. Hydrophilic substances typically have polar bonds or charged groups. The term mea…
Hydrophilic Colloids
Colloidal dispersions in which the dispersed phase has a strong affinity for water. Hydrophilic colloids are stabilized by their interaction with water molecules. Examples include …
Hydrophobic
Describes molecules or groups that repel water and do not dissolve in water. Hydrophobic substances are typically nonpolar. The term means "water-fearing." Hydrophobic interactions…
Hydroxide
A compound containing the hydroxide ion (OH⁻) or the -OH group. Metal hydroxides are bases that dissociate to give OH⁻ ions in water. Sodium hydroxide (NaOH) and potassium hydroxid…
Hydroxyl Group
The functional group -OH consisting of an oxygen atom bonded to a hydrogen atom. The hydroxyl group makes compounds more water-soluble and more reactive. It is found in alcohols, p…
About Chemicals Starting With H
This page lists all chemicals in our database beginning with the letter H. Each entry provides the chemical formula, CAS registry number, physical and chemical properties, common uses, and safety information. Use the alphabetical navigation above to browse other letters, or use the search function to find a specific chemical quickly.
Our chemical glossary covers acids, bases, salts, organic compounds, inorganic compounds, solvents, and many more categories. Click on any chemical name to view its full detailed profile.