Chemistry terms: H

H

Haber processalso Haber–Bosch process, Haber-Bosch process

The industrial synthesis of ammonia, ${{\mathrm{N}{\vphantom{X}}_{\smash[t]{2}}\mskip2mu (\mathrm{g}) {}+{} 3\,\mathrm{H}{\vphantom{X}}_{\smash[t]{2}}\mskip2mu (\mathrm{g})\xrightleftharpoons{}2\,\mathrm{NH}{\vphantom{X}}_{\smash[t]{3}}\mskip2mu (\mathrm{g})}}$, using an iron catalyst at about 400–450 °C and about 200 atm, with unreacted gases recycled.

Introduced in R2.3 How far? The extent of chemical change

Half-cellalso half-cells, half cell

One half of a voltaic cell: an electrode in contact with an electrolyte, such as a metal dipping into a solution of its own ions, where either oxidation or reduction occurs.

Introduced in R3.2 Electron transfer reactions

Half-equationalso half-equations, half equation, ionic half-equation

An equation showing either the oxidation or the reduction part of a redox reaction, with the electrons lost or gained, for example ${{\mathrm{Cu}{\vphantom{X}}^{2+} {}+{} 2\,\mathrm{e}{\vphantom{X}}^{-}\xrightarrow{}\mathrm{Cu}}}$

Half-equivalence pointHL only · also half equivalence point, half-neutralization point, half-equivalence

The point in a titration at which half the volume needed for equivalence has been added. For a weak acid, $[{{\mathrm{HA}}}] = [{{\mathrm{A}{\vphantom{X}}^{-}}}]$ there, so $\text{pH} = \text{p}K_\text{a}$

Half-lifeHL only · also half-lives, half life

The time taken for the concentration of a reactant to fall to half of its value. It is constant for a first-order reactant.

Introduced in R2.2 How fast? The rate of chemical change

Halogenalso halogens, group 17 elements

An element of group 17 (fluorine, chlorine, bromine, iodine, astatine): a reactive non-metal with seven valence electrons that exists as diatomic molecules ${{\mathrm{X}{\vphantom{X}}_{\smash[t]{2}}}}$ and forms ${{\mathrm{X}{\vphantom{X}}^{-}}}$

Halogen carrierHL only · also halogen carriers

A Lewis acid catalyst such as ${{\mathrm{FeBr}{\vphantom{X}}_{\smash[t]{3}}}}$ or ${{\mathrm{AlCl}{\vphantom{X}}_{\smash[t]{3}}}}$

halogenationalso chlorination, bromination

A reaction that introduces one or more halogen atoms into a molecule, for example the chlorination of methane to chloromethane.

Introduced in R3.3 Electron sharing reactions

halogeno groupalso halogeno, halogen atom

The functional group –X, where X is F, Cl, Br or I, bonded to carbon; it defines the halogenoalkanes.

Introduced in S3.2 Functional groups: classification of organic compounds

halogenoalkanealso halogenoalkanes, haloalkane

A compound in which one or more H atoms of an alkane are replaced by halogen atoms, general formula ${{\mathrm{C}{\vphantom{X}}_{\smash[t]{n }}\mathrm{H}{\vphantom{X}}_{\smash[t]{2n +1}}\mathrm{X}}}$

heatalso heat energy, heat transferred

Energy transferred between two bodies because of a difference in temperature, from the hotter to the colder; symbol $Q$, measured in joules.

Introduced in R1.1 Measuring enthalpy changes

heating curvealso heating curves

A graph of temperature against time for a substance heated at a constant rate; the flat parts (plateaus) show changes of state, during which the temperature is constant.

Introduced in S1.1 Introduction to the particulate nature of matter

Heating to constant massalso constant mass

Repeated heating, cooling and weighing of a sample until two consecutive masses agree, showing that a change such as the loss of water is complete.

Introduced in S1.4 Counting particles by mass: the mole

Henderson–Hasselbalch equationHL only · also Henderson-Hasselbalch equation, buffer equation

The logarithmic form of the $K_\text{a}$ expression used for buffers: $\text{pH} = \text{p}K_\text{a} + \log_{10}([{{\mathrm{A}{\vphantom{X}}^{-}}}]/[{{\mathrm{HA}}}])$

Hess's lawalso Hess’s law, Hess law

The enthalpy change for a reaction is independent of the route taken between the initial and final states; a consequence of the conservation of energy.

Introduced in R1.2 Energy cycles in reactions

Heterogeneous catalystalso heterogeneous catalysts, heterogeneous catalysis

A catalyst in a different phase from the reactants, usually a solid on whose surface gases or dissolved substances react, such as iron in the Haber process.

Introduced in R2.2 How fast? The rate of chemical change

heterogeneous equilibriumalso heterogeneous equilibria

An equilibrium involving species in more than one phase, such as ${{\mathrm{CO}{\vphantom{X}}_{\smash[t]{2}}\mskip2mu (\mathrm{g})\xrightleftharpoons{}\mathrm{CO}{\vphantom{X}}_{\smash[t]{2}}\mskip2mu (\mathrm{aq})}}$

heterogeneous mixturealso heterogeneous mixtures, heterogeneous

A mixture whose composition is not uniform, with two or more regions (phases) of different composition, for example sand in water or oil and water.

Introduced in S1.1 Introduction to the particulate nature of matter

heterolytic fissionalso heterolysis, heterolytic bond fission

The breaking of a covalent bond in which both bonding electrons stay with one fragment, forming a cation and an anion, for example ${{\mathrm{HBr}\xrightarrow{}\mathrm{H}{\vphantom{X}}^{+} {}+{} \mathrm{Br}{\vphantom{X}}^{-}}}$

high-resistance voltmeterHL only · also high resistance voltmeter

A voltmeter that lets almost no current flow, so that no reaction occurs in the cell and the maximum potential difference between its electrodes is measured.

Introduced in R3.2 Electron transfer reactions

Hofmann voltameterHL only · also Hofmann apparatus, Hoffmann voltameter

An apparatus with two graduated tubes joined to a central reservoir, used to electrolyse water and collect and measure the hydrogen and oxygen formed.

Introduced in R3.2 Electron transfer reactions

Homogeneous catalystalso homogeneous catalysts, homogeneous catalysis

A catalyst in the same phase as the reactants, such as ${{\mathrm{H}{\vphantom{X}}^{+}\mskip2mu (\mathrm{aq})}}$ in esterification.

Introduced in R2.2 How fast? The rate of chemical change

homogeneous equilibriumalso homogeneous equilibria

An equilibrium in which all the reactants and products are in the same phase, for example all gases or all in one solution.

Introduced in R2.3 How far? The extent of chemical change

homogeneous mixturealso homogeneous mixtures, homogeneous

A mixture with a uniform composition and properties throughout (one phase), so that any sample has the same ratio of components, for example a salt solution, air or brass.

Introduced in S1.1 Introduction to the particulate nature of matter

homologous seriesalso homologue, homologues

A family of compounds with the same functional group and general formula, in which successive members differ by ${{\mathrm{CH}{\vphantom{X}}_{\smash[t]{2}}}}$; members have similar chemical properties and a gradual trend in physical properties.

Introduced in S3.2 Functional groups: classification of organic compounds

homolytic fissionalso homolysis, homolytic bond fission

The breaking of a covalent bond in which each atom keeps one electron of the shared pair, forming two radicals or atoms.

Introduced in R1.2 Energy cycles in reactions

Hund's rulealso Hund’s rule, Hunds rule

Electrons occupy the orbitals of a sublevel singly, with parallel spins, before any orbital gains a second electron.

Introduced in S1.3 Electron configurations

hybrid orbitalHL only · also hybrid orbitals

One of a set of equivalent orbitals formed by mixing s and p orbitals on an atom; each has one large lobe pointing in a definite direction.

Introduced in S2.2 The covalent model

hybridizationalso hybridisation, hybridized, hybridised

The mixing of atomic orbitals on one atom to form an equal number of new, equivalent hybrid orbitals used to form sigma bonds and hold lone pairs.

Introduced in S2 Models of bonding and structure

Hydrated ionalso hydrated ions

An ion surrounded by water molecules attracted to it: the $\delta-$ oxygen atoms face a cation and the $\delta+$ hydrogen atoms face an anion.

Introduced in S2.1 The ionic model

Hydrated saltalso hydrate, hydrates, hydrated salts

A salt whose crystals contain water of crystallization, such as blue copper(II) sulfate pentahydrate, ${{\mathrm{CuSO}{\vphantom{X}}_{\smash[t]{4}}\,{\cdot}\,5\,\mathrm{H}{\vphantom{X}}_{\smash[t]{2}}\mathrm{O}}}$

hydrationalso hydration of alkenes

The addition of water across a C=C double bond to form an alcohol, as in ${{\mathrm{CH}{\vphantom{X}}_{\smash[t]{2}}{=}\mathrm{CH}{\vphantom{X}}_{\smash[t]{2}} {}+{} \mathrm{H}{\vphantom{X}}_{\smash[t]{2}}\mathrm{O}\xrightarrow{}\mathrm{CH}{\vphantom{X}}_{\smash[t]{3}}\mathrm{CH}{\vphantom{X}}_{\smash[t]{2}}\mathrm{OH}}}$

Hydride ionalso hydride ions

The ion ${{\mathrm{H}{\vphantom{X}}^{-}}}$, a hydrogen atom with an extra electron. Supplied by reducing agents, it adds to the partially positive carbon of a carbonyl group, reducing it.

Introduced in S3.1 The periodic table: classification of elements

hydrocarbonalso hydrocarbons

A compound that contains carbon and hydrogen only, such as methane, ${{\mathrm{CH}{\vphantom{X}}_{\smash[t]{4}}}}$, or ethene, ${{\mathrm{C}{\vphantom{X}}_{\smash[t]{2}}\mathrm{H}{\vphantom{X}}_{\smash[t]{4}}}}$

hydrofluorocarbonalso HFC, HFCs, hydrofluorocarbons

A compound of carbon, hydrogen and fluorine, such as ${{\mathrm{CH}{\vphantom{X}}_{\smash[t]{2}}\mathrm{FCF}{\vphantom{X}}_{\smash[t]{3}}}}$

hydrogen bondalso hydrogen bonds, hydrogen bonding

An attraction between a hydrogen atom covalently bonded to N, O or F and a lone pair on an N, O or F atom of a neighbouring molecule; the strongest intermolecular force.

Introduced in S2.2 The covalent model

Hydrogenationalso hydrogenated, catalytic hydrogenation

The addition of hydrogen across a carbon–carbon double or triple bond, using a metal catalyst such as nickel; it is a reduction that lowers the degree of unsaturation.

Introduced in R3.2 Electron transfer reactions

hydrolysisalso hydrolyse, hydrolysed

A reaction in which a bond is broken by reaction with water (or hydroxide ions), as when a halogenoalkane is converted into an alcohol.

Introduced in R3.4 Electron-pair sharing reactions

hydroxyl groupalso hydroxyl, –OH group

The functional group –OH covalently bonded to a carbon atom that carries no C=O; it defines the alcohols. It is not the hydroxide ion.

Introduced in S3.2 Functional groups: classification of organic compounds

Hypothesisalso hypotheses

A provisional, testable explanation of an observation or relationship, from which a specific prediction about the results can be made.