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- half life.
- The half life of a reaction is the time required for the amount of reactant to drop to one half its initial value.
- halide. halide ion.
- A compound or ion containing fluorine, chlorine, bromine, iodine, or astatine.
- halogen. group VIIA; group 18.
- An element of group VIIA (a. k. a. Group 18). The name means "salt former"; halogens react with metals to form binary ionic compounds. Fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At) are known at this time.
- hartree. (Eh) Compare with atomic unit.
- The atomic unit of energy, equal to 4.359 743 81 × 10-18 J ± 0.000 000 34 × 10-18 J [1998 CODATA values].
- heat capacity. Compare with molar heat capacity and specific heat.
- The heat required to raise the temperature of an object by 1°C is called the heat capacity of the object. Heat capacity is an extensive property with units of J K-1.
- heat. Compare with work, energy, enthalpy, and temperature.
- Heat is a transfer of energy that occurs when objects with different temperatures are placed into contact. Heat is a process, not a property of a material.
- heavy water. (D2O)
- Water that contains 2H, rather than 1H. Heavy water is about 11% denser than ordinary water.
- helium. He.
- Element 2, atomic weight 4.0026. A colorless, odorless, inert gas, first discovered in the emission spectrum of the sun in 1868.
- Helmholtz free energy. (A) Arbeitfunktion.
- A thermodynamic property that can be used to predict whether a process will occur spontaneously at constant volume and temperature. Helmholtz free energy A is defined as A = U - TS where U, T and S are the internal energy, temperature, and entropy. Changes in A correspond to changes in free energy for processes occuring at constant temperature and volume. The sign of DeltaA is negative for spontaneous processes and zero for processes at equilibrium.
- Henry's Law. Henry's law constant.
- Henry's law predicts that the solubility (C) of a gas or volatile substance in a liquid is proportional to the partial pressure (P) of the substance over the liquid:
P = k Cwhere k is called the Henry's law constant and is characteristic of the solvent and the solute.
- Hertz. (Hz, s-1) frequency.
- The SI unit of frequency, equal to one cycle of the wave per second (s-1).
- Hess's law. law of constant heat summation; Hess's law of heat summation.
- The heat released or absorbed by a process is the same no matter how many steps the process takes. For example, given a reaction A B, Hess's law says that H for the reaction is the same whether the reaction is written as A C B or as A B. This is the same as writing that H(A B) = H(A C) + H(C B).
- heterocyclic. heterocycle; heterocyclic ring.
- An organic group or molecule containing rings with at least one noncarbon atom on the ring.
- heterogenous mixture. heterogenous.
- A common misspelling of heterogeneous.
- heterogeneous mixture. heterogeneous. Compare with homogeneous mixture, solution, element, and compound.
- A sample of matter consisting of more than one pure substance and more than one phase. Blood, protoplasm, milk, chocolate, smoke, and chicken soup are examples of heterogeneous mixtures.
- high performance liquid chromatography. HPLC.
- An efficient form of column chromatography that pumps a liquid solution of the sample at very high pressure through a column packed with a stationary phase made of very tiny particles. The high pressure pumps required make HPLC an expensive technique.
- high spin complex. high-spin complex.
- A metal-ligand complex with the same number of unpaired electrons as the uncomplexed metal ion. When a weak ligand complexes the metal ion, the crystal field splitting is small and the electrons can still occupy all of the d orbitals without pairing.
- homogeneous. Compare with heterogeneous.
- Having uniform properties or composition.
- A common misspelling of homogeneous.
- homogeneous mixture. solution. Compare with heterogeneous mixture, element and compound.
- A sample of matter consisting of more than one pure substance with properties that do not vary within the sample.
- homolog. homologue; homologous; homologous series.
- A compound belonging to a series of compounds that differ by a repeating group. For example, propanol (CH3CH2CH2OH), n-butanol (CH3CH2CH2CH2OH), and n-pentanol (CH3CH2CH2CH2CH2OH) are homologs; they belong to a homologous series CH3(CH2)nOH.
- A molecule produced by endocrine glands that controls specific biological processes like growth and metabolism.
- A substance that absorbs or retains moisture, added to a product to keep it from drying out.
- Hund's rule. rule of maximum multiplicity.
- A rule of thumb stating that subshells fill so that the number of unpaired spins is maximized, or "spread them out and line them up."
- hybridization. hybrid orbitals; hybridized orbitals.
- The combination of a set of atomic orbitals to produce a new set of "hybrid" orbitals. Hybridized orbitals are theoretical constructions that make molecular structures easier to explain. For example, combining the valence s and p orbitals of carbon produces a set of four "sp3" hybrid orbitals that can be used to explain the tetrahedral bonding in CH4.
- hydrate. Compare with addition compound.
- A hydrate is an addition compound that contains water in weak chemical combination with another compound. For example, crystals of CuSO4·5 H2O (copper sulfate pentahydrate) are made of regularly repeating units, each containing 5 molecules of water weakly bound to a copper(II) ion and a sulfate ion.
- Combination with water.
- hydrazine. (NH2NH2)
- A colorless, fuming, corrosive liquid that is a powerful reducing agent. NH2NH2 is used in jet and rocket fuels, and as an intermediate in the manufacture of agricultural, textile, photographic, and industrial chemicals.
- hydride ion. (H-) hydride.
- A -1 ion formed from hydrogen. Hydride ions and hydride ionic compounds react instantly and sometimes violently with water.
- hydrocarbon. Compare with alkane, alkene, alkyne, and organic.
- Hydrocarbons are organic compounds that contain only hydrogen and carbon. The simplest hydrocarbons are the alkanes.
- hydrogen. (H) H; protium.
- Element 1, atomic weight 1.00797. The most abundant element in the universe. Hydrogen occurs as H2 at ambient temperature and pressure, a colorless, odorless, and extremely flammable gas. Discovered in 1766 by Cavendish.
- hydrogen bond. hydrogen bonding.
- An especially strong dipole-dipole force between molecules X-H...Y, where X and Y are small electronegative atoms (usually F, N, or O) and ... denotes the hydrogen bond. Hydrogen bonds are responsible for the unique properties of water and they loosely pin biological polymers like proteins and DNA into their characteristic shapes.
- A catch-all term for any reaction in which the water molecule is split.
- An instrument for measuring the specific gravity of liquids. A hydrometer is a weight with a vertical scale attached. When placed into a liquid, the hydrometer bobs upright, and sinks to a certain level. The specific gravity or solution composition can be read from the liquid level on the vertical scale. Hydrometers are often calibrated in degrees Baumé.
- hydronium ion. (H3O+) hydronium.
- The H3O+ ion, formed by capture of a hydrogen ion by a water molecule. A strong covalent bond is formed between the hydrogen ion and water oxygen; all hydrogen ions in aqueous solution are bound inside hydronium ions.
- hydrophilic. hydrophilicity; hydrophilic group.
- A polar molecule or group that can form strong hydrogen bonds with water.
- hydrophobic. hydrophobicity; hydrophobic group.
- A nonpolar molecule or group that has little affinity for water. Hydrophobic groups on molecules in solution tend to turn in on themselves or clump together with other hydrophobic groups because they are unable to disrupt the network of strong hydrogen bonds in the water around them.
- hydroxyl. Compare with hydroxide.
- 1. An -OH group within a molecule. 2. A free radical formed by abstraction of a hydrogen atom from water.
- hydroxide. (OH-) hydroxide ion. Compare with hydroxyl.
- 1. The OH- ion. 2. Compounds containing the OH- ion. See also: hydroxide compounds.
- Able to absorb moisture from air. For example, sodium hydroxide pellets are so hygroscopic that they dissolve in the water they absorb from the air.
- By absorbing moisture from air.
- The ability of a substance to absorb moisture from air. For example, sodium hydroxide pellets are so hygroscopic that they dissolve in the water they absorb from the air.
- hypergolic mixture. hypergolic fuel; hypergolic.
- An oxidizing agent and a fuel that react or ignite instantly and spontaneously on contact. Methylhydrazine and dinitrogen tetroxide is a hypergolic mixture used as a propellant in the space shuttle's orbital maneuvering engines.
- hypertonic. Compare with osmotic pressure.
- Describes a solution which has higher osmotic pressure than some other solution (usually, higher osmotic pressure than cell or body fluids). Freshwater fish die if placed in seawater because the seawater is hypertonic, and causes water to leave the cells in fish's body.
- hypotonic. Compare with osmotic pressure.
- Describes a solution which has lower osmotic pressure than some other solution (usually, lower osmotic pressure than cell or body fluids). Washing your contact lenses with distilled water rather than saline is painful because distilled water is hypotonic; it causes water to move into cells, and they swell and burst.
- hypothesis. hypotheses. Compare with theory.
- A hypothesis is a conjecture designed to guide experimentation. Hypotheses are extremely useful in problem solving, and are essential in developing new theories.