H
half-life
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In chemical kinetics, the time it takes for one half of the limiting reactant to be consumed. In nuclear chemistry, the time for half of a sample to undergo radioactive decay.
half-reaction
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A portion of an oxidation-reduction reaction involving only the oxidation or only the reduction portion; useful in balancing oxidation-reduction reactions and assigning electrode potentials.
halogen
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One of the elements in the same column of the periodic table as fluorine.
hard water
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Water containing high concentrations of cations having charge greater than +1; hardness can be removed by ion exchange.
hcp
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Abbreviation for hexagonal closest packed; one of two schemes for closest packing of spheres; described as "aba" to indicate that the atoms of the third layer lie directly above the atoms of the first layer.
heat
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Energy transferred as a result of a temperature difference; a form of energy stored in the movement of atomic-sized particles.
heat capacity
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The quantity of heat needed to raise the temperature of a sample of matter by one unit.
heat of formation
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The heat absorbed during the formation of one mole of a compound from its component elements in their most stable forms.
heat of fusion
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The heat absorbed as a substance melts.
heat of reaction
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The heat absorbed as a reaction occurs.
heat of vaporization
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The heat absorbed as a substance passes from a liquid to a gas.
Henry's law
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The statement that the partial pressure of a gas in equilibrium with a solution is proportional to the concentration of the dissolved gas in the solution.
Hess' law
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The scheme for adding thermodynamic parameters for a reaction when the reactions are added.
heterocyclic
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A cyclic organic compound containing rings formed by carbon and other atoms.
heterogeneous mixture
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A mixture in which regions differ in composition or state of matter.
hexagonal closest packed
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One of two schemes for closest packing of spheres; described as "aba" to indicate that the atoms of the third layer lie directly above the atoms of the first layer.
homogeneous mixture
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A mixture in which all regions have the same composition.
homologous series
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A sequence of compounds having the same functional groups but differing in the length of the chain of carbon atoms.
Hund's rule
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The statement that electrons singly occupy all degenerate orbitals before doubling up in an orbital.
hybrid orbital
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An orbital formed by mathematically combining atomic orbitals to generate an equivalent set of orbitals more consistent with the observed bonding geometry.
hybridization
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The process of mathematically combining atomic orbitals to generate an equivalent set of orbitals more consistent with the observed bonding geometry; a description (sp, etc.) of what orbitals were combined.
hydrate
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A substance that solidifies so as to include water of crystallization.
hydration energy
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The energy absorbed as a substance is dissolved in water.
hydrocarbon
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A compound containing only the elements carbon and hydrogen.
hydrogen bond
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An attractive force, either intramolecular or intermolecular, between an electronegative atom and a hydrogen atom attached to another electronegative atom.
hydrogenation
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Any reaction of hydrogen with an organic compound.
hydrolysis
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The reaction of an anion with water to form the conjugate acid and hydroxide ion; a reaction in which water reacts with a substance to break it into components.
hydrolyze
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To react with water so that the water molecule is split into two parts, H and OH.
hydrophilic
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Water-loving; attracted to water molecules and polar molecules.
hydrophobic
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Water-hating; not attracted to water molecules or polar molecules.
hydroxyl group
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The functional group of an oxygen atom bonded to an hydrogen atom, -OH; found in alcohols.
hygroscopic
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Absorbing moisture from the air.
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