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AllChemicals.info - Chemical glossary Category: Sciences > Chemical Date & country: 10/12/2007, UK Words: 611

Rate-law Expression
Equation relating the rate of a reaction to the concentrations of the reactants and the specific rate of the constant.

Reactants
Substances consumed in a chemical reaction.

Reaction Quotient
The mass action expression under any set of conditions (not necessarily equlibrium), its magnitude relative to K determines the direction in which the reaction must occur to establish equilibrium.

Reaction Ratio
The relative amounts of reactants and products involved in a reaction, maybe the ratio of moles. millimoles, or masses.

Reaction Stoichiometry
Description of the quantitative relationships among substances as they participate in chemical reactions.

Reducing Agent
The substance that reduces another substance and is oxidized.

Resonance
The concept in which two or more equivalent dot formulas for the same arrangement of atoms (resonance structures) are necessary to describe the bonding in a molecule or ion.

Reverse Osmosis
Forcing solvent molecules to flow through a semipermable membrane from a concentated solution into a dilute solution by the application of greater hydrostatic pressure on concentrated side than the osmotic pressure opposing it.

Reversible Reaction
Reactions that do not go to completion and occur in both the forward and reverse direction.

S Orbital
A spherically symmetrical atomic orbital, one per energy level.

Saccharate
a compound formed by interaction of sucrose with a metallic oxide, usually lime, and useful in the purification of sugar.

Saccharic
of or derived from saccharin or a saccharine substance.

Saccharic acid
a white, needlelike, crystalline, water-soluble solid or syrup, C₆H₁₀O₈, usually made by the oxidation of cane sugar, glucose, or starch by nitric acid. Also called 'Glucaric acid.'

Saccharide
an organic compound containing a sugar or sugars.

Safranine
Also called 'phenosafranine'. A purplish-red, water-soluble dye, C_18H14N₄, used for textiles and as a stain in microscopy.

Salicylaldehyde
an oily, slightly water-soluble liquid, C₇H₆O₂, having an almondlike odor: used chiefly in perfumery and in the synthesis of coumarin.

Salicylate
a salt or ester of salicylic acid.

Salinometer
an instrument for measuring the amount of salt in a solution. Also,'salimeter, salometer.'

Salt Bridge
A U-shaped tube containing electrolyte, which connects two half-cells of a voltaic cell.

Saponification
Hydrolysis of esters in the presence of strong soluable bases.

Saturated Hydrocarbons
Hydrocarbons that contain only single bonds. They are also called alkanes or paraffin hydrocarbons.

Saturated Solution
Solution in which no more solute will dissolve.

Second Law of Thermodynamics
The universe tends toward a state of greater diorder in spontaneous processes.

Secondary Standard
a solution that has been titrated against a primary standard. A standard solution is a secondary standard.

Secondary Voltaic Cells
Voltaic cells that can be recharged, original reactanats can be regenerated be reversing the direction of the current flow.

Semiconductor
A substance that does not conduct electricity at low temperatures but does so at higher temperatures.

Semipermable Membrane
A thin partition between two solutions through which certain molecules can pass but others cannot.

Shielding Effect
Electrons in filled sets of s , p orbitals between the nucleus and outer shell electrons shield the outer shell electrons somewhat from the effect of protons in the nucleus, also called screening effect.

Sigma Bonds
Bonds resulting from the head-on overlap of atomic orbitals, in which the region of electron sharing is along and (cylindrically) symmetrical to the imaginary line connecting the bonded atoms.

Sigma Orbital
Molecular orbital resulting from head-on overlap of two atomic orbitals.

Silicones
Polymeric organosilicon compounds, contain individual or cross-linked Si-O chains or rings in which some oxygens of SiO4 tetrahedra are replaced by other groups.

Single Bond
Covalent bond resulting from the sharing of two electrons (one pair) between two atoms.

Solubility Product Constant
Equilibrium constant that applies to the dissolution of a slightly soluble compound.

Solubility Product Principle
The solubility product constant expression for a slightly soluble compound is the product of the concentrations of the constituent ions, each raised to the power that corresponds to the number of ions in one formula unit.

Solute
The dispersed (dissolved) phase of a solution.

Solution
Homogeneous mixture of two or more substances.

Solvation
The process by which solvent molecules surround and interact with solute ions or molecules.

Solvent
The dispersing medium of a solution.

Solvolysis
The reaction of a substance with the solvent in which it is dissolved.

Specific Gravity
The ratio of the density of a substance to the density of water.

Specific Heat
The amount of heat required to raise the temperature of one gram of substance one degree Celsius.

Specific Rate Constant
An experimentally determined (proportionality) constant, which is different for different reactions and which changes only with temperature, k in the rate-law expression: Rate = k [A] x [B]v.

Spectator Ions
Ions in a solution that do not participate in a chemical reaction.

Spectral Line
Any of a number of lines corresponding to definite wavelengths of an atomic emission or absorption spectrum, represents the energy difference between two energy levels.

Spectrochemical Series
Arrangement of ligands in order of increasing ligand field strength.

Spectrum
Display of component wavelengths (colours) of electromagnetic radiation.

Square Planar
A term used to describe molecules and polyatomic ions that have one atom in the center and four atoms at the corners of a square.

Square Planar Complex
Complex in which the metal is in the center of a square plane, with ligand donor atoms at each of the four corners.

Standard Electrode Potential
By convention , potential, Eo, of a half-reaction as a reduction relative to the standard hydrogen electrode when all species are present at unit activity.

Standard Electrodes
Half-cells in which the oxidized and reduced forms of a species are present at unit activity, 1.0M solutions of dissolved ions, 1.0atm partial pressure of gases, and pure solids and liquids.

Standard Entropy
The absolute entropy of a substance in its standard state at 298 K.

Standard Molar Volume
The volume occupied by one mole of an ideal gas under standard conditions, 22.4liters.

Standard Reaction
A reaction in which the numbers of moles of reactants shown in the balanced equation, all in their standard states, are completely converted to the numbers of moles of products shown in the balanced equation, also sall at their standard state.

Stereoisomers
Isomers that differ only in the way that atoms are oriented in space, consist of geometrical and optical isomers.

Stoichiometry
Description of the quantitative relationships among elements and compounds as they undergo chemical changes.

Strong Electrolyte
A substance that conducts electricity well in a dilute aqueous solution.

Strong Field Ligand
Ligand that exerts a strong crystal or ligand electrical field and generally forms low spin complexes with metal ions when possible.

Structural Isomers
Compounds that contain the same number of the same kinds of atoms in different geometric arrangements.

Sublimation
The direct vaporization of a sold by heating without passing through the liquid state.

Substance
Any kind of matter all specimens of which have the same chemical composition and physical properties.

Substitution Reaction
A reaction in which an atom or a group of atoms is replaced by another atom or group of atoms.

Sulfuric Acid
H₂SO₄: colorless, oily liquid, boiling point 330C. A 96 percent solution is used in the laboratory.Fire Hazard: This is a very powerful, acidic oxidizer which can Ignite or even explode on contact with many materials, i.e. acetic acid ,acetone+ HNOs, alcohols, + H202, NH4OH, HCL, NaOH, and others.

Supercooled Liquids
Liquids that, when cooled, apparently solidify but actually continue to flow very slowly under the influence of gravity e.g glass.

Supercritical Fluid
A substance at temperature above its critical temperature.

Supersaturated Solution
A solution that contains a higher than saturation concentration of solute, slight disturbance or seeding causes crystallization of excess solute.

Surface Tension
It is the force in dynes acting along the surface of the liquid 1cm in length and perpendicular to it.

Suspension
A heterogeneous mixture in which solute-like particles settle out of solvent-like phase some time after their introduction.

Temperature
A measure of the intensity of heat, i.e. the hotness or coldness of a sample. or object.

Ternary Acid
A ternary compound containing H, O, and another element, often a nonmetal.

Ternary Compound
A compound consisting of three elements, may be ionic or covalent.

Tetrahedral
A term used to describe molecules and polyatomic ions that have one atom in center and four atoms at the corners of a tetrahedron.

Theoretical Yield
Maximum amount of a specified product that could be obtained from specified amounts of reactants, assuming complete consumption of limiting reactant according to only one reaction and complete recovery of product.

Thermal Cracking
Decomposition by heating a substance in the presence of a catalyst and in the absence of air.

Thermodynamics
The study of the energy transfers accompanying physical and chemical processes.

Thermonuclear Energy
The energy stored in the nucleus of an atom and released through fission, fusion, or radioactivity . In these processes a small amount of mass is converted to energy according to the relationship E = mc², where E is energy, m is mass, and c is the speed of light.

Third Law of Thermodynamics
The entropy of a hypothetical pure, perfect, crystalline sustance at absolute zero temperature is zero.

Titration
A Procedure in which one solution is added to another solution until the chemical reaction between the two solutes is complete, the concentration of one solution is known and that of the other is unknown.

Total Ionic Equation
Equation for a chemical reaction written to show the predominant form of all species in aqueous solution or in contact with water.

Transition State Theory
Theory of reaction rates that states that reactants pass through high-energy transition states before forming products.

Tyndall Effect
The scattering of light by colloidal particles.

Unsaturated Hydrocarbons
Hydrocarbons that contain double or triple carbon-carbon bonds.

Valence Bond Theory
Assumes that covalent bonds are formed when atomic orbitals on different atoms overlap and the electrons are shared.

Valence Electrons
Outermost electrons of atoms, usually those involved in bonding.

van der Waals' Equation
An equation of state that extends the ideal gas law to real gases by inclusion of two empirically determined parameters, which are different for different gases.

Vapor
A gas formed by boiling or evaporating a liquid.

Vapor Pressure
The particle pressure of a vapor at the surface of its parent liquid.

Viscosity
Resistance offered by the molecules of a liquid to flow is termed as viscosity.

Voltage
Potential difference between two electrodes, a measure of the chemical potential for a redox reaction to occur.

Voltaic Cells
Electrochemical cells in which spontaneous chemical reactions produce electricity, also called galvanic cells.

Water Equivalent
The amount of water that would absorb the same amount of heat as the calorimeter per degree temperature increase.

Weak Electrolyte
A substance that conducts electricity poorly in a dilute aqueous solution.

Weak Field Ligand
A Ligand that exerts a weak crystal or ligand field and ge- nerally forms high spin complexes with metals.

xanthate
a salt or ester of xanthic acid.

xanthene dye
any of a group of dyes having a molecular structure related to that of xanthene in which the aromatic (C₆H₄) groups are the chromophore.

xenon
a heavy, colorless, chemically inactive, monatomic gaseous element used for filling radio, television, and luminescent tubes. Symbol: Xe, at. wt.: 131.30, at. no.: 54.

xenon hexafluoride
a colorless, crystalline compound, XeF₆, that melts at 50°C to a yellow liquid, and boils at 75°C.

xenon tetrafluoride
a colorless, crystalline compound, XeF₄, prepared by heating a gaseous mixture of fluorine and xenon.

xenon trioxide
a colorless, nonvolatile solid, XeO₃, explosive when dry: in solution it is called xenic acid.

xylan
the pentosan occurring in woody tissue that hydrolyzes to xylose: used as a source of furfural.

xylene
any of three oily, colorless, water-insoluble, flammable, toxic, isomeric liquids, C₈H₁₀, of the benzene series, obtained mostly from coal tar: used chiefly in the manufacture of dyes.