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Glossary: R


racemic. racemic mixture.
A mixture of equal parts of the levorotatory* and dextrorotatory* isomers of the the same substance. Racemic mixtures are not optically active*.

radian. (rad) Compare with steradian*.
An angle with vertex at the center of a circle of radius r that encompasses an arc of length r.

radiant intensity. (P,I) radiant power; intensity; radiation intensity.
Energy of radiation striking a unit area per unit time. The SI* unit of radiant power is J m-2 s-1.

radioactivity. radiation; radioactive.
Spontaneous* emission of particles or high-energy electromagnetic radiation* from the nuclei of unstable atoms. "Radiation" refers to the emissions, and "radioactive source" refers to the source of the radiation.

radioisotope. Compare with isotope*.
A radioactive* isotope*. For example, tritium* is a radioisotope of hydrogen.

random error. indeterminate error. Compare with systematic error*, gross error* and mistake*.
Random errors are errors that affect the precision of a set of measurements. Random error scatters measurements above and below the mean, with small random errors being more likely than large ones.

Raoult's law.
The vapor pressure* of a solvent in an ideal solution* equals the mole fraction of the solvent times the vapor pressure of the pure solvent.

rare earth.
An oxide* of a rare earth element*.

rare earth element. rare earth metal.
A metallic element that belongs to Group 3B or to the lanthanide* series.

rate constant. (k)
A rate constant is a proportionality constant that appears in a rate law*. For example, k is the rate constant in the rate law d[A]/dt = k[A]. Rate constants are independent of concentration but depend on other factors, most notably temperature.

rate law.
A rate law or rate equation relates reaction rate* with the concentrations of reactants, catalysts, and inhibitors. For example, the rate law for the one-step reaction A + B rightarrow C is d[C]/dt = k[A][B].

reactant. Compare with product*.
A substance that is consumed during a chemical change*.

reaction mechanism. mechanism.
A list of all elementary reactions* that occur in the course of an overall chemical reaction*.

reaction quotient. (Q) Compare with equilibrium constant*.
The product of the concentrations of the products*, divided by the product of the concentrations of the reactants*, for a chemical reaction which is not necessarily at equilibrium*. For example, the reaction quotient for A + B = C + D is equal to (C)(D) / ((A)(B)), where the parentheses indicate concentrations. Each concentration is raised to a power equal to its stoichiometric coefficient* in the expression. The reaction quotient for A + 2B = 3C is equal to (C)3/((A)(B)2). For gas phase reactions, partial pressures* can be used in the reaction quotient in place of concentrations.

reaction rate.
A reaction rate is the speed at which reactants are converted into products in a chemical reaction. The reaction rate is given as the instantaneous rate of change for any reactant or product, and is usually written as a derivative (e. g. d[A]/dt) with units of concentration per unit time (e. g. mol L-1 s-1).

A substance or mixture that is useful in chemical analysis or synthesis.

rearrangement reaction. isomerization; isomerize.
A reaction in which a reactant and product are isomers* of each other. Chemical bonds within the reactant are broken and reformed to produce the product.

receptor. receptor site.
A molecule or surface in a cell that recognizes and binds to a specific messenger molecule, leading to a biological response.

redox indicator. oxidation-reduction indicator.
An organic molecule that has reduced and oxidized forms with different colors; interconversion of the reduced and oxidized forms of the indicator must be reversible. Ferroin* is an example.

redox reaction. electrochemical reaction; oxidation-reduction reaction; redox.
A reaction that involves transfer of electrons from one substance to another. Redox reactions always involve a change in oxidation number for at least two elements in the reactants.

redox titration. oxidation-reduction titration.
A titration* based on a redox* reaction. For example, iron in water can be determined by converting dissolved iron to Fe2+ and titrating the solution with potassium permanganate (KMnO4), a powerful oxidizing agent*.

reducing agent. reductant. Compare with oxidizing agent*.
A reducing agent is a substance that reduce* another substance by supplying electrons to it. Reducing agents cause other substances to be reduced in chemical reactions while they themselves are oxidized. For example, tin(II) is a reducing agent in the following reaction:

Sn2+(aq) + 2 Fe3+(aq) rightarrow Sn4+(aq) + 2 Fe2+(aq)

reduction. reduce; reduced; reducing. Compare with oxidation*.
Reduction is gain of one or more electrons by an atom, molecule, or ion. Reduction is accompanied by a decrease in oxidation number.

reduction half reaction. reduction half-reaction. Compare with oxidation half reaction*.
That part of a redox reaction* that involves gain of electrons. In the oxidation half reaction, the oxidation number* of one or more atoms within the reactants is reduced.

relative error. relative uncertainty. Compare with absolute error*.
The uncertainty in a measurement compared to the size of the measurement. For example, if three replicate weights for an object are 2.00 g, 2.05 g, and 1.95 g, the absolute error can be expressed as ± 0.05 g and the relative error is ± 0.05 g / 2.00 g = 0.025 = 2.5%.

relative standard deviation. (RSD) Compare with standard deviation*.
The relative standard deviation is a measure of precision*, calculated by dividing the standard deviation* for a series of measurements by the average measurement.

1. The substances left after an evaporation* or distillation*. 2. A recognizable molecular fragment embedded in a larger molecule; for example, amino acid* "residues" within a protein*.

Description of the ground state of a molecule with delocalized electrons as an average of several Lewis structures. The actual ground state doesn't switch rapidly between the separate structures: it is an average.

resonance effect. mesomeric effect.
If electron density at a particular point in a molecule is higher or lower than what you'd expect from a single Lewis structure, and various canonical structures can be drawn to show how electron delocalization will explain the discrepancy, the difference in electron density is called a "resonance effect" or "mesomeric effect".

reverse osmosis. Compare with osmosis*.
Solvent molecules flow spontaneously from a dilute solution through a semipermeable membrane* to a more concentrated solution (osmosis). In reverse osmosis, pressure is applied to the more concentrated solution to force the flow of solvent to go from more concentrated to more dilute solution. Reverse osmosis is used to produce fresh water from sea water.

reversible. (A process or reaction that can be reversed by an infinitesimally small change in conditions. For example, ice and water coexist at 1 atm and 0°C; a very slight temperature increase causes the ice to melt; a tiny temperature decrease causes the water to freeze. Melting or freezing under these conditions can be considered reversible. Reversible processes are infinitesimally close to equilibrium. ) reversible process; reversible reaction. Compare with irreversible* and irreversible process*.

RNA. ribonucleic acid. Compare with nucleic acid* and nucleotide*.
A nucleic acid* with D-ribose as the sugar component in its nucleotides*.


General Chemistry Online! Glossary: R

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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/glossary/r.shtml