Chemistry - A Molecular Science

Glossary

A

quantum number

is a number (usually an integer)

that designates an allowed state. All atomic and molecular states (e.g., el

ectronic, vibrational,

rotational, and nuclear) are described by quantum numbers.

R

A

radioactive

nucleus is unstable and disintegrates

spontaneously to another nucleus by emitting or capturing particles.

Radioactive dating

is the determination of the age of a

material from the amount of material involved in the radioactive decay of one of its components.

Radiochemistry

See nuclear chemistry.

Radioisotopes

are radioactive nuclei.

The

rate of change

of a quantity is the rate at which it

changes as a function of the change in another quantity.

A

rate constant (k)

is the proportionality constant

between the concentrations of the components (usually reactants) of a reaction and the rate of reaction.

The

rate-determining step (RDS)

is the elementary

reaction in a mechanism that is so much slower than the other elementary reactions that it dictates the rate of the overall reaction.

The

rate law

expresses the rate of a reaction as a

function of the concentrations of the substances (usually reactants) involved in the reaction.

The

rate of disappearance

is the rate at which a

reactant reacts.

The

rate of formation

or

appearance

is the rate at

which a product is produced.

The

rate of reaction

is the rate at which a product is

produced or a reactant

reacts divided by its

coefficient in the chemical equation.

A

reactant order

is the exponent of the concentration

of a reactant in the rate equation for a reaction.

The

reaction coordinate

is the combination of

intermolecular distance, bond length and bond angle changes required to convert reactant molecules into product molecules.

A

reaction mechanism

is a series of elementary

processes that leads to the overall reaction.

The

reaction order

is the sum of all of the reactant

orders in a reaction.

The

reaction quotient (

Q)

is expressed as the activities

of the products divided by the activities of the reactants. Each activity is

raised to an exponent

equal to the coefficient

of the substance in the

balanced equation. When the activities are equilibrium activities, the reaction quotient is called the equilibrium constant.

A

redox couple

is the oxidized and reduced forms of

the species involved in a half-reaction. For example, Cu

2+/Cu is a redox couple.

The

redox electrons

are the electrons that are

transferred in a redox reaction.

Redox reactions

involve an electron transfer from a

reductant to an oxidant.

A

reducing agent

or

reductant

is a substance that

promotes reduction in another material. It is oxidized in the process.

Reduction

is the gain of electrons, which results in a

decrease in oxidation state of the species being reduced.

A

residue

in a protein is one of the amino acids

making up the protein.

A

resonance structure

is a Lewis structure of a

molecule that differs from another Lewis structure only in the placement of electrons.

Respiration

is the process wher

eby animals extract

energy from carbohydrates.

Rotational degrees of freedom

are spinning motions

about an axis through the center of mass of the molecule. Linear molecules have two rotational degrees of freedom, and nonlinear molecules have three.

S

Salts

are ionic compounds formed in an Arrhenius acid-base reaction. The anion of a salt is supplied by the acid, and the cation by the base.

A

salt bridge

is a liquid junction that consists of a

saturated solution of a strong electrolyte, such as KCl. Ions enter and leave the bridge so as to maintain electrical neutrality in the two half-cells of an electrochemical cell.

Saturated carbons

are involved in four sigma bonds.

Science

is that branch of knowledge that is gained by

the application of the scientific method.

The

Schrödinger equation

relates the energy of an

electron to its wavefunction.

The

scientific method

is used to further scientific

knowledge. It involves observation, hypothesis formulation, prediction, and testing.

The

second law of thermodynamics

states that the

entropy of the universe increases in all spontaneous processes.

A

semiconductor

is a substance whose electrical

conductivity increases with temperature. Semiconductors are characterized by small but nonzero band gaps.

© by

North

Carolina

State

University