Chemistry
From Wikipedia, the free encyclopedia
Chemistry is the
science concerned with the composition, structure, and properties of
matter, as well as the changes it undergoes during
chemical reactions.
Chemistry (the etymology of the word has been much disputed
[1]) is the
science of
matter and the changes it undergoes. The science of matter is also addressed by
physics, but while physics takes a more general and fundamental approach, chemistry is more specialized, being concerned with the composition, behavior, structure, and properties of
matter, as well as the changes it undergoes during
chemical reactions.
[2] It is a
physical science which studies of various
atoms,
molecules,
crystals and other aggregates of matter whether in isolation or combination, which incorporates the concepts of
energy and
entropy in relation to the
spontaneity of
chemical processes.
Disciplines within chemistry are traditionally grouped by the type of matter being studied or the kind of study. These include
inorganic chemistry, the study of
inorganic matter;
organic chemistry, the study of
organic (carbon based) matter;
biochemistry, the study of
substances found in
biological organisms;
physical chemistry, the study of chemical processes using physical concepts such as
thermodynamics and
quantum mechanics; and
analytical chemistry, the analysis of material samples to gain an understanding of their
chemical composition and
structure. Many more specialized disciplines have emerged in recent years, e.g.
neurochemistry the chemical study of the
nervous system (see
subdisciplines).
Summary
Chemistry is the scientific study of interaction of
chemical substances[3] that are constituted of
atoms or the subatomic particles:
protons,
electrons and
neutrons.
[4] Atoms combine to produce
molecules or
crystals. Chemistry is often called "
the central science" because it connects the other
natural sciences such as
astronomy,
physics,
material science,
biology and
geology.
[5][6]
The genesis of chemistry can be traced to certain practices, known as
alchemy, which had been practiced for several
millennia in various parts of the world, particularly the Middle East.
[7]
The structure of objects we commonly use and the properties of the matter we commonly interact with are a consequence of the properties of chemical substances and their interactions. For example,
steel is
harder than iron because its atoms are bound together in a more rigid
crystalline lattice; wood burns or undergoes rapid
oxidation because it can react spontaneously with
oxygen in a
chemical reaction above a certain
temperature; sugar and salt dissolve in water because their molecular/ionic properties are such that dissolution is preferred under the ambient conditions.
The transformations that are studied in chemistry are a result of interaction either between different chemical substances or between
matter and
energy. Traditional chemistry involves study of
interactions between
substances in a chemistry
laboratory using various forms of
laboratory glassware.
A
chemical reaction is a transformation of some substances into one or more other substances.
[8] It can be symbolically depicted through a
chemical equation. The number of atoms on the left and the right in the equation for a chemical transformation is most often equal. The nature of chemical reactions a substance may undergo and the energy changes that may accompany it are constrained by certain basic rules, known as chemical laws.
Energy and
entropy considerations are invariably important in almost all chemical studies. Chemical substances are classified in terms of their
structure, phase as well as their
chemical compositions. They can be analyzed using the tools of
chemical analysis, e.g.
spectroscopy and
chromatography.
Scientists engaged in chemical
research are known as
chemists.
[9] Most chemists specialize in one or more sub-disciplines.
History
Ancient Egyptians pioneered the art of synthetic "wet" chemistry up to 4,000 years ago.
[10] By 1000 BC ancient civilizations were using technologies that formed the basis of the various branches of chemistry such as; extracting metal from their ores, making pottery and glazes, fermenting beer and wine, making pigments for cosmetics and painting, extracting chemicals from plants for medicine and perfume, making cheese, dying cloth, tanning leather, rendering fat into soap, making glass, and making alloys like bronze.
The genesis of chemistry can be traced to the widely observed phenomenon of
burning that led to
metallurgy—the art and science of processing ores to get metals (e.g.
metallurgy in ancient India). The greed for gold led to the discovery of the process for its purification, even though the underlying principles were not well understood—it was thought to be a transformation rather than purification. Many scholars in those days thought it reasonable to believe that there exist means for transforming cheaper (base) metals into gold. This gave way to alchemy and the search for the
Philosopher's Stone which was believed to bring about such a transformation by mere touch.
[11]
Greek atomism dates back to 440 BC, as what might be indicated by the book
De Rerum Natura (The Nature of Things)
[12] written by the Roman
Lucretius[13] in 50 BC. Much of the early development of purification methods is described by
Pliny the Elder in his
Naturalis Historia.
A tentative outline is as follows:
- Egyptian alchemy [3,000 BCE – 400 BCE], formulate early "element" theories such as the Ogdoad.
- Greek alchemy [332 BCE – 642 CE], the Greek king Alexander the Great conquers Egypt and founds Alexandria, having the world's largest library, where scholars and wise men gather to study.
- Arab alchemy [642 CE – 1200], the Muslim conquest of Egypt (primarily Alexandria); development of the Scientific Method by Alhazen and Jābir ibn Hayyān revolutionise the field of Chemistry. Jābir (Latin name Geber) accepted many of the ideas of Aristotle but also modified Aristotle's ideas.[14]
- The House of Wisdom (Arabic: بيت الحكمة; Bait al-Hikma), Al-Andalus (Arabic: الأندلس) and Alexandria (Arabic: الإسكندرية) become the world leading institutions where scientists of all religious and ethnic backgrounds worked together in harmony expanding the reaches of Chemistry in a time known as the Islamic Golden Age.
- Arabs and Persians continue to dominate the field of Chemistry, mastering it and expanding the boundaries of knowledge and experimentation. Besides technical advances in processes and apparatus, the Arabs had developed and improved the purity of substances such as alcohols, acids, and gunpowder, which were not available to the Europeans.[14]
- European alchemy [1300 – present], Pseudo-Geber builds on Arabic chemistry.[citation needed] From the 12th century, major advances in the chemical arts shifted from Arab lands to western Europe.[14]
- Chemistry [1661], Boyle writes his classic chemistry text The Sceptical Chymist.
- Chemistry [1787], Lavoisier writes his classic Elements of Chemistry.
- Chemistry [1803], Dalton publishes his Atomic Theory.
- Chemistry [1869], Dmitry Mendeleev presented his Periodic Table being the framework of the modern chemistry
The earliest pioneers of Chemistry, and inventors of the modern
scientific method,
[15] were medieval
Arab and Persian scholars. They introduced precise
observation and controlled
experimentation into the field and discovered numerous
Chemical substances.
[16][verification needed]
"Chemistry as a science was almost created by the Muslims; for in this field, where the Greeks (so far as we know) were confined to industrial experience and vague
hypothesis, the
Saracens introduced precise
observation, controlled
experiment, and careful records. They invented and named the
alembic (al-anbiq), chemically analyzed innumerable
substances, composed
lapidaries, distinguished
alkalis and
acids, investigated their affinities, studied and manufactured hundreds of
drugs. Alchemy, which the Muslims inherited from Egypt, contributed to chemistry by a thousand incidental discoveries, and by its method, which was the most scientific of all medieval operations."
[16]
The most influential Muslim chemists were
Jābir ibn Hayyān (Geber, d. 815),
al-Kindi (d. 873),
al-Razi (d. 925),
al-Biruni (d. 1048) and
Alhazen (d. 1039).
[17] The works of Jābir became more widely known in Europe through
Latin translations by a
pseudo-Geber in 14th century
Spain, who also wrote some of his own books under the pen name "Geber". The contribution of
Indian alchemists and metallurgists in the development of chemistry was also quite significant.
[18]
The emergence of chemistry in Europe was primarily due to the recurrent incidence of the
plague and blights there during the so called
Dark Ages.
[citation needed] This gave rise to a need for medicines. It was thought that there exists a universal medicine called the
Elixir of Life that can cure all diseases
[citation needed], but like the Philosopher's Stone, it was never found.
For some practitioners, alchemy was an intellectual pursuit, over time, they got better at it.
Paracelsus (1493–1541), for example, rejected the 4-elemental theory and with only a vague understanding of his chemicals and medicines, formed a hybrid of alchemy and science in what was to be called
iatrochemistry. Similarly, the influences of philosophers such as
Sir Francis Bacon (1561–1626) and
René Descartes (1596–1650), who demanded more rigor in mathematics and in removing bias from scientific observations, led to a
scientific revolution. In chemistry, this began with
Robert Boyle (1627–1691), who came up with an equation known as
Boyle's Law about the characteristics of gaseous state.
[20]
Chemistry indeed came of age when
Antoine Lavoisier (1743–1794), developed the theory of
Conservation of mass in 1783; and the development of the
Atomic Theory by
John Dalton around 1800. The Law of Conservation of Mass resulted in the reformulation of chemistry based on this law
[citation needed] and the oxygen theory of combustion, which was largely based on the work of Lavoisier. Lavoisier's fundamental contributions to chemistry were a result of a conscious effort
[citation needed] to fit all experiments into the framework of a single theory. He established the consistent use of the chemical balance, used oxygen to overthrow the
phlogiston theory, and developed a new system of chemical nomenclature and made contribution to the modern metric system. Lavoisier also worked to translate the archaic and technical language of chemistry into something that could be easily understood by the largely uneducated masses, leading to an increased public interest in chemistry. All these advances in chemistry led to what is usually called the
chemical revolution. The contributions of Lavoisier led to what is now called modern chemistry—the chemistry that is studied in educational institutions all over the world. It is because of these and other contributions that
Antoine Lavoisier is often celebrated as the "
Father of Modern Chemistry".
[21] The later discovery of
Friedrich Wöhler that many natural substances,
organic compounds, can indeed be synthesized in a chemistry
laboratory also helped the modern chemistry to mature from its infancy.
[22]
The
discovery of the chemical elements has a long history from the days of alchemy and culminating in the discovery of the
periodic table of the chemical elements by
Dmitri Mendeleev (1834–1907)
[23] and later discoveries of some
synthetic elements.
Etymology
The word
chemistry comes from the earlier study of alchemy, which is a set of practices that encompasses elements of chemistry, metallurgy, philosophy, astrology, astronomy, mysticism and medicine. Alchemy in turn is derived from the
Arabic word "كيمياء" meaning "value", it is commonly thought of as the quest to turn lead or another common starting material into gold.
[24] This linguistic relation between the pursuit of value and alchemy is thought to have
Egyptian origins. Many believe that the Arabic word "alchemy" is derived from the word
Chemi or
Kimi, which is the ancient name of
Egypt in
Egyptian.
[25][26][27] The word was subsequently borrowed by the
Greeks, and from the Greeks by the Arabs when they occupied
Alexandria (
Egypt) in the 7th century. The Arabs added the Arabic definite article "al" to the word, resulting in the word "الكيمياء" (al-kīmiyā). Thus, an alchemist was called a 'chemist' in popular speech, and later the suffix "-ry" was added to this to describe the art of the chemist as "chemistry".
Definitions
In retrospect, the definition of chemistry seems to invariably change per decade, as new discoveries and theories add to the functionality of the science. Shown below are some of the standard definitions used by various noted chemists:
- Alchemy (330) – the study of the composition of waters, movement, growth, embodying, disembodying, drawing the spirits from bodies and bonding the spirits within bodies (Zosimos).[28]
- Chymistry (1661) – the subject of the material principles of mixt bodies (Boyle).[29]
- Chymistry (1663) – a scientific art, by which one learns to dissolve bodies, and draw from them the different substances on their composition, and how to unite them again, and exalt them to a higher perfection (Glaser).[30]
- Chemistry (1730) – the art of resolving mixt, compound, or aggregate bodies into their principles; and of composing such bodies from those principles (Stahl).[31]
- Chemistry (1837) – the science concerned with the laws and effects of molecular forces (Dumas).[32]
- Chemistry (1947) – the science of substances: their structure, their properties, and the reactions that change them into other substances (Pauling).[33]
- Chemistry (1998) – the study of matter and the changes it undergoes (Chang).[34]
Basic concepts
Several
concepts are essential for the study of chemistry; some of them are:
[35]
Atom
An
atom is the basic unit of chemistry. It consists of a positively charged core (the
atomic nucleus) which contains
protons and
neutrons, and which maintains a number of
electrons to balance the positive charge in the nucleus. The atom is also the smallest entity that can be envisaged to retain some of the
chemical properties of the element, such as
electronegativity,
ionization potential, preferred
oxidation state(s),
coordination number, and preferred types of
bonds to form (e.g.,
metallic,
ionic,
covalent).
Element
The concept of
chemical element is related to that of chemical substance. A chemical element is specifically a substance which is composed of a single type of atom. A chemical element is characterized by a particular number of
protons in the
nuclei of its atoms. This number is known as the
atomic number of the element. For example, all atoms with 6 protons in their nuclei are atoms of the chemical element
carbon, and all atoms with 92 protons in their nuclei are atoms of the element
uranium. 94 different chemical elements or types of atoms based on the number of protons exist naturally. A further 18 have been recognised by
IUPAC as existing artificially only. Although all the nuclei of all atoms belonging to one element will have the same number of protons, they may not necessarily have the same number of
neutrons, such atoms are termed
isotopes. In fact several
isotopes of an element may exist.
The most convenient presentation of the chemical elements is in the
periodic table of the chemical elements, which groups elements by atomic number. Due to its ingenious arrangement,
groups, or columns, and
periods, or rows, of elements in the table either share several chemical properties, or follow a certain trend in characteristics such as
atomic radius,
electronegativity, etc. Lists of the elements
by name,
by symbol, and by
atomic number are also available.
Compound
A
compound is a substance with a
particular ratio of atoms of particular
chemical elements which determines its composition, and a particular
organization which determines chemical properties. For example,
water is a compound containing
hydrogen and
oxygen in the ratio of two to one, with the oxygen atom between the two hydrogen atoms, and an angle of 104.5° between them. Compounds are formed and interconverted by
chemical reactions.
Substance
A chemical substance is a kind of matter with a definite
composition and set of
properties.
[36] Strictly speaking, a mixture of compounds, elements or compounds and elements is not a chemical substance, but it may be called a chemical. Most of the substances we encounter in our daily life are some kind of mixture; for example:
air,
alloys,
biomass, etc.
Nomenclature of substances is a critical part of the language of chemistry. Generally it refers to a system for naming
chemical compounds. Earlier in the history of chemistry substances were given name by their discoverer, which often led to some confusion and difficulty. However, today the IUPAC system of chemical nomenclature allows chemists to specify by name specific compounds amongst the vast variety of possible chemicals. The standard nomenclature of chemical substances is set by the
International Union of Pure and Applied Chemistry (IUPAC). There are well-defined systems in place for naming chemical species.
Organic compounds are named according to the
organic nomenclature system.
[37] Inorganic compounds are named according to the
inorganic nomenclature system.
[38] In addition the
Chemical Abstracts Service has devised a method to index chemical substance. In this scheme each chemical substance is identifiable by a number known as
CAS registry number.
Molecule
A
molecule is the smallest indivisible portion of a pure
chemical substance that has its unique set of chemical properties, that is, its potential to undergo a certain set of chemical reactions with other substances. Molecules can exist as electrically neutral units unlike
ions. Molecules are typically a set of atoms bound together by
covalent bonds, such that the structure is electrically neutral and all valence electrons are paired with other electrons either in bonds or in
lone pairs.
A molecular structure depicts the bonds and relative positions of atoms in a molecule such as that in
Paclitaxel shown here
Not all substances consist of discrete molecules. Most chemical elements are composed of lone atoms as their smallest discrete unit. Other types of substances, such as
ionic compounds and
network solids, are organized in such a way as to lack the existence of identifiable molecules
per se. Instead, these substances are discussed in terms of
formula units or
unit cells as the smallest repeating structure within the substance; as they lack identifiable molecules.
One of the main characteristic of a molecule is its geometry often called its
structure. While the structure of diatomic, triatomic or tetra atomic molecules may be trivial, (linear, angular pyramidal etc.) the structure of polyatomic molecules, that are constituted of more than six atoms (of several elements) can be crucial for its chemical nature.
Mole and amount of substance
Main article:
Mole (unit)Mole is a unit to measure
amount of substance (also called chemical amount). A
mole is the amount of a
substance that contains as many elementary entities (atoms, molecules or ions) as there are atoms in 0.012
kilogram (or 12
grams) of
carbon-12, where the carbon-12 atoms are unbound, at rest and in their
ground state.
[39] The number of entities per mole is known as the
Avogadro constant, and is determined empirically. The currently accepted value is 6.02214179(30) × 10
23 mol
−1 (2007
CODATA). One way to understand the meaning of the term "mole" is to compare and contrast it to terms such as
dozen. Just as one dozen eggs contains 12 individual eggs, one mole contains 6.02214179(30) × 10
23 atoms, molecules or other particles. The term is used because it is much easier to say, for example, 1 mole of carbon, than it is to say 6.02214179(30) × 10
23 carbon atoms, and because moles of chemicals represent a scale that is easy to experience.
The amount of substance of a
solute per volume of
solution is known as amount of substance concentration, or
molarity for short. Molarity is the quantity most commonly used to express the
concentration of a solution in the chemical laboratory. The most commonly used units for molarity are mol/L (the official SI units are mol/m
3).
Ions and salts
An
ion is a charged species, an atom or a molecule, that has lost or gained one or more electrons. Positively charged
cations (e.g.
sodium cation Na
+) and negatively charged
anions (e.g.
chloride Cl
−) can form a crystalline lattice of neutral
salts (e.g.
sodium chloride NaCl). Examples of
polyatomic ions that do not split up during
acid-base reactions are
hydroxide (OH
−) and
phosphate (PO
43−).
Ions in the gaseous phase are often known as
plasma.
Acidity and basicity
A substance can often be classified as an
acid or a
base. There are several different theories which explain acid-base behavior. The simplest is
Arrhenius theory, which states than an acid is a substance that produces
hydronium ions when it is dissolved in water, and a base is one that produces
hydroxide ions when dissolved in water. According to
Brønsted–Lowry acid-base theory, acids are substances that donate a positive
hydrogen ion to another substance in a chemical reaction; by extension, a base is the substance which receives that hydrogen ion. A third common theory is
Lewis acid-base theory, which is based on the formation of new
chemical bonds. Lewis theory explains that an acid is a substance which is capable of accepting a pair of electrons from another substance during the process of bond formation, while a base is a substance which can provide a pair of electrons to form a new bond. According to concept as per Lewis, the crucial things being exchanged are charges.
[40][unreliable source?] There are several other ways in which a substance may be classified as an acid or a base, as is evident in the history of this concept
[41]
Acid strength is commonly measured by two methods. One measurement, based on the Arrhenius definition of acidity, is
pH, which is a measurement of the hydronium ion concentration in a solution, as expressed on a negative
logarithmic scale. Thus, solutions that have a low pH have a high hydronium ion concentration, and can be said to be more acidic. The other measurement, based on the Brønsted–Lowry definition, is the
acid dissociation constant (K
a), which measure the relative ability of a substance to act as an acid under the Brønsted–Lowry definition of an acid. That is, substances with a higher K
a are more likely to donate hydrogen ions in chemical reactions than those with lower K
a values.
Phase
Main article:
Phase (matter)In addition to the specific chemical properties that distinguish different chemical classifications chemicals can exist in several phases. For the most part, the chemical classifications are independent of these bulk phase classifications; however, some more exotic phases are incompatible with certain chemical properties. A
phase is a set of states of a chemical system that have similar bulk structural properties, over a range of conditions, such as
pressure or
temperature. Physical properties, such as
density and
refractive index tend to fall within values characteristic of the phase. The phase of matter is defined by the
phase transition, which is when energy put into or taken out of the system goes into rearranging the structure of the system, instead of changing the bulk conditions.
Sometimes the distinction between phases can be continuous instead of having a discrete boundary, in this case the matter is considered to be in a
supercritical state. When three states meet based on the conditions, it is known as a
triple point and since this is invariant, it is a convenient way to define a set of conditions.
The most familiar examples of phases are
solids,
liquids, and
gases. Many substances exhibit multiple solid phases. For example, there are three phases of solid
iron (alpha, gamma, and delta) that vary based on temperature and pressure. A principal difference between solid phases is the
crystal structure, or arrangement, of the atoms. Another phase commonly encountered in the study of chemistry is the
aqueous phase, which is the state of substances dissolved in
aqueous solution (that is, in water). Less familiar phases include
plasmas,
Bose-Einstein condensates and
fermionic condensates and the
paramagnetic and
ferromagnetic phases of
magnetic materials. While most familiar phases deal with three-dimensional systems, it is also possible to define analogs in two-dimensional systems, which has received attention for its relevance to systems in
biology.
Redox
It is a concept related to the ability of atoms of various substances to lose or gain electrons. Substances that have the ability to oxidize other substances are said to be oxidative and are known as
oxidizing agents, oxidants or oxidizers. An oxidant removes electrons from another substance. Similarly, substances that have the ability to reduce other substances are said to be reductive and are known as
reducing agents, reductants, or reducers. A reductant transfers electrons to another substance, and is thus oxidized itself. And because it "donates" electrons it is also called an electron donor. Oxidation and reduction properly refer to a change in oxidation number—the actual transfer of electrons may never occur. Thus, oxidation is better defined as an increase in
oxidation number, and reduction as a decrease in oxidation number.
Bonding
Main article:
Chemical bondAtoms sticking together in molecules or crystals are said to be bonded with one another. A chemical bond may be visualized as the
multipole balance between the positive charges in the nuclei and the negative charges oscillating about them.
[42] More than simple attraction and repulsion, the energies and distributions characterize the availability of an electron to bond to another atom.
A
chemical bond can be a
covalent bond, an
ionic bond, a
hydrogen bond or just because of
Van der Waals force. Each of these kind of bond is ascribed to some potential. These potentials create the
interactions which hold
atoms together in
molecules or
crystals. In many simple compounds,
Valence Bond Theory, the Valence Shell Electron Pair Repulsion model (
VSEPR), and the concept of
oxidation number can be used to explain molecular structure and composition. Similarly, theories from
classical physics can be used to predict many ionic structures. With more complicated compounds, such as
metal complexes, valence bond theory is less applicable and alternative approaches, such as the
molecular orbital theory, are generally used. See diagram on electronic orbitals.
Reaction
When a chemical substance is transformed as a result of its interaction with another or energy, a chemical reaction is said to have occurred.
Chemical reaction is therefore a concept related to the 'reaction' of a substance when it comes in close contact with another, whether as a mixture or a
solution; exposure to some form of energy, or both. It results in some energy exchange between the constituents of the reaction as well with the system environment which may be a designed vessels which are often
laboratory glassware. Chemical reactions can result in the formation or
dissociation of molecules, that is, molecules breaking apart to form two or more smaller molecules, or rearrangement of
atoms within or across molecules. Chemical reactions usually involve the making or breaking of
chemical bonds.
Oxidation, reduction,
dissociation, acid-base
neutralization and molecular
rearrangement are some of the commonly used kinds of chemical reactions.
A chemical reaction can be symbolically depicted through a
chemical equation. While in a non-nuclear chemical reaction the number and kind of atoms on both sides of the equation are equal, for a nuclear reaction this holds true only for the nuclear particles viz. protons and neutrons.
[43]
The sequence of steps in which the reorganization of chemical bonds may be taking place in the course of a chemical reaction is called its
mechanism. A chemical reaction can be envisioned to take place in a number of steps, each of which may have a different speed. Many
reaction intermediates with variable stability can thus be envisaged during the course of a reaction. Reaction mechanisms are proposed to explain the
kinetics and the relative product mix of a reaction. Many
physical chemists specialize in exploring and proposing the mechanisms of various chemical reactions. Several empirical rules, like the
Woodward-Hoffmann rules often come handy while proposing a mechanism for a chemical reaction.
According to the
IUPAC gold book a chemical reaction is a process that results in the interconversion of chemical species".
[44] Accordingly, a chemical reaction may be an
elementary reaction or a
stepwise reaction. An additional caveat is made, in that this definition includes cases where the
interconversion of conformers is experimentally observable. Such detectable chemical reactions normally involve sets of molecular entities as indicated by this definition, but it is often conceptually convenient to use the term also for changes involving single molecular entities (i.e. 'microscopic chemical events').
Equilibrium
Although the concept of
equilibrium is widely used across sciences, in the context of chemistry, it arises whenever a number of different states of the chemical composition are possible. For example, in a mixture of several chemical compounds that can react with one another, or when a substance can be present in more than one kind of phase. A system of chemical substances at equilibrium even though having an unchanging composition is most often not
static; molecules of the substances continue to react with one another thus giving rise to a
dynamic equilibrium. Thus the concept describes the state in which the parameters such as chemical composition remain unchanged over time. Chemicals present in biological systems are invariably not at equilibrium; rather they are far from equilibrium.
Energy
In the context of chemistry, energy is an attribute of a substance as a consequence of its
atomic,
molecular or aggregate
structure. Since a chemical transformation is accompanied by a change in one or more of these kinds of structure, it is invariably accompanied by an
increase or
decrease of
energy of the substances involved. Some energy is transferred between the surroundings and the reactants of the reaction in the form of
heat or
light; thus the products of a reaction may have more or less energy than the reactants. A reaction is said to be
exergonic if the final state is lower on the energy scale than the initial state; in the case of
endergonic reactions the situation is the reverse. A reaction is said to be
exothermic if the reaction releases heat to the surroundings; in the case of
endothermic reactions, the reaction absorbs heat from the surroundings.
Chemical reactions are invariably not possible unless the reactants surmount an energy barrier known as the
activation energy. The
speed of a chemical reaction (at given temperature T) is related to the activation energy E, by the Boltzmann's population factor
e − E / kT - that is the probability of molecule to have energy greater than or equal to E at the given temperature T. This exponential dependence of a reaction rate on temperature is known as the
Arrhenius equation. The activation energy necessary for a chemical reaction can be in the form of heat, light,
electricity or mechanical
force in the form of
ultrasound.
[45]
A related concept
free energy, which also incorporates entropy considerations, is a very useful means for predicting the feasibility of a reaction and determining the state of equilibrium of a chemical reaction, in
chemical thermodynamics. A reaction is feasible only if the total change in the
Gibbs free energy is negative,
; if it is equal to zero the chemical reaction is said to be at
equilibrium.
There exist only limited possible states of energy for electrons, atoms and molecules. These are determined by the rules of
quantum mechanics, which require
quantization of energy of a bound system. The atoms/molecules in a higher energy state are said to be excited. The molecules/atoms of substance in an excited energy state are often much more reactive; that is, more amenable to chemical reactions.
The phase of a substance is invariably determined by its energy and the energy of its surroundings. When the intermolecular forces of a substance are such that the energy of the surroundings is not sufficient to overcome them, it occurs in a more ordered phase like liquid or solid as is the case with water (H
2O); a liquid at room temperature because its molecules are bound by
hydrogen bonds.
[46] Whereas
hydrogen sulfide (H
2S) is a gas at room temperature and standard pressure, as its molecules are bound by weaker
dipole-dipole interactions.
The transfer of energy from one chemical substance to another depends on the
size of energy
quanta emitted from one substance. However, heat energy is often transferred more easily from almost any substance to another because the
phonons responsible for vibrational and rotational energy levels in a substance have much less energy than
photons invoked for the electronic energy transfer. Thus, because vibrational and rotational energy levels are more closely spaced than electronic energy levels, heat is more easily transferred between substances relative to light or other forms of electronic energy. For example, ultraviolet electromagnetic radiation is not transferred with as much efficacy from one substance to another as thermal or electrical energy.
The existence of characteristic energy levels for different
chemical substances is useful for their identification by the analysis of
spectral lines. Different kinds of spectra are often used in chemical
spectroscopy, e.g.
IR,
microwave,
NMR,
ESR, etc. Spectroscopy is also used to identify the composition of remote objects - like stars and distant galaxies - by analyzing their radiation spectra.
Emission spectrum of
iron The term
chemical energy is often used to indicate the potential of a chemical substance to undergo a transformation through a
chemical reaction or to transform other chemical substances.
Chemical laws
Main article:
Chemical lawChemical reactions are governed by certain laws, which have become fundamental concepts in chemistry. Some of them are:
Subdisciplines
Chemistry is typically divided into several major sub-disciplines. There are also several main cross-disciplinary and more specialized fields of chemistry.
[47]
- Analytical chemistry is the analysis of material samples to gain an understanding of their chemical composition and structure. Analytical chemistry incorporates standardized experimental methods in chemistry. These methods may be used in all subdisciplines of chemistry, excluding purely theoretical chemistry.
- Inorganic chemistry is the study of the properties and reactions of inorganic compounds. The distinction between organic and inorganic disciplines is not absolute and there is much overlap, most importantly in the sub-discipline of organometallic chemistry.
- Materials chemistry is the preparation, characterization, and understanding of substances with a useful function. The field is a new breadth of study in graduate programs, and it integrates elements from all classical areas of chemistry with a focus on fundamental issues that are unique to materials. Primary systems of study include the chemistry of condensed phases (solids, liquids, polymers) and interfaces between different phases.
- Neurochemistry is the study of neurochemicals; including transmitters, peptides, proteins, lipids, sugars, and nucleic acids; their interactions, and the roles they play in forming, maintaining, and modifying the nervous system.
Other fields include
agrochemistry,
astrochemistry (and
cosmochemistry),
atmospheric chemistry,
chemical engineering,
chemical biology,
chemo-informatics,
electrochemistry,
environmental chemistry,
femtochemistry,
flavor chemistry,
flow chemistry,
geochemistry,
green chemistry,
histochemistry,
history of chemistry,
hydrogenation chemistry,
immunochemistry,
marine chemistry,
materials science,
mathematical chemistry,
mechanochemistry,
medicinal chemistry,
molecular biology,
molecular mechanics,
nanotechnology,
natural product chemistry,
oenology,
organometallic chemistry,
petrochemistry,
pharmacology,
photochemistry,
physical organic chemistry,
phytochemistry,
polymer chemistry,
radiochemistry,
solid-state chemistry,
sonochemistry,
supramolecular chemistry,
surface chemistry,
synthetic chemistry,
thermochemistry, and many others.
Chemical industry
The
chemical industry represents an important economic activity. The global top 50 chemical producers in 2004 had sales of 587 billion
US dollars with a profit margin of 8.1% and
research and development spending of 2.1% of total chemical sales.
From Wikipedia
