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فرهادبیژنی

                  InthenameofGod

Skills to be tested

  1. Identify the central atom in a molecule containing more than two atoms as a start.
  2. Identify the number of valence electrons of any element. This concept is important, because you need to know the number of valence electrons in order to write a Lewis dot structure for the molecule in question.
  3. Count the number of VSEPR pairs or steric number (SN) for the central atom in a molecule. You need this number in order to describe or predict the shape of the molecule in question.
  4. Determine the number of lone electron pairs that are not shared with other atoms. Often, a Lewis dot structure is useful to help you count this number.
  5. Predict the shape of molecules or inos as the key concept of VSEPR theory. From the shape and by applying the idea that lone electron pairs takes up more space, you can predict the bond angles withing 5% of the observed values.
  6. Predict the values of bond angles and describe the hybrid orbitals used by the central atoms in the molecules or ions.

Valence-Shell Electron-Pair Repulsion (VSEPR) Models

The 3-dimensional structure of BF3 is different from PF3, and this is difficult to comphrend by considering their formulas alone. However, the Lewis dot structure for them are different, and the electron pair in :PF3 is the reason for its structure being different from BF3 (no lone pair).

Three-dimensional arrangements of atoms or bonds in molecules are important properties as are bondlengths, bond angles and bond energies. The Lewis dot symbols led us to see the non-bonding electron pairs, whose role in determining the shape of a molecule was examined by N.V. Sidgwick and H.E. Powell in 1940, and later by R.S. Nyholm and R.J. Gillespie. They have developed an extensive rationale called valence-shell electron-pair repulsion (VSEPR) model of molecular geometry.

Molecular shapes and steric numbers (SN)
Example SN Descriptor
BeCl2, CO2 2 Linear
BF3, SO3
SO2E, OO2E
3 Trigonal planar
bent
CH4
NH3E
H2OE2
4 Tetrahedral
pyramidal
bent
PF5
SF4E
ClF3E2
5 Trigonal bypyramidal
butterfly
T-shape
SF6, OIF5
BrF5E
XeF4E2
6 octahedral
pyramidal
square planar
E represents a lone electron pair.
SN is also called the number of VSEPR pairs
or number of electron pairs.
The Valence-Shell Electron-Pair Repulsion (VSEPR) models consider the unshared pairs (or lone electron pairs) and the bonding electrons. This considerations of lone and bonding electron pairs give an excellent explanation about the molecular shapes. The VSEPR model counts both bonding and nonbonding (lone) electron pairs, and call the total number of pairs the steric number (SN). If the element A has m atoms bonded to it and n nonbonding pairs, then

SN = m + n SN is useful for predicting shapes of molecules. If X is any atom bonded to A (in single, double, or tripple bond), a molecule may be represented by AXmEn where E denote a lone electron pair. This formula enable us to predict its geometry. The common SN, descriptor, and examples are given in the table on the right.

Note that the SN is also called the number of VSEPR pairs or number of electron pairs. The VSEPR model has another general rule:

lone pairs of electrons take up more space than bonded pairs making the bond angle, say H-O-H for water less than the tetrahedral angle of 109.5 °. Actually, the H-O-H angle in water is 105 °.
The geometry of the molecules with their SNs equal to 2 to 6 are given in the Table. The frist line for each is the sape including the lone electron pair(s). If the lone electron pairs are ignored, the geometry of the molecule is given by another descriptor.

To get an idea about the shapes of molecules and ions, three dimensional models are the best to use. However, good computer graphics sometimes also illustrate very well. The link VSEPR Illustration: View and manipulate molecular models gives excellent graphics, and you may enjoy seeing some of the graphics of the molecules.

Another link showing beautiful photographs of models of Molecular Geometry is also interesting. One of its photograph has been used in the previous page Hybrid orbitals.

VSEPR in French

Molecular database without transition elements is a very nice site in French, and it is very interesting to read for those who does not know French.

Confidence Building Questions

  • What is the central atom in CCl2O?
     

    Skill:
    Identify the central atom is a good start to draw a structure:

    Cl
      >C=O
    Cl
    

  • What is the number of valence electrons in carbon?
     

    Skill:
    Give the number of valence electrons of any main-group element.

  • What is the number of lone pair electron around the central atom S in SCl2O?
     

    Skill:
    Draw a Lewis dot structure, and express it in the form CCl2=O.

         ..
        :Cl:
     .  /
    :O=C      the structure of phosgene.
     '  \..
        :Cl:
    
    This carbonyl chloride is also called phosgene, a colourless, chemically reactive, highly toxic gas having an odour like that of musty hay. It was used during World War I against troops.

  • What is the central atom in SCl2O?
     

    Skill:
    Identify the central atom is a good start to draw a structure:

    Cl
      >S=O
    Cl
    

  • What is the number of valence electrons in sulfur?
     

    Skill:
    Give the number of valence electrons of any main-group element.

  • What is the number of lone pair electron around the central atom S in SCl2O?
     

    Skill:
    Draw a Lewis dot structure, and express in the form :SCl2=O or SCl2=OE.

        Cl
       /
    O=S:    the structure.
       \
        Cl
    

  • What is the number of lone pairs of electrons around O in H2O?
     

    Skill:
    The structure is

    H  ..
     >O
    H  ''

  • How many lone electron pairs are there around C in CO2?
     

    Skill:
    From the structure of O=C=O, figure out the number of lone electron pairs. CO2 is linear, so is H-CN.

  • What is the molecular shape of ClF3 (chlorine-trifluoride)?
     

    Skill:
    Predict the shape of a molecule ::ClF3 or ClF3E2.

         F
         |
    F---Cl---F
       .. ..
    

  • What is the bond angles (in degrees) in a CH4 molecule?
     

    Skill:
    Calculate bond angles from the geometry.

  • What is the shape of SF6? Give a geometric term for the configuration of the 6 F's around S.
     

    Skill:
    Apply terms linear, T shape, seesaw, trigonal planar, trigonal bipyramidal, tetrahedral, octahedral etc. to describe the geometric shapes of molecules.

 
+ نوشته شده در  دوشنبه بیست و هفتم تیر 1390ساعت 15:52  توسط فرهادبیژنی  | 

رسم ساختار لوویس :

ساختارهای لوویس برای نشان دادن توزیع الکترون های ظرفیت و تئوری VSEPR برای توصیف شکل مولکول هاست.

رسم ساختار لوویس :

1-    محاسبه ی مجموع الکترون های ظرفیت با جمع شماره گروه اتم ها. ( برای شماره گروه های    دورقمی از آخرین رقم استفاده می شود.)

2-    برای آنیون های چند اتمی بار منفی به تعداد الکترون ها اضافه شده و برای کاتیون های چند   اتمی بار مثبت از تعداد الکترون ها کم می شود.

3-    تعیین اتم مرکزی ,

        اتم مرکزی اغلب اولین اتمی است که در فرمول نوشته می شود مانند S در SO2                 . ( البته اتم با الکترونگاتیوی کم تر اتم مرکزی است.)                                                                                                  هیدروژن همواره اتم متصل به اتم مرکزی است، اکسیژن و هالوژن ها نیزمعمولا همین طور هستند.

4-  بین اتم ها با اتم مرکزی یک جفت الکترون پیوندی ( یا یک خط پیوند ) قرار می گیرد. الکترون های باقی مانده ابتدا در اطراف اتم های متصل به اتم مرکزی ( به جز هیدروژن ) قرار می گیرد تا قاعده ی اکتت رعایت شود و سپس اگر الکترونی باقی ماند بر روی اتم مرکزی.

5-    اگر اتم مرکزی هشتایی نشود یک یا چند جفت الکترون از اتم های اطراف برداشته شده و بین اتم های اطراف با اتم مرکزی پیوندهای دوگانه یا سه گانه برقرار می شود.

معمولا اتم های S , O , N , C تمایل به تشکیل پیوندهای دوگانه یا سه گانه دارند. نقض قاعده ی اکتت :

ترکیب هایی با یک الکترون  ( NO2 , NO )

اتم مرکزی با دو یا سه پیوند ( BeF2 , BF3 )

اتم مرکزی با بیش از چهار پیوند( PCl5 , SF6 , XeF4 )


+ نوشته شده در  دوشنبه بیست و هفتم تیر 1390ساعت 15:43  توسط فرهادبیژنی  | 

Chemistry

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Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions.
During chemical reactions, bonds between atoms break and form, resulting in different substances with different properties. In a blast furnace, iron oxide, a compound, reacts with carbon monoxide to form iron, one of the chemical elements, and carbon dioxide.

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 (or reaction), structure, and properties of matter, as well as the changes it undergoes during chemical reactions.[2] It is a physical science which studies various substances, atoms, molecules, crystals and other aggregates of matter whether in isolation or combination, and 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).

Contents

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Summary

Chemistry is the scientific study of interaction of chemical substances that are constituted of atoms or the subatomic particles:[3] protons, electrons and neutrons.[4] Atoms combine to produce molecules or crystals. Chemistry is sometimes 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.

Laboratory, Institute of Biochemistry, University of Cologne

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.

Democritus' atomist philosophy was later adopted by Epicurus (341–270 BCE).

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 in 50 BC.[13] Much of the early development of purification methods is described by Pliny the Elder in his Naturalis Historia.

A tentative outline is as follows:

  1. Egyptian alchemy [3,000 BCE – 400 BCE], formulate early "element" theories such as the Ogdoad.
  2. 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.
  3. Arab alchemy [642 CE – 1200], the Muslim conquest of Egypt; development of alchemy by Jābir ibn Hayyān, al-Razi and others; Jābir modifies Aristotle's theories; advances in processes and apparatus.[14]
  4. 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]
  5. Chemistry [1661], Boyle writes his classic chemistry text The Sceptical Chymist.
  6. Chemistry [1787], Lavoisier writes his classic Elements of Chemistry.
  7. Chemistry [1803], Dalton publishes his Atomic Theory.
  8. 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.

Antoine-Laurent de Lavoisier is considered the "Father of Modern Chemistry".[19]

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 word alchemy, an earlier set of practices that encompassed elements of chemistry, metallurgy, philosophy, astrology, astronomy, mysticism and medicine; it is commonly thought of as the quest to turn lead or another common starting material into gold.[24] The word alchemy in turn is derived from the Arabic word al-kīmīā (الكيمياء), meaning alchemy. The Arabic term is borrowed from the Greek χημία or χημεία.[25][26] This may have Egyptian origins. Many believe that al-kīmīā is derived from χημία, which is in turn derived from the word Chemi or Kimi, which is the ancient name of Egypt in Egyptian.[25] Alternately, al-kīmīā may be derived from χημεία, meaning "cast together".[27]

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 has changed over time, 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 mixed 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. Ninety–four 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

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)×1023 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)×1023 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)×1023 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/m3).

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 (PO43−).

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 (Ka), 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 Ka are more likely to donate hydrogen ions in chemical reactions than those with lower Ka values.

Phase

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

Atoms 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,  \Delta G \le 0 \,; 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 (H2O); a liquid at room temperature because its molecules are bound by hydrogen bonds.[46] Whereas hydrogen sulfide (H2S) 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

+ نوشته شده در  دوشنبه بیست و هفتم تیر 1390ساعت 15:39  توسط فرهادبیژنی  | 

شوخی های شیمیایی

درمسابقات آزمایشگاهی یکی از مدارس کشور دبیر شیمی از دانش آموزی پرسید : با طراحی یک آزمایش تعیین کن که چگونه می توان درصد شکر موجود در یک آدامس را مشخص کنیم ؟دانش آموز کمی فکر کرد و بعد گفت :این که کاری نداره  اول خودمونو وزن می کنیم بعد از جویدن کامل  آدامس وبی مزه شدنش آن را از دهان خارج نموده و دوباره خودمان را وزن می کنیم با استفاده از اضافه وزنمان نسبت به قبل از جویدن آدامس جرم شکر آن را بدست می آوریم ودر پایان درصد شکر را محاسبه می کنیم .

 

 

 

+ نوشته شده در  دوشنبه بیست و هفتم تیر 1390ساعت 15:24  توسط فرهادبیژنی  | 

                                                   به نام خدا

شیمی:

از ویکی‌پدیا، دانشنامهٔ آزاد
پرش به: ناوبری, جستجو
شیمی مطالعهٔ ساختار، خواص، ترکیبات، و تغییر شکل مواد است.

شیمی (به پارسی سره: کیمیا)[نیازمند منبع]و(به انگلیسی: Chemistry) مطالعهٔ ساختار، خواص، ترکیبات، و تغییر شکل مواد است. این علم مربوط می‌شود به عناصر شیمیایی و ترکیبات شیمیایی که شامل اتمها، مولکولها، و برهم‌کنش میان آنهاست.


محتویات

[نهفتن]

تاریخچه [ویرایش]

نوشتار اصلی: تاریخ شیمی

در اوایل، تلاش برای بیان طبیعت مواد و چگونگی دگرگونی آن‌ها ناموفق بود. دانش پیشرفته‌تر کیمیا نیز در این مورد ناتوان بود. به هرحال دانش کیمیا به کمک انجام تحقیقات اولیه و ثبت نتیجه‌ها، پایه‌گذار شیمی مدرن بود. تغییر نگرش در شناخت مواد زمانی شروع شد که رابرت بویل در سال ۱۶۶۱ در کتاب شیمی‌دان شکاک میان شیمی و کیمیا تفاوت قائل شد.[۱][۲] پس از آن شیمی با تلاش‌های آنتوان لاووازیه و ارائه قانون پایستگی جرم، به یک دانش تکامل‌یافته تبدیل شد. دغدغهٔ هر دو دانش کیمیا و شیمی شناخت طبیعت مواد و چگونگی دگرگونی آن‌ها بود، اما تنها شیمی از شیوه‌های علمی قوی بهره‌مند شد.[۲] با کوشش‌های ویژهٔ جوسایا ویلارد گیبز تاریخ شیمی با ترمودیناک رابطهٔ عمیقی پیدا کرد.[۳]

نگاه گذرا [ویرایش]

تئوری اتمی پایه و اساس علم شیمی است. این تئوری بیان می‌دارد که تمام مواد از واحدهای بسیار کوچکی به نام اتم تشکیل شده‌اند. یکی از اصول و قوانینی که در مطرح شدن شیمی به عنوان یک علم تأثیر به‌سزایی داشته، اصل بقای جرم است. این قانون بیان می‌کند که در طول انجام یک واکنش شیمیایی معمولی، مقدار ماده تغییر نمی‌کند. (امروزه فیزیک مدرن ثابت کرده که در واقع این انرژی است که بدون تغییر می‌ماند و همچنین انرژی و جرم با یکدیگر رابطه دارند.)

این مطلب به طور ساده به این معنی است که اگر ده‌هزار اتم داشته باشیم و مقدار زیادی واکنش شیمیایی انجام پذیرد، در پایان ما همچنان بطور دقیق ده‌هزار اتم خواهیم داشت. اگر انرژی از دست رفته یا به‌دست‌آمده را مد نظر قرار دهیم، مقدار جرم نیز تغییر نمی‌کند. شیمی کنش و واکنش میان اتم‌ها را به تنهایی یا در بیشتر موارد به‌همراه دیگر اتم‌ها و به‌صورت یون یا مولکول (ترکیب) بررسی می‌کند.

این اتم‌ها اغلب با اتم‌های دیگر واکنش‌هایی را انجام می‌دهند. (برای نمونه زمانی‌که آتش چوب را می‌سوزاند واکنشی است بین اتم‌های اکسیژن موجود در هوا و مواد آلی چوب. که نور بر روی مواد شیمیایی فیلم عکاسی ایجاد می‌کند شکل می‌گیرد.)

یکی از یافته‌های بنیادین و جالب دانش شیمی این بوده‌است که اتم‌ها روی‌هم‌رفته همیشه به نسبت برابر با یکدیگر ترکیب می‌شوند. سیلیس دارای ساختمانی است که نسبت اتم‌های سیلیسیوم به اکسیژن در آن یک به دو است. امروزه ثابت شده‌است که استثناهایی در زمینهٔ قانون نسبت‌های معین وجود دارد(مواد غیر استوکیومتری).

یکی دیگر از یافته‌های کلیدی شیمی این بود که زمانی که یک واکنش شیمیایی مشخص رخ می‌دهد، مقدار انرژی که بدست می‌آید یا از دست می‌رود همواره یکسان است. این امر ما را به مفاهیم مهمی مانند تعادل، ترمودینامیک می‌رساند.

شیمی فیزیک بر پایهٔ فیزیک پیشرفته (مدرن) بنا شده‌است. اصولاً می‌توان تمام سیستم‌های شیمیایی را با استفاده از تئوری مکانیک کوانتوم شرح داد. این تئوری از لحاظ ریاضی پیچیده بوده و عمیقاً شهودی است. به هر حال در عمل و بطور واقعی تنها بررسی سیستم‌های سادهٔ شیمیایی قابل بررسی با مفاهیم مکانیکی کوانتوم امکان‌پذیر است و در اکثر مواقع باید از تقریب استفاده کرد(مانند تئوری کاری دانسیته). بنابراین درک کامل مکانیک کوانتوم برای تمامی مباحث شیمی کاربرد ندارد؛ زیرا نتایج مهم این تئوری (بخصوص اربیتال اتمی) با استفاده از مفاهیم ساده‌تری قابل درک و به‌کارگیری هستند.

با اینکه در بسیاری موارد ممکن است مکانیک کوانتوم نادیده گرفته شود، اما از مفهوم اساسی آن، یعنی کوانتومی کردن انرژی، نمی‌توان صرف نظر کرد. شیمی‌دان‌ها برای بکارگیری کلیه روش‌های طیف نمایی به آثار و نتایج کوانتوم وابسته‌اند. علم فیزیک هم ممکن است مورد بی توجهی واقع شود، اما به هر حال برآیند نهایی آن (مانند رزونانس مغناطیسی هسته‌ای) پژوهیده و مطالعه می‌شود.

یکی دیگر از تئوری‌های اصلی فیزیک مدرن که نباید نادیده گرفته شود نظریه نسبیت است. این نظریه که از دیدگاه ریاضی پیچیده‌است، شرح کامل فیزیکی علم شیمی است. مفاهیم نسبیتی تنها در برخی از محاسبات خیلی دقیق ساختمان هسته، به‌ویژه در عناصر سنگین‌تر، کاربرد دارند و در عمل تقریباً با شیمی پیوند ندارند.

بخش‌های اصلی دانش شیمی عبارت‌اند از:

  • شیمی تجزیه، که به تعیین ترکیبات مواد و اجزای تشکیل دهنده آن‌ها می‌پردازد.
  • شیمی آلی، که به مطالعهٔ ترکیبات کربن‌دار، غیر از ترکیباتی چون دو اکسید کربن (دی اکسید کربن) می‌پردازد.
  • شیمی معدنی، که به اکثریت عناصری که در شیمی آلی روی آنها تاکید نشده و برخی خواص مولکولها می‌پردازد.
  • شیمی فیزیک، که پایه و اساس کلیهٔ شاخه‌های دیگر را تشکیل می‌دهد، و شامل ویژگی‌های فیزیکی مواد و ابزار تئوری بررسی آنهاست.

دیگر رشته‌های مطالعاتی و شاخه‌های تخصصی که با شیمی پیوند دارند عبارت‌اند از: علم مواد، مهندسی شیمی، شیمی بسپار، شیمی محیط زیست و داروسازی.

شاخه‌های شیمی [ویرایش]

جستارهای وابسته [ویرایش]

+ نوشته شده در  دوشنبه بیست و هفتم تیر 1390ساعت 14:46  توسط فرهادبیژنی  |