Chemistry Topics

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Chemistry Topics

Chemistry

Kimia and Greek χημεία khēmeía meaning ایمیک Chemistry (from Persian language

“alchemy”) is the science of matter at the atomic to molecular scale, dealing primarily with collections of atoms, such as gases, molecules, crystals, and metals. Chemistry deals with the composition and statistical properties of such structures, as well as their transformations and interactions to become materials encountered in everyday life. Chemistry also deals with understanding the properties and interactions of individual atoms with the purpose of applying that knowledge at macroscopic levels. According to modern chemistry, the physical properties of materials are generally determined by their structure at the atomic scale, which is itself defined by interatomic electromagnetic forces and laws of quantum mechanics.

Introduction

Chemistry is often called the “central science” because it connects other sciences, such as physics, material science, nanotechnology, biology, pharmacy, bioinformatics, and geology.

These connections are formed through various sub-disciplines that utilize concepts from multiple scientific disciplines. For example, physical chemistry involves applying the principles of physics to materials at the atomic and molecular level. The precise nature of the theoretical connection that chemistry (along with the other so-called special sciences) has with physics is a matter of research in philosophy of science.

© 2014 All Star Training, Inc. Page 1 Chemistry pertains to the interactions of matter. These interactions may be between two material substances or between matter and energy, especially in conjunction with the First Law of

Thermodynamics. Traditional chemistry involves interactions between substances in chemical reactions, where one or more substances become one or more other substances. Sometimes these reactions are driven by energetic (enthalpic) considerations, such as when two highly energetic substances such as elemental hydrogen and oxygen react to form the less energetic substance water. Chemical reactions may be facilitated by a catalyst, which is generally another chemical substance present within the reaction media but unconsumed (such as sulfuric acid catalyzing the electrolysis of water) or a non-material phenomenon (such as electromagnetic radiation in photochemical reactions). Traditional chemistry also deals with the analysis of chemicals both in and apart from a reaction, as in spectroscopy.

All ordinary matter consists of atoms or the subatomic components that make up atoms; protons, electrons and neutrons. Atoms may be combined to produce more complex forms of matter such as ions, molecules or crystals. The structure of the world we commonly experience and the properties of the matter we commonly interact with are determined by properties of chemical substances and their interactions. 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.

Substances tend to be classified in terms of their energy or phase as well as their chemical compositions. The three phases of matter at low energy are Solid, Liquid and Gas. Solids have fixed structures at room temperature which can resist gravity and other weak forces attempting to

© 2014 All Star Training, Inc. Page 2 rearrange them, due to their tight bonds. Liquids have limited bonds, with no structure and flow with gravity. Gases have no bonds and act as free particles. Another way to view the three phases is by volume and shape: roughly speaking, solids have fixed volume and shape, liquids have fixed volume but no fixed shape, and gases have neither fixed volume nor fixed shape.

Water (H2O) is a liquid at room temperature because its molecules are bound by intermolecular forces called Hydrogen bonds. Hydrogen sulfide (H2S) on the other hand is a gas at room temperature and standard pressure, as its molecules are bound by weaker dipole-dipole interactions. The hydrogen bonds in water have enough energy to keep the water molecules from separating from each other but not from sliding around, making it a liquid at temperatures between 0 °C and 100 °C at sea level. Lowering the temperature or energy further, allows for a tighter organization to form, creating a solid, and releasing energy. Increasing the energy (see heat of fusion) will melt the ice although the temperature will not change until all the ice is melted. Increasing the temperature of the water will eventually cause boiling when there is enough energy to overcome the polar attractions between individual water molecules (100 °C at 1 atmosphere of pressure), allowing the H2O molecules to disperse enough to be a gas. Note that in each case there is energy required to overcome the intermolecular attractions and thus allow the molecules to move away from each other.

Scientists who study chemistry are known as chemists. Most chemists specialize in one or more sub-disciplines. The chemistry taught at the high school or early college level is often called

© 2014 All Star Training, Inc. Page 3 “general chemistry” and is intended to be an introduction to a wide variety of fundamental concepts and to give the student the tools to continue on to more advanced subjects. Many concepts presented at this level are often incomplete and technically inaccurate, yet they are of extraordinary utility. Chemists regularly use these simple, elegant tools and explanations in their work because they have been proven to accurately model a very wide array of chemical reactivity, are generally sufficient, and more precise solutions may be prohibitively difficult to obtain.

The science of chemistry is historically a recent development but has its roots in alchemy which has been practiced for millennia throughout the world. The word chemistry is directly derived from the word alchemy; however, the etymology of alchemy is unclear.

History

Robert Boyle - founder of modern chemistry through use of controlled experiments, as contrasted with earlier rudimentary alchemical methods.

The roots of chemistry can be traced to several phenomena. First is that of burning. This led to metallurgy. First, metals were purified from their ores, and later on alloys were created as a means of strengthening metals. This was a process that happened over thousands of years.

Gold had been purified long before the first alloys were created. However, the underlying process for purifying gold was not well understood. It was thought to be a transformation rather than purification. Many scholars in those days thought it reasonable to find a means for

© 2014 All Star Training, Inc. Page 4 transforming cheaper (base) metals into gold. This led to the rise of alchemy, and the search for the Philosopher’s Stone, believed to help create such a transformation.

Another force gave rise to alchemy: the plagues and

blights that rocked Europe during what have been called

the Dark Ages. This gave rise to a need for medicines. It

was thought that there might exist a cure-all for all disease,

called the Elixir of Life. However, like the Philosopher’s

Stone, neither one were ever found.

Alchemy for many was an avenue for charlatans to create fake medicines and counterfeit money.

For others, it was an intellectual pursuit that could not separate superstition from scientific inquiry. Over time, practitioners got better at it. Paracelsus (1493-1541) rejected the 4-element 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.

Following the influences of philosophers such as Sir Francis Bacon (1561-1626) and René

Descartes (1596-1650), a scientific revolution ensued. These philosophers demanded more rigor in mathematics and in removing bias from scientific observations. In chemistry, this began with

Robert Boyle (1627-1691), who discovered gases, and came up with equations that were known as Boyle’s Law. The person celebrated as the Father of Chemistry was Antoine Lavoisier (1743-

1794), who developed the theory of Conservation of mass in 1783. Equally important was the development of the Atomic Theory, principly by John Dalton (1766-1844) around 1800.

The discoveries of the chemical elements has a long history from the days of alchemy and culminating in the creation of the periodic table of the chemical elements by Dmitri Mendeleyev

(1834-1907). The Nobel Prize in Chemistry created in 1901 gives an excellent overview of chemical discovery in the past 100 years.

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, for example, are some of the standard definitions used by various noted chemists:

. Alchemy (330) – the study of the composition of waters, movement, growth, embodying

and disembodying, drawing the spirits from bodies and bonding the spirits within bodies

(Zosimos).

. Chymistry (1661) – the subject of the material principles of mixt bodies (Boyle).

. Chemistry (1837) – the science concerned with the laws and effects of molecular forces

(Dumas).

. Chemistry (1947) – the science of substances: their

structure, their properties, and the reactions that change

them into other substances (Pauling).

. Chemistry (1998) – the study of matter and the changes it

undergoes (Chang).

Chemical Industry

© 2014 All Star Training, Inc. Page 6 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.

Subdisciplines

Chemistry typically is divided into several major sub-disciplines. There are also several main cross-disciplinary and more specialized fields of chemistry.

. 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.

. Biochemistry is the study of the chemicals, chemical reactions and chemical interactions

that take place in living organisms. Biochemistry and organic chemistry are closely

related, as in medicinal chemistry or neurochemistry. Biochemistry is also associated

with molecular biology and genetics.

. 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.

. Organic chemistry is the study of the

structure, properties, composition,

mechanisms, and reactions of organic

compounds. An organic compound is

defined as any compound based on a carbon skeleton.

systems and processes. In particular, the energetics and dynamics of such systems and

processes are of interest to physical chemists. Important areas of study include chemical

thermodynamics, chemical kinetics, electrochemistry, statistical mechanics, and

spectroscopy. Physical chemistry has large overlap with molecular physics. Physical

chemistry involves the use of calculus in deriving equations. It is usually associated with

quantum chemistry and theoretical chemistry. Physical chemistry is a distinct discipline

from chemical physics.

. Theoretical chemistry is the study of chemistry via fundamental theoretical reasoning

(usually within mathematics or physics). In particular the application of quantum

mechanics to chemistry is called quantum chemistry. Since the end of the Second World

War, the development of computers has allowed a systematic development of

computational chemistry, which is the art of developing and applying computer programs

for solving chemical problems. Theoretical chemistry has large overlap with (theoretical

and experimental) condensed matter physics and molecular physics. Essentially from

reductionism theoretical chemistry is just physics, just like fundamental biology is just

chemistry and physics.

. Nuclear chemistry is the study of how subatomic particles come together and make

nuclei. Modern Transmutation is a large component of nuclear chemistry, and the table of

nuclides is an important result and tool for this field.

Fundamental Concepts

Nomenclature

© 2014 All Star Training, Inc. Page 8 Nomenclature refers to the system for naming chemical compounds. There are well-defined systems in place for naming chemical species. Organic compounds are named according to the organic nomenclature system. Inorganic compounds are named according to the inorganic nomenclature system.

Atoms

An atom is a collection of matter consisting 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 portion into which an element can be divided and still retain its properties, made up of a dense, positively charged nucleus surrounded by a system of electrons.

Elements

An element is a class of atoms which have the same number of protons in the nucleus. 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.

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,

Ions

An ion is a charged species, or 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 neutral salts (e.g. sodium chloride NaCl). Examples of polyatomic

− 3− ions that do not split up during acid-base reactions are hydroxide (OH ) and phosphate (PO4 ).

Chemical Compound

A chemical compound is a chemical substance consisting of two or more different chemically bonded chemical elements, with a fixed ratio determining the composition. The ratio of each element is usually expressed by chemical formula. For example, water (H2O) is a compound consisting of two hydrogen atoms bonded to an oxygen atom.

The atoms within a compound can be held together by a variety of interactions, ranging from covalent bonds to electrostatic forces in ionic bonds. A continuum of bond polarities exist between the purely covalent bond (as in H2) and ionic bonds. For example H2O is held together by polar covalent bonds. Sodium chloride is an example of an ionic compound.

Formulas

Chemists describe compounds using formula in various formats. For molecules, the formula for the molecular unit is shown. For polymeric materials, such as minerals and many metal oxides,

of the elements in molecular and empirical formulas is C, then H

and then alphabetical. Trifluoroacetic acid is thus described as

C2HF3O2. More descriptive formulas convey structure information, illustrated again with trifluoroacetic acid. CF3CO2H. On the other hand, formulas for inorganic compounds often do not convey structural information, as illustrated by H2SO4 for a molecule that has no H-S bonds. A more descriptive presentation would be O2S(OH)2.

Phases and Thermal Properties

Compounds may have several possible phases. All compounds can exist as solids, at least at low enough temperatures. Molecular compounds may also exist as liquids, gases, and, in some cases, even plasmas. All compounds decompose upon applying heat. The temperature at which such fragmentation occurs is often called the decomposition temperature. Decomposition temperatures are not sharp and depend on the rate of heating. At sufficiently high temperatures, all compounds, either after they have decomposed somehow or in the act of decomposing, fragment into smaller compounds or to individual atoms.

Molecule

In science, a molecule is a combination of two or more atoms in a definite arrangement held together by chemical bonds. Chemical substances are not infinitely divisible into smaller fractions of the same substance: a molecule is generally considered the smallest particle of a pure substance that still retains its composition and chemical properties. Certain pure substances (e.g.,

In the molecular sciences, a “molecule” is a sufficiently stable, electrically neutral entity composed of two or more atoms The concept of a single-atom or monatomic molecule, as found in noble gases, is used almost exclusively in the kinetic theory of gases, where the fundamental gas particles are conventionally termed “molecules” regardless of their composition.

History

Although the concept of molecules was first introduced in 1811 by Avogadro, and was accepted by many chemists as a result of Dalton’s laws of Definite and Multiple Proportions (1803-1808), with notable exceptions (Boltzmann, Maxwell, Gibbs), the existence of molecules as anything other than convenient mathematical constructs was still an open debate in the physics community until the work of Perrin (1911), and was strenuously resisted by early positivists such as Mach. The modern theory of molecules makes great use of the many numerical techniques offered by computational chemistry. Dozens of molecules have now been identified in interstellar space by microwave spectroscopy.

Overview

The science of molecules is called molecular chemistry or molecular physics, depending on the focus. Molecular chemistry deals with the laws governing the interaction between molecules that results in the formation and breakage of chemical bonds,

© 2014 All Star Training, Inc. Page 12 while molecular physics deals with the laws governing their structure and properties. In practice, however, this distinction is vague. In molecular sciences, a molecule consists of a stable system

(bound state) comprising two or more atoms. Polyatomic ions may sometimes be usefully thought of as electrically charged molecules. The term unstable molecule is used for very reactive species, i.e., short-lived assemblies (resonances) of electrons and nuclei, such as radicals, molecular ions, Rydberg molecules, transition states, Van der Waals complexes, or systems of colliding atoms as in Bose-Einstein condensates.

A peculiar use of the term molecular is as a synonym to covalent, which arises from the fact that, unlike molecular covalent compounds, ionic compounds do not yield well-defined smallest particles that would be consistent with the definition above. However, the same problem also arises for some (but not all) covalent compounds. No typical “smallest particle” can be defined for covalent crystals, or network solids, which are composed of repeating unit cells that extend indefinitely either in a plane (such as in graphite) or three-dimensionally (such as in diamond).

While all gases exist as molecules by definition (as the term for gas particles), not all solids and liquids do. In fact, many of the most familiar substances in ordinary experience, such as rocks, crystals, and metals, are composed of atoms or ions, but are not made of molecules.

In a molecule, the atoms are joined by shared pairs of electrons in a chemical bond. It may consist of atoms of the same chemical element, as with oxygen (O2), or of different elements, as with water (H2O).

Molecular Size

© 2014 All Star Training, Inc. Page 13 Most molecules are far too small to be seen with the naked eye, but there are exceptions. DNA, a macromolecule, can reach macroscopic sizes, as can molecules of many polymers. The smallest

+ of all molecules is the hydrogen ion molecule H2 , comprised of two protons bonded together by the sharing of one electron. The next largest molecule is the hydrogen molecule H2, with a length roughly twice the 74 picometres distance between the two hydrogen nuclei; but as with all molecules, however, the exact size of its electron cloud is difficult to define precisely. Single molecules cannot usually be observed by light (as noted above), but small molecules and even the outlines of individual atoms may be traced in some circumstances by use of an atomic force microscope. Some of the largest molecules are supermolecules.

Molecular Formula

The empirical formula of a molecule is the simplest integer ratio of the chemical elements that constitute the compound. For example, in their pure forms, water is always composed of a 2:1 ratio of hydrogen to oxygen, and ethyl alcohol or ethanol is always composed of carbon, hydrogen, and oxygen in a 2:6:1 ratio. However, this does not determine the kind of molecule uniquely - dimethyl ether has the same ratio as ethanol, for instance. Molecules with the same atoms in different arrangements are called isomers. The empirical formula is often the same as the molecular formula but not always. For example the molecule acetylene has molecular formula C2H2, but the simplest integer ratio of elements is CH.

The molecular formula reflects the exact number of atoms that compose a molecule.

© 2014 All Star Training, Inc. Page 14 The molecular mass can be calculated from the chemical formula and is expressed in conventional atomic mass units equal to 1/12th of the mass of a neutral carbon-12 (12C isotope) atom. For network solids, the term formula unit is used in stoichiometric calculations.

Molecular Geometry

Molecules have fixed equilibrium geometries—bond lengths and angles— about which they continuously oscillate through vibrational and rotational motions. A pure substance is composed of molecules with the same average geometrical structure. The chemical formula and the structure of a molecule are the two important factors that determine its properties, particularly its reactivity. Isomers share a chemical formula but normally have very different properties because of their different structures. Stereoisomers, a particular type of isomers, may have very similar physico-chemical properties and at the same time very different biochemical activities.

Molecular Spectroscopy

Molecular spectroscopy deals with the response (spectrum) of molecules interacting with probing signals of known energy (or frequency, according to Planck’s formula). Scattering theory provides the theoretical background for spectroscopy.

The probing signal used in spectroscopy can be an electromagnetic wave or a beam of particles

(electrons, positrons, etc.) The molecular response can consist of signal absorption (absorption spectroscopy), the emission of another signal (emission spectroscopy), fragmentation, or chemical changes.

© 2014 All Star Training, Inc. Page 15 Spectroscopy is recognized as a powerful tool in investigating the microscopic properties of molecules, in particular their energy levels. In order to extract maximum microscopic information from experimental results, spectroscopy is often coupled with chemical computations.

Substance

A chemical substance can be an element, compound or a mixture of compounds, elements or compounds and elements.

Most of the matter we encounter in our daily life are one or another kind of mixtures, e.g. air, alloys, biomass etc.