79-589: Phosfolan ( chemical formula : C 7 H 14 NO 3 PS 2 ) is a chemical compound used as an insecticide . This article about an organic compound is a stub . You can help Misplaced Pages by expanding it . Chemical formula A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule , using chemical element symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, commas and plus (+) and minus (−) signs. These are limited to
158-498: A double bond connects the atoms on either side of them. A triple bond may be expressed with three lines ( HC≡CH ) or three pairs of dots ( HC:::CH ), and if there may be ambiguity, a single line or pair of dots may be used to indicate a single bond. Molecules with multiple functional groups that are the same may be expressed by enclosing the repeated group in round brackets . For example, isobutane may be written (CH 3 ) 3 CH . This condensed structural formula implies
237-470: A double displacement reaction , the anions and cations of two compounds switch places and form two entirely different compounds. These reactions are in the general form: AB + CD ⟶ AD + CB {\displaystyle {\ce {AB + CD->AD + CB}}} For example, when barium chloride (BaCl 2 ) and magnesium sulfate (MgSO 4 ) react, the SO 4 anion switches places with
316-434: A first-order reaction , which could be the disintegration of a substance A, is given by: Its integration yields: Here k is the first-order rate constant, having dimension 1/time, [A]( t ) is the concentration at a time t and [A] 0 is the initial concentration. The rate of a first-order reaction depends only on the concentration and the properties of the involved substance, and the reaction itself can be described with
395-736: A single displacement reaction , a single uncombined element replaces another in a compound; in other words, one element trades places with another element in a compound These reactions come in the general form of: A + BC ⟶ AC + B {\displaystyle {\ce {A + BC->AC + B}}} One example of a single displacement reaction is when magnesium replaces hydrogen in water to make solid magnesium hydroxide and hydrogen gas: Mg + 2 H 2 O ⟶ Mg ( OH ) 2 ↓ + H 2 ↑ {\displaystyle {\ce {Mg + 2H2O->Mg(OH)2 (v) + H2 (^)}}} In
474-620: A structural formula is useful, as it illustrates which atoms are bonded to which other ones. From the connectivity, it is often possible to deduce the approximate shape of the molecule . A condensed (or semi-structural) formula may represent the types and spatial arrangement of bonds in a simple chemical substance, though it does not necessarily specify isomers or complex structures. For example, ethane consists of two carbon atoms single-bonded to each other, with each carbon atom having three hydrogen atoms bonded to it. Its chemical formula can be rendered as CH 3 CH 3 . In ethylene there
553-623: A "vital force" and distinguished from inorganic materials. This separation was ended however by the synthesis of urea from inorganic precursors by Friedrich Wöhler in 1828. Other chemists who brought major contributions to organic chemistry include Alexander William Williamson with his synthesis of ethers and Christopher Kelk Ingold , who, among many discoveries, established the mechanisms of substitution reactions . The general characteristics of chemical reactions are: Chemical equations are used to graphically illustrate chemical reactions. They consist of chemical or structural formulas of
632-536: A characteristic half-life . More than one time constant is needed when describing reactions of higher order. The temperature dependence of the rate constant usually follows the Arrhenius equation : where E a is the activation energy and k B is the Boltzmann constant . One of the simplest models of reaction rate is the collision theory . More realistic models are tailored to a specific problem and include
711-443: A chemical formula is complicated by being written as a condensed formula (or condensed molecular formula, occasionally called a "semi-structural formula"), which conveys additional information about the particular ways in which the atoms are chemically bonded together, either in covalent bonds , ionic bonds , or various combinations of these types. This is possible if the relevant bonding is easy to show in one dimension. An example
790-735: A correct structural formula. For example, ethanol may be represented by the condensed chemical formula CH 3 CH 2 OH , and dimethyl ether by the condensed formula CH 3 OCH 3 . These two molecules have the same empirical and molecular formulae ( C 2 H 6 O ), but may be differentiated by the condensed formulae shown, which are sufficient to represent the full structure of these simple organic compounds . Condensed chemical formulae may also be used to represent ionic compounds that do not exist as discrete molecules, but nonetheless do contain covalently bound clusters within them. These polyatomic ions are groups of atoms that are covalently bound together and have an overall ionic charge, such as
869-499: A different connectivity from other molecules that can be formed using the same atoms in the same proportions ( isomers ). The formula (CH 3 ) 3 CH implies a central carbon atom connected to one hydrogen atom and three methyl groups ( CH 3 ). The same number of atoms of each element (10 hydrogens and 4 carbons, or C 4 H 10 ) may be used to make a straight chain molecule, n - butane : CH 3 CH 2 CH 2 CH 3 . The alkene called but-2-ene has two isomers, which
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#1732780822377948-399: A few molecules, usually one or two, because of the low probability for several molecules to meet at a certain time. The most important elementary reactions are unimolecular and bimolecular reactions. Only one molecule is involved in a unimolecular reaction; it is transformed by isomerization or a dissociation into one or more other molecules. Such reactions require the addition of energy in
1027-405: A fire-like element called "phlogiston", which was contained within combustible bodies and released during combustion . This proved to be false in 1785 by Antoine Lavoisier who found the correct explanation of the combustion as a reaction with oxygen from the air. Joseph Louis Gay-Lussac recognized in 1808 that gases always react in a certain relationship with each other. Based on this idea and
1106-479: A formula might be written using decimal fractions , as in Fe 0.95 O , or it might include a variable part represented by a letter, as in Fe 1− x O , where x is normally much less than 1. A chemical formula used for a series of compounds that differ from each other by a constant unit is called a general formula . It generates a homologous series of chemical formulae. For example, alcohols may be represented by
1185-521: A given temperature and chemical concentration. Some reactions produce heat and are called exothermic reactions , while others may require heat to enable the reaction to occur, which are called endothermic reactions . Typically, reaction rates increase with increasing temperature because there is more thermal energy available to reach the activation energy necessary for breaking bonds between atoms. A reaction may be classified as redox in which oxidation and reduction occur or non-redox in which there
1264-400: A molecular formula of C 6 H 14 , and (for one of its isomers, n-hexane) a structural formula CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 , implying that it has a chain structure of 6 carbon atoms, and 14 hydrogen atoms. However, the empirical formula for hexane is C 3 H 7 . Likewise the empirical formula for hydrogen peroxide , H 2 O 2 , is simply HO , expressing
1343-482: A molecule than its empirical formula, but is more difficult to establish. In addition to indicating the number of atoms of each elementa molecule, a structural formula indicates how the atoms are organized, and shows (or implies) the chemical bonds between the atoms. There are multiple types of structural formulas focused on different aspects of the molecular structure. The two diagrams show two molecules which are structural isomers of each other, since they both have
1422-438: A neutral radical . In the second case, both electrons of the chemical bond remain with one of the products, resulting in charged ions . Dissociation plays an important role in triggering chain reactions , such as hydrogen–oxygen or polymerization reactions. For bimolecular reactions, two molecules collide and react with each other. Their merger is called chemical synthesis or an addition reaction . Another possibility
1501-503: A number of other sugars , including fructose , galactose and mannose . Linear equivalent chemical names exist that can and do specify uniquely any complex structural formula (see chemical nomenclature ), but such names must use many terms (words), rather than the simple element symbols, numbers, and simple typographical symbols that define a chemical formula. Chemical formulae may be used in chemical equations to describe chemical reactions and other chemical transformations, such as
1580-432: A prefixed superscript in a chemical formula. For example, the phosphate ion containing radioactive phosphorus-32 is [ PO 4 ] . Also a study involving stable isotope ratios might include the molecule O O . A left-hand subscript is sometimes used redundantly to indicate the atomic number . For example, 8 O 2 for dioxygen, and 8 O 2 for the most abundant isotopic species of dioxygen. This
1659-415: A sequence of individual sub-steps, the so-called elementary reactions , and the information on the precise course of action is part of the reaction mechanism . Chemical reactions are described with chemical equations , which symbolically present the starting materials, end products, and sometimes intermediate products and reaction conditions. Chemical reactions happen at a characteristic reaction rate at
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#17327808223771738-617: A single typographic line of symbols, which may include subscripts and superscripts . A chemical formula is not a chemical name since it does not contain any words. Although a chemical formula may imply certain simple chemical structures , it is not the same as a full chemical structural formula . Chemical formulae can fully specify the structure of only the simplest of molecules and chemical substances , and are generally more limited in power than chemical names and structural formulae. The simplest types of chemical formulae are called empirical formulae , which use letters and numbers indicating
1817-453: A true structural formula, which is a graphical representation of the spatial relationship between atoms in chemical compounds (see for example the figure for butane structural and chemical formulae, at right). For reasons of structural complexity, a single condensed chemical formula (or semi-structural formula) may correspond to different molecules, known as isomers . For example, glucose shares its molecular formula C 6 H 12 O 6 with
1896-413: Is boron carbide , whose formula of CB n is a variable non-whole number ratio with n ranging from over 4 to more than 6.5. When the chemical compound of the formula consists of simple molecules , chemical formulae often employ ways to suggest the structure of the molecule. These types of formulae are variously known as molecular formulae and condensed formulae . A molecular formula enumerates
1975-420: Is a double bond between the carbon atoms (and thus each carbon only has two hydrogens), therefore the chemical formula may be written: CH 2 CH 2 , and the fact that there is a double bond between the carbons is implicit because carbon has a valence of four. However, a more explicit method is to write H 2 C=CH 2 or less commonly H 2 C::CH 2 . The two lines (or two pairs of dots) indicate that
2054-444: Is another way to identify a synthesis reaction. One example of a synthesis reaction is the combination of iron and sulfur to form iron(II) sulfide : 8 Fe + S 8 ⟶ 8 FeS {\displaystyle {\ce {8Fe + S8->8FeS}}} Another example is simple hydrogen gas combined with simple oxygen gas to produce a more complex substance, such as water. A decomposition reaction
2133-440: Is convenient when writing equations for nuclear reactions , in order to show the balance of charge more clearly. The @ symbol ( at sign ) indicates an atom or molecule trapped inside a cage but not chemically bound to it. For example, a buckminsterfullerene ( C 60 ) with an atom (M) would simply be represented as MC 60 regardless of whether M was inside the fullerene without chemical bonding or outside, bound to one of
2212-559: Is endothermic at low temperatures, becoming less so with increasing temperature. Δ H ° is zero at 1855 K , and the reaction becomes exothermic above that temperature. Changes in temperature can also reverse the direction tendency of a reaction. For example, the water gas shift reaction is favored by low temperatures, but its reverse is favored by high temperatures. The shift in reaction direction tendency occurs at 1100 K . Reactions can also be characterized by their internal energy change, which takes into account changes in
2291-499: Is no oxidation and reduction occurring. Most simple redox reactions may be classified as a combination, decomposition, or single displacement reaction. Different chemical reactions are used during chemical synthesis in order to obtain the desired product. In biochemistry , a consecutive series of chemical reactions (where the product of one reaction is the reactant of the next reaction) form metabolic pathways . These reactions are often catalyzed by protein enzymes . Enzymes increase
2370-576: Is possible to collate chemical formulae into what is known as Hill system order. The Hill system was first published by Edwin A. Hill of the United States Patent and Trademark Office in 1900. It is the most commonly used system in chemical databases and printed indexes to sort lists of compounds. A list of formulae in Hill system order is arranged alphabetically, as above, with single-letter elements coming before two-letter symbols when
2449-427: Is released. Typical examples of exothermic reactions are combustion , precipitation and crystallization , in which ordered solids are formed from disordered gaseous or liquid phases. In contrast, in endothermic reactions, heat is consumed from the environment. This can occur by increasing the entropy of the system, often through the formation of gaseous or dissolved reaction products, which have higher entropy. Since
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2528-537: Is that only a portion of one molecule is transferred to the other molecule. This type of reaction occurs, for example, in redox and acid-base reactions. In redox reactions, the transferred particle is an electron, whereas in acid-base reactions it is a proton. This type of reaction is also called metathesis . for example Most chemical reactions are reversible; that is, they can and do run in both directions. The forward and reverse reactions are competing with each other and differ in reaction rates . These rates depend on
2607-454: Is the condensed molecular/chemical formula for ethanol , which is CH 3 −CH 2 −OH or CH 3 CH 2 OH . However, even a condensed chemical formula is necessarily limited in its ability to show complex bonding relationships between atoms, especially atoms that have bonds to four or more different substituents . Since a chemical formula must be expressed as a single line of chemical element symbols , it often cannot be as informative as
2686-413: Is used in retro reactions. The elementary reaction is the smallest division into which a chemical reaction can be decomposed, it has no intermediate products. Most experimentally observed reactions are built up from many elementary reactions that occur in parallel or sequentially. The actual sequence of the individual elementary reactions is known as reaction mechanism . An elementary reaction involves
2765-532: Is when a more complex substance breaks down into its more simple parts. It is thus the opposite of a synthesis reaction and can be written as AB ⟶ A + B {\displaystyle {\ce {AB->A + B}}} One example of a decomposition reaction is the electrolysis of water to make oxygen and hydrogen gas: 2 H 2 O ⟶ 2 H 2 + O 2 {\displaystyle {\ce {2H2O->2H2 + O2}}} In
2844-497: The Le Chatelier's principle . For example, an increase in pressure due to decreasing volume causes the reaction to shift to the side with fewer moles of gas. The reaction yield stabilizes at equilibrium but can be increased by removing the product from the reaction mixture or changed by increasing the temperature or pressure. A change in the concentrations of the reactants does not affect the equilibrium constant but does affect
2923-458: The contact process in the 1880s, and the Haber process was developed in 1909–1910 for ammonia synthesis. From the 16th century, researchers including Jan Baptist van Helmont , Robert Boyle , and Isaac Newton tried to establish theories of experimentally observed chemical transformations. The phlogiston theory was proposed in 1667 by Johann Joachim Becher . It postulated the existence of
3002-471: The stoichiometry , the number of atoms of each species should be the same on both sides of the equation. This is achieved by scaling the number of involved molecules (A, B, C and D in a schematic example below) by the appropriate integers a, b, c and d . More elaborate reactions are represented by reaction schemes, which in addition to starting materials and products show important intermediates or transition states . Also, some relatively minor additions to
3081-523: The sulfate [SO 4 ] ion. Each polyatomic ion in a compound is written individually in order to illustrate the separate groupings. For example, the compound dichlorine hexoxide has an empirical formula ClO 3 , and molecular formula Cl 2 O 6 , but in liquid or solid forms, this compound is more correctly shown by an ionic condensed formula [ClO 2 ] [ClO 4 ] , which illustrates that this compound consists of [ClO 2 ] ions and [ClO 4 ] ions. In such cases,
3160-562: The transition state theory , the calculation of the potential energy surface , the Marcus theory and the Rice–Ramsperger–Kassel–Marcus (RRKM) theory . In a synthesis reaction, two or more simple substances combine to form a more complex substance. These reactions are in the general form: A + B ⟶ AB {\displaystyle {\ce {A + B->AB}}} Two or more reactants yielding one product
3239-486: The 17th century, Johann Rudolph Glauber produced hydrochloric acid and sodium sulfate by reacting sulfuric acid and sodium chloride . With the development of the lead chamber process in 1746 and the Leblanc process , allowing large-scale production of sulfuric acid and sodium carbonate , respectively, chemical reactions became implemented into the industry. Further optimization of sulfuric acid technology resulted in
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3318-486: The 1:1 ratio of component elements. Formaldehyde and acetic acid have the same empirical formula, CH 2 O . This is also the molecular formula for formaldehyde, but acetic acid has double the number of atoms. Like the other formula types detailed below, an empirical formula shows the number of elements in a molecule, and determines whether it is a binary compound , ternary compound , quaternary compound , or has even more elements. Molecular formulae simply indicate
3397-555: The 2Cl anion, giving the compounds BaSO 4 and MgCl 2 . Another example of a double displacement reaction is the reaction of lead(II) nitrate with potassium iodide to form lead(II) iodide and potassium nitrate : Pb ( NO 3 ) 2 + 2 KI ⟶ PbI 2 ↓ + 2 KNO 3 {\displaystyle {\ce {Pb(NO3)2 + 2KI->PbI2(v) + 2KNO3}}} According to Le Chatelier's Principle , reactions may proceed in
3476-408: The atomic theory of John Dalton , Joseph Proust had developed the law of definite proportions , which later resulted in the concepts of stoichiometry and chemical equations . Regarding the organic chemistry , it was long believed that compounds obtained from living organisms were too complex to be obtained synthetically . According to the concept of vitalism , organic matter was endowed with
3555-468: The available resources used above in simple condensed formulae. See IUPAC nomenclature of organic chemistry and IUPAC nomenclature of inorganic chemistry 2005 for examples. In addition, linear naming systems such as International Chemical Identifier (InChI) allow a computer to construct a structural formula, and simplified molecular-input line-entry system (SMILES) allows a more human-readable ASCII input. However, all these nomenclature systems go beyond
3634-468: The carbon atoms. Using the @ symbol, this would be denoted M@C 60 if M was inside the carbon network. A non-fullerene example is [As@Ni 12 As 20 ] , an ion in which one arsenic (As) atom is trapped in a cage formed by the other 32 atoms. This notation was proposed in 1991 with the discovery of fullerene cages ( endohedral fullerenes ), which can trap atoms such as La to form, for example, La@C 60 or La@C 82 . The choice of
3713-399: The charge on a particular atom may be denoted with a right-hand superscript. For example, Na , or Cu . The total charge on a charged molecule or a polyatomic ion may also be shown in this way, such as for hydronium , H 3 O , or sulfate , SO 2− 4 . Here + and − are used in place of +1 and −1, respectively. For more complex ions, brackets [ ] are often used to enclose
3792-480: The chemical formula CH 3 CH=CHCH 3 does not identify. The relative position of the two methyl groups must be indicated by additional notation denoting whether the methyl groups are on the same side of the double bond ( cis or Z ) or on the opposite sides from each other ( trans or E ). As noted above, in order to represent the full structural formulae of many complex organic and inorganic compounds, chemical nomenclature may be needed which goes well beyond
3871-423: The chemical reactions of unstable and radioactive elements where both electronic and nuclear changes can occur. The substance (or substances) initially involved in a chemical reaction are called reactants or reagents . Chemical reactions are usually characterized by a chemical change , and they yield one or more products , which usually have properties different from the reactants. Reactions often consist of
3950-532: The concentration and therefore change with the time of the reaction: the reverse rate gradually increases and becomes equal to the rate of the forward reaction, establishing the so-called chemical equilibrium. The time to reach equilibrium depends on parameters such as temperature, pressure, and the materials involved, and is determined by the minimum free energy . In equilibrium, the Gibbs free energy of reaction must be zero. The pressure dependence can be explained with
4029-455: The condensed formula only need be complex enough to show at least one of each ionic species. Chemical formulae as described here are distinct from the far more complex chemical systematic names that are used in various systems of chemical nomenclature . For example, one systematic name for glucose is (2 R ,3 S ,4 R ,5 R )-2,3,4,5,6-pentahydroxyhexanal. This name, interpreted by the rules behind it, fully specifies glucose's structural formula, but
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#17327808223774108-551: The dissolving of ionic compounds into solution. While, as noted, chemical formulae do not have the full power of structural formulae to show chemical relationships between atoms, they are sufficient to keep track of numbers of atoms and numbers of electrical charges in chemical reactions, thus balancing chemical equations so that these equations can be used in chemical problems involving conservation of atoms, and conservation of electric charge. A chemical formula identifies each constituent element by its chemical symbol and indicates
4187-457: The entire formula of the ion with charge +3. This is strictly optional; a chemical formula is valid with or without ionization information, and Hexamminecobalt(III) chloride may be written as [Co(NH 3 ) 6 ] Cl − 3 or [Co(NH 3 ) 6 ]Cl 3 . Brackets, like parentheses, behave in chemistry as they do in mathematics, grouping terms together – they are not specifically employed only for ionization states. In
4266-455: The entropy term in the free-energy change increases with temperature, many endothermic reactions preferably take place at high temperatures. On the contrary, many exothermic reactions such as crystallization occur preferably at lower temperatures. A change in temperature can sometimes reverse the sign of the enthalpy of a reaction, as for the carbon monoxide reduction of molybdenum dioxide : This reaction to form carbon dioxide and molybdenum
4345-405: The entropy, volume and chemical potentials . The latter depends, among other things, on the activities of the involved substances. The speed at which reactions take place is studied by reaction kinetics . The rate depends on various parameters, such as: Several theories allow calculating the reaction rates at the molecular level. This field is referred to as reaction dynamics. The rate v of
4424-406: The equilibrium position. Chemical reactions are determined by the laws of thermodynamics . Reactions can proceed by themselves if they are exergonic , that is if they release free energy. The associated free energy change of the reaction is composed of the changes of two different thermodynamic quantities, enthalpy and entropy : Reactions can be exothermic , where Δ H is negative and energy
4503-448: The form of heat or light. A typical example of a unimolecular reaction is the cis–trans isomerization , in which the cis-form of a compound converts to the trans-form or vice versa. In a typical dissociation reaction, a bond in a molecule splits ( ruptures ) resulting in two molecular fragments. The splitting can be homolytic or heterolytic . In the first case, the bond is divided so that each product retains an electron and becomes
4582-402: The formula C n H 2 n + 1 OH ( n ≥ 1), giving the homologs methanol , ethanol , propanol for 1 ≤ n ≤ 3. The Hill system (or Hill notation) is a system of writing empirical chemical formulae, molecular chemical formulae and components of a condensed formula such that the number of carbon atoms in a molecule is indicated first, the number of hydrogen atoms next, and then
4661-407: The forward or reverse direction until they end or reach equilibrium . Reactions that proceed in the forward direction (from left to right) to approach equilibrium are often called spontaneous reactions , that is, Δ G {\displaystyle \Delta G} is negative, which means that if they occur at constant temperature and pressure, they decrease the Gibbs free energy of
4740-499: The ionic formula, as in [B 12 H 12 ] , which is found in compounds such as caesium dodecaborate , Cs 2 [B 12 H 12 ] . Parentheses ( ) can be nested inside brackets to indicate a repeating unit, as in Hexamminecobalt(III) chloride , [Co(NH 3 ) 6 ] Cl − 3 . Here, (NH 3 ) 6 indicates that the ion contains six ammine groups ( NH 3 ) bonded to cobalt , and [ ] encloses
4819-477: The latter case here, the parentheses indicate 6 groups all of the same shape, bonded to another group of size 1 (the cobalt atom), and then the entire bundle, as a group, is bonded to 3 chlorine atoms. In the former case, it is clearer that the bond connecting the chlorines is ionic , rather than covalent . Although isotopes are more relevant to nuclear chemistry or stable isotope chemistry than to conventional chemistry, different isotopes may be indicated with
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#17327808223774898-405: The name is not a chemical formula as usually understood, and uses terms and words not used in chemical formulae. Such names, unlike basic formulae, may be able to represent full structural formulae without graphs. In chemistry , the empirical formula of a chemical is a simple expression of the relative number of each type of atom or ratio of the elements in the compound. Empirical formulae are
4977-531: The number of all other chemical elements subsequently, in alphabetical order of the chemical symbols . When the formula contains no carbon, all the elements, including hydrogen, are listed alphabetically. By sorting formulae according to the number of atoms of each element present in the formula according to these rules, with differences in earlier elements or numbers being treated as more significant than differences in any later element or number—like sorting text strings into lexicographical order —it
5056-433: The number of atoms to reflect those in the molecule, so that the molecular formula for glucose is C 6 H 12 O 6 rather than the glucose empirical formula, which is CH 2 O . However, except for very simple substances, molecular chemical formulae lack needed structural information, and are ambiguous. For simple molecules, a condensed (or semi-structural) formula is a type of chemical formula that may fully imply
5135-494: The numbers of each type of atom in a molecule of a molecular substance. They are the same as empirical formulae for molecules that only have one atom of a particular type, but otherwise may have larger numbers. An example of the difference is the empirical formula for glucose, which is CH 2 O ( ratio 1:2:1), while its molecular formula is C 6 H 12 O 6 ( number of atoms 6:12:6). For water, both formulae are H 2 O . A molecular formula provides more information about
5214-409: The numerical proportions of atoms of each type. Molecular formulae indicate the simple numbers of each type of atom in a molecule, with no information on structure. For example, the empirical formula for glucose is CH 2 O (twice as many hydrogen atoms as carbon and oxygen ), while its molecular formula is C 6 H 12 O 6 (12 hydrogen atoms, six carbon and oxygen atoms). Sometimes
5293-603: The proportionate number of atoms of each element. In empirical formulae, these proportions begin with a key element and then assign numbers of atoms of the other elements in the compound, by ratios to the key element. For molecular compounds, these ratio numbers can all be expressed as whole numbers. For example, the empirical formula of ethanol may be written C 2 H 6 O because the molecules of ethanol all contain two carbon atoms, six hydrogen atoms, and one oxygen atom. Some types of ionic compounds, however, cannot be written with entirely whole-number empirical formulae. An example
5372-513: The rates of biochemical reactions, so that metabolic syntheses and decompositions impossible under ordinary conditions can occur at the temperature and concentrations present within a cell . The general concept of a chemical reaction has been extended to reactions between entities smaller than atoms, including nuclear reactions , radioactive decays and reactions between elementary particles , as described by quantum field theory . Chemical reactions such as combustion in fire, fermentation and
5451-404: The reactants on the left and those of the products on the right. They are separated by an arrow (→) which indicates the direction and type of the reaction; the arrow is read as the word "yields". The tip of the arrow points in the direction in which the reaction proceeds. A double arrow (⇌) pointing in opposite directions is used for equilibrium reactions . Equations should be balanced according to
5530-457: The reaction can be indicated above the reaction arrow; examples of such additions are water, heat, illumination, a catalyst , etc. Similarly, some minor products can be placed below the arrow, often with a minus sign. Retrosynthetic analysis can be applied to design a complex synthesis reaction. Here the analysis starts from the products, for example by splitting selected chemical bonds, to arrive at plausible initial reagents. A special arrow (⇒)
5609-432: The reaction is accompanied by an energy change as new products are generated. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms , with no change to the nuclei (no change to the elements present), and can often be described by a chemical equation . Nuclear chemistry is a sub-discipline of chemistry that involves
5688-477: The reaction. They require less energy to proceed in the forward direction. Reactions are usually written as forward reactions in the direction in which they are spontaneous. Examples: Reactions that proceed in the backward direction to approach equilibrium are often called non-spontaneous reactions , that is, Δ G {\displaystyle \Delta G} is positive, which means that if they occur at constant temperature and pressure, they increase
5767-703: The reduction of ores to metals were known since antiquity. Initial theories of transformation of materials were developed by Greek philosophers, such as the Four-Element Theory of Empedocles stating that any substance is composed of the four basic elements – fire, water, air and earth. In the Middle Ages , chemical transformations were studied by alchemists . They attempted, in particular, to convert lead into gold , for which purpose they used reactions of lead and lead-copper alloys with sulfur . The artificial production of chemical substances already
5846-463: The same molecular formula C 4 H 10 , but they have different structural formulas as shown. The connectivity of a molecule often has a strong influence on its physical and chemical properties and behavior. Two molecules composed of the same numbers of the same types of atoms (i.e. a pair of isomers ) might have completely different chemical and/or physical properties if the atoms are connected differently or in different positions. In such cases,
5925-426: The standard for ionic compounds , such as CaCl 2 , and for macromolecules, such as SiO 2 . An empirical formula makes no reference to isomerism , structure, or absolute number of atoms. The term empirical refers to the process of elemental analysis , a technique of analytical chemistry used to determine the relative percent composition of a pure chemical substance by element. For example, hexane has
6004-498: The standards of chemical formulae, and technically are chemical naming systems, not formula systems. For polymers in condensed chemical formulae, parentheses are placed around the repeating unit. For example, a hydrocarbon molecule that is described as CH 3 (CH 2 ) 50 CH 3 , is a molecule with fifty repeating units. If the number of repeating units is unknown or variable, the letter n may be used to indicate this formula: CH 3 (CH 2 ) n CH 3 . For ions ,
6083-527: The symbol has been explained by the authors as being concise, readily printed and transmitted electronically (the at sign is included in ASCII , which most modern character encoding schemes are based on), and the visual aspects suggesting the structure of an endohedral fullerene. Chemical formulae most often use integers for each element. However, there is a class of compounds, called non-stoichiometric compounds , that cannot be represented by small integers. Such
6162-405: The symbols begin with the same letter (so "B" comes before "Be", which comes before "Br"). The following example formulae are written using the Hill system, and listed in Hill order: Chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and
6241-456: Was a central goal for medieval alchemists. Examples include the synthesis of ammonium chloride from organic substances as described in the works (c. 850–950) attributed to Jābir ibn Ḥayyān , or the production of mineral acids such as sulfuric and nitric acids by later alchemists, starting from c. 1300. The production of mineral acids involved the heating of sulfate and nitrate minerals such as copper sulfate , alum and saltpeter . In
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