Stoichiometric

Stoichiometry is the branch of chemistry dealing with the relative amounts of reactants and products in chemical reactions. In a balanced chemical reaction, the relationship between the amount of reactants and products typically form a ratio of integers. For example, in the reaction to form ammonia (NH3), exactly one molecule of nitrogen (N2) reacts with three molecules of hydrogen (H2) to produce two molecules of NH3:N2 + 3H2 → 2NH3Stoichiometry can be used to find a number such as the number of products (mass, moles, volume, etc) that can be produced with given reactants and percent yield (the percentage of reactants are given that are made into products). Stoichiometric calculations can predict how elements and components dissolved in the standard solution react in experimental conditions. Stoichiometry based on the law of conservation of mass: mass of the reactants equals the mass of products.Reaction stoichiometry describes the quantitative relationship between substances as they participate in a chemical reaction. In the example above, the reaction stoichiometry describes the ratio of 01:03:02 molecular nitrogen, hydrogen dan.amoniaThe composition describes stoichiometric (mass) the quantitative relationship between the elements in the compound. For example, the composition describes stoichiometric ratio of hydrogen in ammonia nitrogen compounds: 1 mol of ammonia consists of 1 mole of nitrogen and 3 moles of hydrogen. As the nitrogen atom is about 14 times heavier than a hydrogen atom, the mass ratio is 14:3, so 17 kg contains 14 kg of ammonia nitrogen and 3 kg of hydrogen.Stoichiometric amounts of reagents or stoichiometric ratio is the ratio of the number of optimal or where, assuming that the reaction proceeds to completion:1. All reagents consumed,2. There is no shortage of reagents,3. No excess reagent.Non-stoichiometric mixture, where the reaction has gone to completion, would have only a limiting reagent consumed completely.While almost all the reactions have integer-ratio of the number of units of material stoichiometry (moles, number of particles), some nonstoichiometric compounds are known that can not be represented by the ratio of natural numbers are well defined. These materials thus breaking the law of definite proportions forms the basis of the stoichiometry along with the law multiple proportions.Gas stoichiometry associated with reactions involving gas, where gas is at pressure, known temperature, and volume, and can be assumed to be an ideal gas. For gas, the volume ratio is ideally equal to the ideal gas law, but the ratio of the mass of a single reaction should be calculated from the molecular mass of the reactants and products. In practice, because of the isotope, the molar mass is used as a replacement when calculating the ratio of the masses.Contents• 1 Etymology• 2 Definitions• Balancing Chemical Reactions 3• 4 different stoichiometries in competing reactions• 5 stoichiometric coefficients• 6 Stoichiometry matrix• 7 Gas stoichiometry• 8 Stoichiometry burning• 9 References• 10 External linksEtymologyStoichiometric term derived from the Greek word στοιχεῖον stoicheion "element" and μέτρον metron "measure". In Greek patristic, said Stoichiometria used by Nicephorus to refer to the number of the number of rows of the canonical New Testament and some of the Apocrypha.DefinitionStoichiometry rests on a very basic laws that help to better understand, namely, the law of conservation of mass, law of definite proportions (ie, the law of constant composition) and the law of multiple proportions. In general, chemical reactions combine in specific ratios of chemicals. Since the chemical reaction can not create or destroy matter, or change one element into another, the amount of each element must be the same throughout the whole reaction. For example, the number of elements of X on the side of the reactants must equal the number of elements of X on the product side.Chemical reactions, such as macroscopic operating unit, consisting of only a very large number of elementary reactions, in which one molecule reacts with other molecules. As the molecules react (or group) consists of a particular set of atoms in the integer ratio, the ratio between the reactants in the reaction was also complete in integer ratios. The reaction can consume more than one molecule, and the number of stoichiometric calculate this amount, defined as positive for the product (added) and negative for reactants (deleted) [1].Different elements have different atomic masses, and as a collection of single atoms, molecules have a definite molar mass, measured in moles (6.02 × 1023 individual molecules, Avogadro's constant). By definition, carbon-12 has a molar mass of 12 g / mol. So to calculate the stoichiometry of the mass, the number of molecules needed for each reactant expressed in moles and multiplied by the molar mass of each to give the mass of each reactant per mole of reaction. Mass ratio can be calculated by dividing each with a total overall reaction.Balancing chemical reactionsStoichiometry is often used to balance chemical equations (stoichiometric reaction). For example, the two diatomic gases, hydrogen and oxygen, can combine to form a liquid, water, the exothermic reaction, as described by the following equation:2H2 + O2 → 2H2OReaction stoichiometry describes the ratio of 02:01:02 molecular hydrogen, oxygen, and water in the above equation.Stoichiometric term is also often used for the proportion of elements in a compound molar stoichiometric (stoichiometry composition). For example, the stoichiometry of hydrogen and oxygen in H2O is 2:1. In stoichiometric compounds, molar proportions are integers.Stoichiometry is not only used to balance chemical equations, but also used in the conversion, ie, convert from grams to moles, or from grams to milliliters. For example, to find the number of moles in 2.00 g NaCl, people will do the following:
 In the example above, when written in the form of fractions, multiplication identity forming units of grams, which is equivalent to one (g / g = 1), the amount generated from the mole (units required), as shown in the following equation,
 Stoichiometry are also used to find the right amount of reagents to be used in a chemical reaction (stoichiometric amount). An example is shown below using a thermite reaction,
 This equation shows that 1 mol of aluminum oxide and 2 moles of iron will be produced by 1 mol of iron (III) oxide and 2 moles of aluminum. So, to completely react with 85.0 g of iron (III) oxide (0.532 mol), 28.7 g (1.06 mol) of aluminum required.
 Different stoichiometries in competing reactionsOften, more than one reaction may be given the same starting material. Reactions may differ in their stoichiometry. For example, methylation of benzene (), through the Friedel-Crafts reaction using a catalyst, alcohol can produce single, double alcohol, or a product that is alcohol still more, as shown in the following example,

 In this example, where the reaction is controlled in part by the relative concentrations of the reactants.Stoichiometric coefficientsIn layman's terms, the stoichiometric coefficients (or stoichiometric number in the IUPAC nomenclature [2]) of any given component is the number of molecules participating in the reaction as written.For example, in the reaction CH4 + 2 O2 → CO2 + H2O 2, the stoichiometric coefficients of CH4 will be 1 and the stoichiometric coefficients of O2 would be 2.In technical terms-more precisely, the stoichiometric coefficients in a chemical reaction system of the i-th component is defined as

 where Ni is the number of molecules of i, and ξ is the progress variable or extent of reaction (Prigogine & Defay, p 18;. Prigogine, pp. 4-7,. Guggenheim, p 37 & 62).Ξ reaction rates can be considered as a product (or hypothetical) real, a molecule that is produced each time the reaction event occurs. It is the vast amount illustrate the progress of chemical reactions equal to the number of chemical transformations, as shown by the reaction equation on a molecular scale, divided by the Avogadro constant (essentially the number of chemical transformations). Changes in the extent of reaction given by dξ = DnB / νB, where νB is the stoichiometric number of each entity B reaction (reactants or products) DnB an appropriate amount. [3]Stoichiometric coefficients νi is the extent to which a chemical species participating in the reaction. The convention is to assign negative coefficients to reactants (consumed) and a positive effect on the product. However, any reaction can be seen as a "walk" in the opposite direction, and all the coefficients then change sign (as well as free energy). Is the reaction will actually go forward direction arbitrarily-selected or not depends on the amount of a substance that is present at any given time, which determines the kinetics and thermodynamics, ie, whether the equilibrium lies to the right or left.If one considers the actual reaction mechanisms, stoichiometric coefficients will always be integers, since elementary reactions always involve the whole molecule. If someone is using a composite representation of the reaction of the "whole", some might be a rational fraction. There are chemical species that do not participate in the reaction, because it's their stoichiometric coefficients zero. Each chemical species is regenerated, such as catalysts, also has the stoichiometric coefficients of zero.The simplest case is probably one of isomerism
 where νB = 1 since one molecule of B is produced every time a reaction occurs, while νA = -1 since the molecule must be consumed. In any chemical reaction, the total mass is not only preserved, but also the number of atoms of each type are preserved, and it is appropriate to impose constraints on the possible values ​​of the stoichiometric coefficients.Usually there are some reactions run simultaneously in a natural reaction systems, including those in biology. Because each component can participate in chemical reactions simultaneously, the stoichiometric coefficients of the i-th component of the k-th reaction is defined as
 so that the total (differential) change in the number of i-th component isThe size of the reaction provides the most obvious and most explicitly represent changes in the composition, even though they are not yet widely used.With a complex reaction system, it is often useful to consider both the representation of the system response in terms of number of chemicals present {} Ni (state variables), and the representation of the actual composition in terms of degree of freedom, as expressed by the extent of the reaction {} ξk. The transformation of the area into a vector expressing vector expressing the number of square matrix whose elements are the stoichiometric coefficients [νi k].Maximum and minimum for each ξk occurs whenever the first reactant depleted for the forward reaction, or the first of the "product" depleted if the reaction is seen as driven in the reverse direction. This is a purely kinematic restrictions on the reaction simplex, a hyperplane in composition space, or space N, the dimension equal to the number of linear-independent chemical reactions. It's certainly less than the number of chemical components, for each reaction manifests the relationship between at least two chemicals. Accessible region of the hyperplane depends on the amount of each chemical species actually present, contingent facts. The amount is different even can produce different hyperplanes, which all share the same algebraic stoichiometry.In accordance with the principles of chemical kinetics and thermodynamic equilibrium, every chemical reaction is reversible, at least to some degree, so any equilibrium must be an interior point of the simplex. As a result, extreme ξ for it will not happen unless the experimental system is prepared with zero initial amounts of several products.The number of independent physical reactions may be greater than the number of chemical components, and depends on a variety of reaction mechanisms. For example, there may be two (or more) reaction paths for the isomerism above. The reaction can occur by itself, but faster and with different intermediates, in the presence of a catalyst.The (dimensionless) "units" can be taken to be molecules or moles. Moles are most commonly used, but more suggestive envision additional chemical reactions in molecular terms. The s N and ξ are reduced to molar units by dividing by Avogadro's number. While mass unit dimensions can be used, comments on integers is then no longer valid.Stoichiometric matrix.In complex reactions, stoichiometries often represented in a more compact form called the stoichiometry matrix. Stoichiometric matrix is ​​denoted by the symbol,.If the network has a reaction and the reaction of the participating molecular species stoichiometric matrix will have corresponding rows and columns.For example, consider the reaction system is shown below:S1 → S2S2 + 5S3 + 4S3 → 2S2S3 S4 →S4 → S5.This system consists of four reactions and five different molecular species. Stoichiometric matrix for this system can be written as:
 where rows correspond to S1, S2, S3, S4 and S5, respectively. Note that the conversion process a matrix of stoichiometric reaction scheme could be a lossy transformation, for example, stoichiometries in the second reaction simplify when included in the matrix. This means that it is not always possible to recover the original scheme of the reaction stoichiometry matrix.Often the stoichiometry matrix combined with rate vector, v to form a compact equation describing the rate of change of the molecular species:Gas stoichiometryGas stoichiometry is the quantitative relationship (ratio) between the reactants and products in a chemical reaction with the reaction that produces gas. Gas stoichiometry applies when the resulting gas is assumed ideal, and the temperature, pressure and gas volume are all known. Ideal gas law is used for these calculations. Often, but not always, standard temperature and pressure (STP), which is taken as 0 ° C and 1 bar, and is used as a condition for gas stoichiometry calculations.Gas stoichiometry calculations solve known volume or mass of gaseous products or reactants. For example, if we want to calculate the volume of NO2 gas generated from the combustion of 100 g of NH3, the reaction:4NH3 (g) + 7O2 (g) → 4NO2 (g) + 6H2O (l)we will do the following calculation:
 There is a 1:1 molar ratio of NH3 to NO2 in the above balanced combustion reaction, so 5.871 mol NO2 is formed. We will use the ideal gas law to solve the volume at 0 ° C (273.15 K) and 1 atmosphere using the gas law constant R = 0.08206 L • atm • K-1 • mol-1:
 Gas stoichiometry often involves the need to know the molar mass of gas, given the density of the gas. Ideal gas law can be rearranged to obtain the relationship between the density and the molar mass of an ideal gas:and thus:where:
 = Absolute pressure of gas
 = Volume of gas
 = Number of moles
 = Universal, the ideal gas law constant = absolute temperature of gas
 = Gas density and
 = Mass of gas
 = Molar mass of gasStoichiometric combustionSee also: Air-fuel ratio and CombustionIn a combustion reaction, oxygen reacts with fuel, and the exact point where the oxygen is all consumed and burned all the fuel is defined as the stoichiometric point. With more oxygen (combustion overstoichiometric), some of them still did not react. Likewise, if combustion is incomplete due to lack of sufficient oxygen, the fuel remains unreacted. (BBM reacted too slowly to keep from burning or less mixing of fuel and oxygen -. Was not because stoichiometric) of different hydrocarbon fuels have different contents of carbon, hydrogen and other elements, so that they vary stoichiometry.With fuel mass [4]With volume [5]Percent fuel by massGasoline 14.7: 1 - 6.8%Natural gas 17.2: 1 7.9: 1 5.8%Propane (LP) 15.67: 1 23.9: 1 6.45%Ethanol 9: 1 - 11.1%6:47 methanol: 1 - 15.6%Hydrogen 34.3: 1 2:39: 1 2.9%Diesel 14.5: 1 0.094: 1 6.8%The gasoline engine can run at the stoichiometric air-to-fuel ratio, because gasoline is quite stable and the mixed (sprayed or carburetted) with air prior to ignition. Diesel engines, in contrast, run lean, with more air available than simple stoichiometry would require. Solar burned less stable and effective as it is injected, leaving little time for evaporation and mixing. Thus, it will form soot (black smoke) in the stoichiometric ratio.