What is stoichiometry

Stoichiometry is the study of quantitative relationships between amounts of reactants used and products formed by a chemical reaction. Stoichiometry is based on the law of conservation of mass. The law of conservation of mass states that matter is neither created nor destroyed. Thus, in a chemical reaction, the mass of the reactants equals the mass of the products. You can use stoichiometry to answer questions about the amounts of reactants used or products formed by reactions. For example, look at the balanced chemical equation for the formation of table salt (NaCl). 

You could use stoichiometry to answer the following questions about the chemical reaction. 

When you look at a balanced equation, there are two ways to interpret the coefficients. The coefficients tell you how many individual particles are interacting in the chemical reaction. For example, from the chemical equation above, you learn that two sodium atoms react with one chlorine molecule to form two formula units of table salt. You also learn that two moles of sodium react with one mole of chlorine to form two moles of salt. What the coefficients do not tell you directly is the masses of the reactants and the products in the chemical reaction. 

Interpret the balanced chemical equation in terms of particles, moles, and mass. Show that the law of conservation of mass is observed. 

Solving Problems A chemistry Handbook Chemistry Matter and Change 113 

Are you interested in taking an active role in the health care of others Would you like to advise physicians as well as patients Then consider a career as a pharmacist. 

Pharmacists must understand the composition and use of prescribed drugs and medicines, and over-the-counter medications. They advise doctors and patients about proper use, harmfui combinations, and possibie side-effects. Aithough pharmaceuticai companies sup-piy most medicines, pharmacists may do the actual mixing of ingredients to form powders, tablets, capsules, ointments, and solutions. 

You can interpret this equation in terms of representative particles by saying that four atoms of iron react with three molecules of oxygen to produce two formula units of iron(III) oxide. But, remember that coefficients in an equation represent not only numbers of individual particles but also numbers of moles of particles. Therefore, you can also say that four moles of iron react with three moles of oxygen to produce two moles of iron(III) oxide. 

Does the chemical equation tell you anything about the masses of the reactants and products Not directly. But as you learned in Chapter 11, the mass of any substance can be determined by multiplying the number of moles of the substance by the conversion factor that relates mass and number of moles, which is the molar mass. Thus, the mass of the reactants can be calculated in this way. 

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For a reaction as complex as catalytic enantioselective cyclopropanation with zinc carbenoids, there are many experimental variables that influence the rate, yield and selectivity of the process. From an empirical point of view, it is important to identify the optimal combination of variables that affords the best results. From a mechanistic point of view, a great deal of valuable information can be gleaned from the response of a complex reaction system to changes in, inter alia, stoichiometry, addition order, solvent, temperature etc. Each of these features provides some insight into how the reagents and substrates interact with the catalyst or even what is the true nature of the catalytic species. 

It is reasonable to ask if it is possible to predict what the stoichiometry and geometry of the product resulting from the interaction of a particular metal ion with a particular ligand (or ligands) is likely to be. Can we make any progress towards this goal from our discussions in the earlier part of this book As will become increasingly clear, the answer is a mixed one sometimes the interplay of d electrons in the valence shell is of prime and direct importance, sometimes of little importance, but more often it is relevant, yet only in an indirect way. 

Are Side Reactions Important What is the Stoichiometry of the Reaction When a mixture of various species is present in a reaction vessel, one often has to worry about the possibility that several reactions, and not just a single reaction, may occur. If one is trying to study one particular reaction, side reactions complicate chemical analysis of the reaction mixture and mathematical analysis of the raw data. The stoichiometry of the reaction involved and the relative importance of the side reactions must be determined by qualitative and quantitative anal-lysis of the products of the reaction at various times. If one is to observe the growth and decay of intermediate products in series reactions, measurements must be made on the reaction system before the reaction goes to completion. 

By how fast , we are in effect asking What is the value of the rate of this reaction The reaction has a 1 1 stoichiometry so, following the model in Equation (8.3), the rate equation of reaction is... 

Strategy. Before we start, we need to know (i) what is the electrode reaction. Next, we need to determine (ii) the number of moles (or fractions thereof) of charge which flows through the cell. This is Faraday s first law in action. Knowing the number of moles, we then invoke Faraday s second law and decide from the reaction stoichiometry (iii) how many moles of metal are formed. Finally (iv), now knowing the number of moles of metal, we can calculate the mass of metal from the known atomic mass. The following procedure is therefore adopted ... 

For hydroxamic acids, it is generally assumed that it is the Af-hydroxyamide/keto form, as opposed to the hydroximic/hydroxyoxime form, that predominates in acid medium, the environment usually required for most precipitates or colors to form . It is in general unknown what is the stoichiometry and structure of most metal hydroxamate complexes in solution. Nevertheless, the reaction of the majority of hydroxamic acids with metal ions can be written schematically as shown in equation 2. 

This is the first time we have encountered multiple reactions. For these in general, if it is necessary to write N stoichiometric equations to describe what is happening, then it is necessary to follow the decomposition of N reaction components to describe the kinetics. Thus, in this system following C, or Cr, or Q alone will not give both ki and k2. At least two components must be followed. Then, from the stoichiometry, noting that + Cr + Q is constant, we can find the concentration of the third component. 

A crucially important finding is that submitochon-drial particles or vesicles from broken chloroplasts will synthesize ATP from ADP and P , when an artificial pH gradient is imposed.172186 Isolated purified FjF0 ATPase from a thermophilic Bacillus has been coreconstituted into liposomes with the light-driven proton pump bacteiiorhodopsin (Chapter 23). Illumination induced ATP synthesis.187 These observations support Mitchell s proposal that the ATP synthase is both spatially separate from the electron carriers in the membrane and utilizes the protonmotive force to make ATP. Thus, the passage of protons from the outside of the mitochondria back in through the ATP synthase induces the formation of ATP. What is the stoichiometry of this process ... 

What is a nonstoichiometric compound Give an example, and account for its lack of stoichiometry in terms of structure. 

The design of chemical reactors encompasses at least three fields of chemical engineering thermodynamics, kinetics, and heat transfer. For example, if a reaction is run in a typical batch reactor, a simple mixing vessel, what is the maximum conversion expected This is a thermodynamic question answered with knowledge of chemical equilibrium. Also, we might like to know how long the reaction should proceed to achieve a desired conversion. This is a kinetic question. We must know not only the stoichiometry of the reaction but also the rates of the forward and the reverse reactions. We might also wish to know how much heat must be transferred to or from the reactor to maintain isothermal conditions. This is a heat transfer problem in combination with a thermodynamic problem. We must know whether the reaction is endothermic or exothermic. 

What is the relationship between the rate of reaction obtained by monitoring the formation of H2 or I2 and the rate obtained by watching HI disappear The stoichiometry of the reaction says that two HI molecules are consumed for every molecule of H2 or I2 produced. This means that the rate of decomposition of HI is twice as fast as the rate at which H2 and I2 are formed. We can translate this relationship into a mathematical equation as follows ... 

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