WILL A REACTION OCCUR

It is possible to write chemical equations for reactions that do not occur, or which occur only to a limited extent. This can be illustrated with a couple of examples. Consider the laboratory problem faced by a technician performing studies of plant nutrient metal ions leached from soil by water. The technician was using atomic absorption analysis, a sensitive instrumental technique for the determination of metal ions in solution. While determining the concentration of zinc ion, Zn +, dissolved in the soil leachate, the technician ran out of standard zinc solution used to provide known concentrations of zinc to calibrate the instrument, so that its readings would give known values from the sample solutions. Each liter of the standard solution contained exactly Img of zinc in the form of dissolved zinc chloride, ZnCl2. The 

When the 100 mg piece of zinc metal was added to some hydrochloric acid in a flask, bubbles of hydrogen gas were evolved, the zinc dissolved as zinc chloride, and the standard solution containing the desired concentration of dissolved zinc was prepared according to the plan. 

A piece of zinc metal in contact with hydrochloric acid reacts rapidly, giving off hydrogen gas and going into solution as ZnCl2. 

A piece of copper metal (wire) placed in hydrochloric acid does not react. 

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In view of the electron affinity of a fluorine atom, we can speculate on what would be the result of a collision between two fluorine atoms. Will a reaction occur The energy is one of the factors which determines the answer. First let us consider a reaction that does not occur spontaneously. 

Will a reaction occur when copper metal is dipped into a solution of silver nitrate Explain. 

Will a reaction occur between (a) nickel metal and hydrochloric acid and (b) tin metal and a solution of aluminum chloride Write balanced equations for the reactions. 

If a copper wire is placed into a solution of lead (11) nitrate, will a reaction occur Explain. 

Discussion of Markovnikovs Rule raises two important general questions about chemical reactions (1) Under what conditions is a reaction likely to proceed (2) How rapidly will a reaction occur We will consider these questions briefly in the next two sections before continuing our survey of the reactions of alkenes. 

Now, will a reaction occur Check the activity series, then answer. 

The last chapter in this introductory part covers the basic physical chemistry that is required for using the rest of the book. The main ideas of this chapter relate to basic thermodynamics and kinetics. The thermodynamic conditions determine whether a reaction will occur spontaneously, and if so whether the reaction releases energy and how much of the products are produced compared to the amount of reactants once the system reaches thermodynamic equilibrium. Kinetics, on the other hand, determine how fast a reaction occurs if it is thermodynamically favorable. In the natural environment, we have systems for which reactions would be thermodynamically favorable, but the kinetics are so slow that the system remains in a state of perpetual disequilibrium. A good example of one such system is our atmosphere, as is also covered later in Chapter 7. As part of the presentation of thermodynamics, a section on oxidation-reduction (redox) is included in this chapter. This is meant primarily as preparation for Chapter 16, but it is important to keep this material in mind for the rest of the book as well, since redox reactions are responsible for many of the elemental transitions in biogeochemical cycles. 

The magnitude of an equilibrium constant tells us nothing about how fast the system will reach equilibrium. Equilibrium constants are thermodynamic quantities, whereas the speed of a reaction is a kinetic quantity. The two are not related. Rather, an equilibrium constant is a measure of the extent to which a reaction occurs. 

In previous chapters, we considered questions like How much energy does a reaction liberate or consume and In which direction will a reaction proceed We then asked questions like To what extent will a reaction proceed in that direction, before it stops and even Why do reactions occur at all In this chapter, we look at a different question How fast does a reaction proceed Straightaway, we make assumptions. Firstly, we need to know whether the reaction under study can occur there is no point in looking at how fast it is not going if a reaction is not thermodynamically feasible So we first assume the reaction can and does occur. 

Although we appreciate from Equation (8.3) that the reaction will stop when one or both of the concentration terms reaches zero, we should also appreciate that the concentration terms reach zero faster if the value of k is large, and the concentrations deplete more slowly if k is smaller. We see how the value of the rate constant is important, because it tells us how fast a reaction occurs. 

Directions No Calculators may be used with Part B. Write the formulas to show the reactants and the products for any FIVE of the laboratory situations described below. Answers to more than five choices will not be graded. In all cases a reaction occurs. Assume that solutions are aqueous unless otherwise indicated. Represent substances in solution as ions if the substances are extensively ionized. Omit formulas for any ions or molecules that are unchanged by the reaction. You need not balance the equations. The Section II score weighting for this question is 15%. 

Many times, we can use thermodynamics to predict whether a reaction will occur spontaneously, but it gives very little information about the speed at which a reaction occurs. Kinetics is the study of the speed of reactions. It is largely an experimental science. Some general qualitative ideas about reaction speed may be applied, but accurate quantitative relationships require that we collect experimental data. 

When a reaction occurs between gaseous species or in solution, chemists usually express the reaction rate as a change in the concentration of the reactant or product per unit time. Recall, from your previous chemistry course, that the concentration of a compound (in mol/L) is symbolized by placing square brackets, [ ], around the chemical formula. The equation below is the equation you will work with most often in this section. 

B. COMPONENT CONTINUITY EQUATIONS (COMPONENT BALANCES). Unlike mass, chemical components are not conserved. If a reaction occurs inside a system, the number of moles of an individual component will increase if it is a... 

The risk of rechallenge has to be very carefully considered. Its use will depend on the severity of the reaction, availability of a specific antidote, ease and speed of reversing the effect and the subject s willingness to be exposed for a second time. RechaUenge is not infrequently imdertaken in Phase I studies when an exaggerated response (Type A reaction) occurs and a smaller dose can... 

Now, this quantity impedance (Z) turns out upon detailed analysis to contain within the characteristics of its variation with frequency,48 properties of the reaction occurring at the electrode/solution interface. For example, if a reaction occurring there has as its rale-determining step the electron transfer, then the variation of the impedance with frequency will have certain characteristics different from those shown in the Z — log to plot if the rate-determining step involves instead diffusion in the solution. So, by working out how Z varies with log CD according to a chosen mechanism... 

Consider a reversible reaction with a ArG° value of-20 kJ mol-1. In which direction will this reaction occur at the point at which the reaction quotient, Qn is 1000 What is the equilibrium reaction constant, KT, of this reaction ... 

A reaction occurring in a bulk phase will show an increase in the rate with the area as shown in Fig. 5.3 for a reaction occurring in the film or at the interface, the rate will be linearly dependent on the interfacial area. The interfacial area in a dispersed two-phase liquid-liquid system can be estimated by measuring the rate of a suitable test reaction in a reactor with the known interfacial area (a flat interface, Section 5.3.2.1), and comparing it with the reaction rate in a dispersed system [6, 15]. A convenient reactive system for this purpose is a formate ester and 1-2 M aqueous NaOH. Formate esters are very reactive to hydroxide ion (fo typically around 25 M 1 s 1), so the reaction is complete inside the diffusion film, and the reaction rate is proportional to the interfacial area. A plot of the interfacial area per unit volume against the agitator speed obtained in this way in the author s laboratory for the equipment shown in Fig. 5.12 is shown in Fig. 5.14 [8]. 

The product ONOO has been suggested to mediate oxidation and nitration of macromolecules and following the observation that SOD enhanced the effect of the EDRF (219, 220) has been invoked to explain the majority of the cytotoxicity of NO in vivo (221). However, the relative pseudo-first-order rate constant is of equal importance as the overall rate constant in determining whether a reaction occurs in vivo. In other words, the impact a particular reaction will have in vivo is governed by the concentration of the reactants as much as by the overall rate constant [for a summary of evaluating RNOS, see Ref. (217)]. 

Since harmonic oscillators are considered in this theory, the bonds will never break, so it is necessary to introduce an ad hoc criterion for when a reaction occurs. Reaction is normally defined to occur when a particular bond length attains a critical value. The bond length cannot be extracted directly from a particular normal-mode coordinate, since these coordinates, typically, involve the motion of several atoms in the molecule. The bond length can, however, be calculated quite readily, by noting that the displacement of a coordinate associated with an atom of mass mr is given by Eq. (E.5) ... 

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