Speed of a reaction

The speed of a reaction (vj, sometimes called the absolute speed, is the derivative in relation to time of the extent of this reaction, so we will write  

The appUcation of relation [1.2] immediately leads to the extent at time t  

Through [1.4] we obtain the amount of any principal component of the reaction at the same time in a closed zone  

The exchange of the components between the zone and the exterior must be taken into account in the open zone, and therefore  

The reaction speed in a monozone reaction is proportional at each moment to the volume (or surface in the case of a siuface zone) of the zone concerned at this instant. For instance, for the gas phase of reaction [l. Rl], the reaction volume is the volume of the reactor that contains the gaseous constituents of the reaction. 

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The reaction should be relatively fast. (Most ionic reactions satisfy this condition.) In some cases the addition of a catalyst may be necessary to increase the speed of a reaction. 

How fast do chemical reactions occur The speed of a reaction is described by its rate. Rate is the number of events per unit time, such as the number of molecules reacting per second. Every elementary reaction has a characteristic rate. Some reactions are so fast that they are complete in the smallest measurable traction of a second, whereas others are so slow that they require almost an eternity to reach completion. The observed rate of an overall chemical reaction is determined by the rates of the elementary reactions that make up the mechanism. 

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. 

The magnitude of the rate constant, k, indicates the speed of a reaction. 

You have seen that a large rate constant indicates a fast reaction, while a small rate constant indicates a slow reaction. It is not always easy, however, to determine how long a reaction will take to he completed just hy looking at the value of k. How can you relate the speed of a reaction to the units of s for a first-order reaction ... 

On the other hand, there are some reactions whose speed is diminished with a rise of temp., and others again in which the reaction is reversed by raising the temp. It is probable that when the speed of a reaction is slow enough for convenient measurement, the relative number of reacting or active molecules is small. The increase in the number of active molecules with temp, increases with the sp. ht. Cv and the increase in the speed of the reaction is greater the greater the value of Cv. 

An important advantage of these techniques is that the measurements of electrical potential can be very accurate, which allows monitoring until almost complete conversion, or until equilibrium in a reversible process. In fact, potentiometry is extremely powerful for obtaining equilibrium constants [25]. However, there are also restrictions and limitations (a) the solution must be conducting (b) the response time of the electrode can be relatively long, so there is a limit to the speed of a reaction which can be monitored and (c) there can be appreciable interference from impurities, or intermediates and products. 

Marcus is a coauthor of the Rice-Ramsperger-Kassel-Marcus theory of molecular reactions. However, his theoretical work in the 1950s on electron transfer reactions started him on his pathway to eminence. Back then, chemists knew electron-transfer processes were occurring during chemical reactions, but they had no way to predict the speed of a reaction or to develop strategic chemical experiments. In addition, some reactions that were predicted to proceed rapidly instead poked along at a snail s pace. 

Most chemical reactions give off heat and are classified as exothermic reactions. The rate of a reaction may be calculated by the Arrhenius equation, which contains absolute temperature, K, equal to the Celsius temperature plus 273, in an exponential term. As a general rule, the speed of a reaction doubles for each 10°C increase in temperature. Reaction rates are important in fires or explosions involving hazardous chemicals. A remarkable aspect of biochemical reactions is that they occur rapidly at very mild conditions, typically at body temperature in humans (see Chapter 3). For example, industrial fixation of atmospheric elemental nitrogen to produce chemically bound nitrogen in ammonia requires very high temperatures and pressures, whereas Rhizobium bacteria accomplish the same thing under ambient conditions. 

Catalyst Substance that brings about a change in the speed of a reaction without being changed itself. 

In this article we approach the topic of coherent control from the perspective of a chemist who wishes to maximize the yield of a particular product of a chemical reaction. The traditional approach to this problem is to utilize the principles of thermodynamics and kinetics to shift the equilibrium and increase the speed of a reaction, perhaps using a catalyst to increase the yield. Powerful as these methods are, however, they have inherent limitations. They are not useful, for example, if one wishes to produce molecules in a single quantum state or aligned along some spatial axis. Even for bulk samples averaged over many quantum states, conventional methods may be ineffective in maximizing the yield of a minor side product. 

Roughly a hundred years earlier Sir Joseph Lister suffered exposure to exactly the same total amount of phenol with no blisters and no scarring at all. Sir Joseph conducted surgical operations under a fine spray of a very dilute solution of phenol. The speed of a reaction in solution depends on the concentration of the solution. Thankfully there are better ways of ensuring hygiene in operating theatres now as phenol is nasty stuff. 

Catalysts and enzymes in very small amounts greatly alter the speed of a reaction. They cannot make impossible reactions work, but can alter the rate of those reaction that are possible. They do not alter the product of the reaction, only the speed you make them. Providing the catalysts are not destroyed or poisoned they can be used over and over again. How can such tiny amounts of a substance be so important As you know, any chemical reaction happens in several steps. We hesitate to repeat them, but we will ... 

An ideal catalyst will increase the speed of a reaction without itself being destroyed or used up during the reaction. 

The speed of a reaction between molecules in a film and those in the underlying solution depends on the rate of approach of the latter to the interface. There are two ways of exactly evaluating the latter, one depending, as we have seen in Section II, on the use of the gas laws and the other using the diffusion coefficient of the molecules in solution. The former method w as suggested by Fosbinder and Rideal (16), who used the Hertz expression,... 

However, the required approximations for the validity of equation (4) are rather difficult to meet in practice. Another integral parameter used for the characterization of the speed of a reaction is the characteristic temperature Ts [3]. When a sample is heated and reaches Ts (starting from a low value To where no decomposition occurs), only 36.8% (100/e %) of the initial sample is supposed to remain un-pyrolysed. This parameter is particularly useful when the sample decomposes rapidly during TRT. Ts depends on the nature of the compound to be pyrolysed (through k), on the heating rate q, and on the starting temperature To. Ts has a meaning only if Ts < Teq. 

The speed of a reaction depends on many factors. Concentrations of reacting species certainly play a major role in speeding up or slowing down a particular reaction (Fig. 18.1). As we will see in Section 18.5, many reaction rates are extremely sensitive to temperature. This means that careful control of temperature is critical... 

A catalyst is a chemical that changes the speed of a reaction, but does not change in the reaction. Some catalysts are a single substance. Other catalysts are a combination of many substances. Catalysts do not start reactions. Most catalysts speed up reactions. Inhibitors are catalysts that slow down reactions. Some inhibitors cause other catalysts to stop working. 

From Table 7-1, the formation of diamond from graphite (the standard state of carbon) is accompanied by a positive AH of 1.88 kJ/mol at 25 C. From Problem 16.1(0, AS for the same process is negative. Since 25 °C is not the transition temperature, the process is not a reversible one. In fact, it is not even a spontaneous irreversible process, and (16-2) does not apply with the inequality sign. On the contrary, the opposite process, the conversion of diamond to graphite at one atmosphere, is thermodynamically spontaneous, and AS for this process would obey (16-2) with the inequality sign. It may come as an unpleasant surprise to jewelers to learn that diamond is unstable. The term spontaneous, however, implies nothing about the speed of a reaction, which in this case is interminably slow at normal temperatures in the absence of a catalyst. 

As a means to predict the properties of drug delivery systems, it is useful to briefly review some basic thermodynamic functions. The rate or speed of a reaction is given by... 

KINETICS, the SUBJECT OF THIS CHAPTER, PROVIDES ANSWERS TO THE QUESTION ABOUT THE SPEED OF A REACTION. ThE LAST QUESTION IS ONE OF MANY ANSWERED BY THE STUDY OF CHEMICAL EQUILIBRIUM, WHICH WILL BE CONSIDERED IN CHAPTERS 14, 15, l6, AND 18. 

Getting Things Moving The metabolic processes of cold-blooded animals like this baby Nile crocodile (Crocodylus niloticus) speed up as the temperatures rise toward midday. In this chapter, you ll see how temperature, as well as several other factors, influences the speed of a reaction. 

The Reaction Must Be Possible. Since catalysts affect only the rate or speed of a reaction and have nothing to do with the inherent tendency of a reaction to proceed, it is obviously of real importance to know whether or not the reaction is at all possible for the conditions of temperature and pressure chosen. The concern is then with chemical affinity and since the free energy change of a definite, specific reaction is a quantitative measure of such chemical forces, it is desirable to have free-energy data on the substances in question if such data can be had. If reliable free-energy data show the reaction to be possible, then experimental work may be started in order to find suitable catalysts that will enable the reaction to proceed at a reasonable rate. 

Finally, a statement is necessary concerning the temperature coefficient of hydrogenation reactions. Barring decrease in activity of the catalyst (such as sintering), few if any hydrogenation reactions ht the generalized statement that the speed of a reaction may be doubled for every 10-15°C increase in temperature. It usually takes 50°C or more to double the speed of a hydrogenation reaction. 

Why does equilibrium occur We saw earlier in this chapter that molecules react by colliding with one another, and that the more collisions, the faster the reaction. This is why the speed of a reaction depends on concentrations. In this case the concentrations of H2O and CO are lowered as the forward reaction occurs—that is, as products are formed. 

The reaction rate law is an experimentally determined mathematical relationship that relates the speed of a reaction to the concentrations of the reactants. 

If a reaction is spontaneous, does that mean it will be fast Thermodynamic spontaneity cannot tell us whether a reaction will he fast. The speed of a reaction is a kinetic property controlled hy the nature of the energy state of the ES complex and the transition state. Enzymes speed up the reaction rate by creating a situation where the distance between the transition state and the ES complex on an energy diagram is reduced. 

Recall Can the thermodynamic property AG° be used to predict the speed of a reaction in a living organism Why or why not ... 

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