Velocity of a reaction

OS 90] [R 31] [P 70] At weak electrical field, the propagation velocity of a reaction front in a capillary-flow reactor was increased or decreased depending on the mutual orientation of the electrical field and the reaction zone propagation [68]. The movement of two reaction fronts was given by optical images in [68]. 

The rate, or velocity, of a reaction is usually defined as the change with time t of the concentration (denoted by square brackets) of one of the reactants or of one of the products of the reaction that is. 

The law of mass action states that the velocity of a reaction at a given temperature is proportional to the product of the active masses of the reacting substances. To illustrate this law consider the reaction... 

Again, since the velocity of a reaction is proportional to the amount of the reacting substances present in the system, the second reaction will be the slowest at the start, and gradually become faster. The speed of formation of the iodine is therefore the resultant velocity of two consecutive reactions and the belated appearance of the iodine—the period of induction—corresponds with the time required for the first reaction to make enough hydriodic acid to enable the second reaction to make sufficient iodine to colour the starch. 

Maximal velocity The rate or velocity of a reaction (v) is the num ber of substrate molecules converted to product per unit time velocity is usually expressed as prnol of product formed per minute. The rate of an enzyme-catalyzed reaction increases with substrate concentration until a maximal velocity (Vmax) is reached (Figure 5.6). The leveling off of the reaction rate at high substrate concentrations reflects the saturation with substrate of all avail able binding sites on the enzyme molecules present. 

This is a very striking calculation of the absolute velocity of a reaction, but we must examine carefully the exact significance of the degree of numerical agreement between the two values of k. 

The first important fact is that, as Menschutkin showed, the velocity of a reaction may vary several hundredfold with change of solvent. No general explanation of this has been found, and it must be concluded that reaction rates are subject to highly specific influences of solvent molecules. 

The velocity of a reaction is therefore proportional to the total amount of enzyme (Et) in the previous reaction. If the activity of dihydrofolate reductase were to be inhibited tenfold by methotrexate, increasing Et tenfold can restore the velocity (amount of tetrahydro-folate made per time). 

In this formulation, the k s are the specific rate constants for the two reversible reactions. The velocity of a reaction is equal to the rate of formation of P minus the rate of disappearance of P and is given by... 

The expression for the reciprocal velocity of a reaction proceeding through a linear sequence of reactions with short-lived intermediates was first published in 1931 (19). In that paper it was used to disentangle the mechanism of the thermal hydrolysis of methanol catalyzed by copper. 

The initial velocity of a reaction catalyzed by a regulatory enzyme was determined over the following range of initial substrate concentrations ... 

So far we have been considering the velocity of a reaction which proceeds simultaneously and uniformly throughout the mass capable of reaction. Only in the discussion of contact action was there any mention of local effects, which, however, in that case remained localized. The phenomena now to be mentioned concern changes brought about by local causes in a substance or mixture capable of reaction, which then spread throughout the mass. 

Velocity of a reaction.—To be definite, let us suppose that the reaction produced in the S3n3tem studied is a combination at a certain instant t the system contains a mass m of the compound formed by this combination this mass increases with the time, so that at an instant later than this mass has a value greater than m. 

The velocity of a reaction, v, increases with the enzyme concentration, [E], if the substrate concentration, [S], is constant. 

The velocity of a reaction increases with temperature until a maximum is reached, after which the velocity decreases due to denaturation of the enzyme. 

The velocity of a reaction between two components A and B, according to the activity theory, is proportional to the activities of these reactants ... 

Let us look back at Figure 1.17. That is a so-called VOLCANO relation happens that if you plot on the ordinate axis the velocity of a reaction and on the ventricle abscissa axis bonding of the named substrates to an important radical bonded to the surface (as an intermediate). Bonding (including the heat of sublimation) of the catalyst substrate, one often finds a volcano shape. Now, in Table 1.3, I have listed the melting points of a number of refractory (i.e., stable) metals. What do you see Bonds to platinum will be in the middle of this group. It will bond well enough, but not too much so that reactants just adhere to the surface and do not desorb. 

It is not strictly true to say that the velocity of a reaction catalyzed by a strong acid or a strong base is universally or exactly proportional to the catalyst concentration. In the first place, this statement ignores the primary salt effect (see below), though deviations attributable to this cause are unlikely to exceed a few per cent in 0.1 N solution. In the second place it is once more becoming fashionable to describe such salts... 

Substrate concentration The rate of all enzymes is dependent on substrate concentration. Enzymes exhibit saturation kinetics their rate increases with increasing substrate concentration [S], but reaches a maximum velocity when the enzyme is saturated with substrate. For many enzymes, the Michaelis-Menten equation describes the relationship between Vi(the initial velocity of a reaction), [S], Vynax, cmd the K (the substrate concentration at which v, = Vi Vynax)-... 

A catalytic action is generally defined as one in which the velocity of a reaction is modified by the presence of a substance which is itself unchanged at the end of the reaction. The substance which causes such an action is called the catalytic agent or catalyst. The following general relations have heretofore been assumed to apply to catalytic actions. 

In the first place, the catalyst has the same chemical composition at the beginning and at the end of the reaction. A small amount of the catalyst is able to effect the transformation of a large amount of the reacting substance. A catalyst can only modify the velocity of a reaction, it is incapable of starting a reaction. A catalytic agent does not affect the final state of equilibrium of a reaction, or, in other words, the velocities of two opposing... 

In place of the relations pointed out, a catalytic action will be taken to be based upon the definition of a catalyst as a substance which may modify the velocity of a reaction without itself undergoing a change in chemical composition. No further limitations will be introduced, and it will be shown how the conclusions from this point of view compare with the conclusions derived from or based upon the description of catalytic reactions used heretofore. The definition given when used with the general equation of a chemical reaction evidently simplifies it from the structural or compositional point of view, because the chemical composition of one of the initial and final products of the reaction is the same. The way in which such a substance may modify the velocity of a reaction must next be considered, and it is this point which forms the crux of the general theory to be used. The general theory consists of what has been called the addition theory of chemical reactions. 

The general conditions which had been assumed to hold for catalytic reactions and which were used to determine whether a substance acted as catalyst, were given earlier in this chapter. They were evolved gradually as more and more reactions of this nature were studied, and it is not surprising therefore that this superstructure of conditions became top-heavy and that some of the conditions assumed to be essential in a catalytic reaction, at times were found not to hold. The question whether the equilibrium point of a reaction is changed by a catalyst is a case in point. If a catalyst only changes the velocity of a reaction, and exerts no other influence whatsoever, as assumed in the 6... 

The statement is sometimes made that the velocity of a reaction doubles for each 10°C rise in temperature. If this were true for the temperatures 298 K and 308 K, what would be the activation energy of the reaction Repeat for 373 and 383 K. By what factor will the rate constant be increased between 298 and 308 K if the activation energy is 40,000 cal/gmol ... 

The concentration of the enzyme-substrate complex influences the velocity of enzymatic reactions. The relationship between the velocity of a reaction and the concentration of substrates is described by the Michaelis-Menton equation ... 

Substrate Inhibition Substrate inhibition represents another example of cooperativity in enzyme kinetic reactions, but of a different profile than described to this point. With substrate inhibition kinetics, the velocity of a reaction increases (as expected for hyperbolic profiles) to an apex, however, beyond this point the velocity of the reaction decreases with increasing substrate concentrations (Fig. 4.7). 

The rate or velocity of a reaction is the change in reaction coordinate, i.e., the concentration of reactant or product, per unit of time. The reaction rate, v, which can be expressed either as the rate of disappearance of the reactant or that of the formation of the product is ... 

The dependence of the velocity of a reaction on concentration follows from the fact that the forces between the molecules are of short range so that reaction will take place only when the reactant molecules are almost in contact. Thus collision of two (or more) molecules must occur for reaction to take place, and since the probability of the two reacting molecules being in a specific place is proportional to the product of their concentrations, the rate of reaction will be proportional to the concentrations of the reactants. 

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